Manual

pagecontentsSection 1Introduction

page contents

1-2 9200 OPTIMOD-AM Digital Audio Processor

1-3 How OPTIMOD-AM Audio Processing Is Different

1-4 Location of OPTIMOD-AM1-4 At the Transmitter is Best1-4 Where Access to the Transmitter Plant is Not Possible

1-5 Studio-Transmitter Link

1-8 Transmitter

1-13 Antenna System

1-14 Monitoring1-14 Modulation Monitors and Their RF Amplifiers1-14 Monitoring on Loudspeakers and Headphones

1-15 EAS Test1-15 Figure 1-1: Frequency Response, Monitor Rolloff Filter

1-17 Why the North American NRSC Standard?

1-18 Security Passcode For PC Control

1-19 Warranty, Feedback

OPTIMOD-AM Digital INTRODUCTION 1-1

9200 OPTIMOD-AM Digital Audio Processor

Orban’s all-digital 9200 OPTIMOD-AM Audio Processor can help you achieve the highestpossible quality in AM medium-wave and long-wave broadcast sound. OPTIMOD-AMdelivers louder, cleaner, brighter, “FM-like” audio with an open, fatigue-free quality thatattracts listeners and holds them. Because all processing is performed by high-speedmathematical calculations within Motorola DSP56004 Digital Signal Processing chips, theprocessing has cleanliness, quality, and stability over time and temperature that is un-matched by analog processors.

OPTIMOD-AM rides gain over an adjustable range of up to 25dB, compressing dynamicrange and compensating for operator gain-riding errors and for gain inconsistencies inautomated systems.

OPTIMOD-AM increases the density and loudness of the program material by multi-bandlimiting and multi-band distortion-canceling clipping — improving the consistency of thestation’s sound and increasing loudness and definition remarkably, without producingaudible side effects.

OPTIMOD-AM precisely controls peak levels to prevent overmodulation.

OPTIMOD-AM comes with eight factory preset sounds to accommodate almost any userrequirement. A single LESS-MORE control easily modifies any factory preset. The presetscan be further customized by the user (by FULL CONTROL), and can be stored and recalledon command.

OPTIMOD-AM can be remote-controlled by 5-12V pulses applied to eight programma-ble, optically-isolated ports. It can also be remote-controlled by an external computerrunning Orban remote software (included) and connected directly or via modem to OPTI-MOD-AM’s RS-232 serial port.

OPTIMOD-AM compensates for the high- and low-frequency rolloffs of typical AMreceivers with a fully-adjustable program equalizer — providing up to 20dB of high-fre-quency boost (at 5kHz) without producing the side effects encountered in conventionalprocessors. This equalizer can thus produce more extreme pre-emphasis appropriate for thevery narrow-band radios often found in countries outside North America. OPTIMOD-AM’sfully parametric low- and mid-frequency equalizers allow you to tailor your air sound toyour precise requirements and desires.

OPTIMOD-AM is designed to be installed at the transmitter, replacing all processingnormally employed at the transmitter site, including compressor, protection peak limiters,clippers, and high- and low-pass filters normally included within the transmitter.

OPTIMOD-AM controls the transmitter bandwidth as necessary to meet governmentregulations, regardless of program material or equalization. OPTIMOD-AM’s high-fre-quency bandwidth can be switched instantly in 500Hz increments between 4.5kHz and9.5kHz. The lower cutoff frequencies meet the output power spectral density requirementsof ITU-R 328-5 without further low-pass filtering at the transmitter, while the 9.5kHz filtermeets the requirements of the NRSC-1 standard (North America).

1-2 INTRODUCTION Orban 9200

OPTIMOD-AM compensates for inaccuracies in the pulse response (tilt, overshoot,ringing) of transmitters and antenna systems with a powerful four-parameter transmitterequalizer.

OPTIMOD-AM comes with a Monitor Rolloff Filter to accurately simulate the frequencyresponse of an “average” receiver, for use in studio monitoring.

OPTIMOD-AM can be equipped with optional AES/EBU Digital Input and Output,enabling the user to maintain an all-digital signal path. Both input and output are equippedwith sample-rate converters, and can operate at 32kHz, 44.1kHz, and 48kHz sample rates.

How OPTIMOD-AM Audio Processing IsDifferent

OPTIMOD-AM is a major breakthrough in broadcast audio processing. It enables you toachieve a level of audio quality on AM that was previously thought to be impossible. It fullyexploits the limits of the standard AM channel. Its capabilities may even exceed the limitsof some AM transmitters and antennas.

Because OPTIMOD-AM incorporates several audio processing innovations exclusiveto Orban products, you should not assume that it can be operated in the same way asless sophisticated processors. If you do, you may get disappointing results.

Take a little time now to familiarize yourself with OPTIMOD-AM — a small investment ofyour time now will yield large dividends in audio quality.

The rest of Section 1 explains how OPTIMOD-AM fits into the AM broadcast plant. Section2 explains how to install it. Section 3 tells how to properly operate OPTIMOD-AM. Section4 through Section 6 provide reference information.

OPTIMOD-AM was designed to deliver a high-quality “FM-like” sound to the listener’s earby pre-processing for the limitations of the average car or table radio (while avoiding audibleside effects and compromises in loudness or coverage). Because such processing can makeaudible many defects ordinarily lost in the usual sea of “AM mud,” it is very important thatthe source audio be as clean as possible. Read the copy of Orban’s Audio Quality in theAM Broadcast Plant enclosed with your OPTIMOD-AM — it contains valuable informa-tion and specific suggestions for improving the quality of your audio.

For best results, feed OPTIMOD-AM unprocessed audio. No other audio processing isnecessary or desirable.

If you wish to place level protection prior to your studio-transmitter link(STL), use the Orban 8200ST OPTIMOD-Studio Compressor/Limiter/HFLimiter/Clipper. The 8200ST can be adjusted so that it substitutes for thebroad-band AGC circuitry in OPTIMOD-AM, which is then defeated.


Location of OPTIMOD-AM

At the Transmitter is Best

The best location for OPTIMOD-AM is as close as possible to the transmitter, so thatOPTIMOD-AM’s output can be connected to the transmitter through a circuit path thatintroduces no change in the HF curve of the carefully peak-limited waveform at OPTIMOD-AM’s output. A shielded wire, with no active circuitry, is ideal.

Severe changes in the shape of the waveform can be caused by passing it through a circuitwith non-constant group delay and/or non-flat frequency response in the 30-9500Hz range.Deviation from flatness and phase-linearity will cause spurious modulation peaks becausethe shape of the peak-limited waveform is changed. Such peaks add nothing to averagemodulation. Thus, the average modulation must be lowered to accommodate those peaks sothat they do not overmodulate. Transformers can cause such problems.

To achieve satisfactory waveform fidelity, with negligible overshoots, the deviation fromlinear phase must be less than ±10 degrees, 30-9500Hz. Frequency response must be lessthan 3dB down at 0.15Hz (this is not a typographical error!), and less than 0.1dB down at9.5kHz.

Where Access to the Transmitter Plant is Not Possible

Sometimes, it is not possible to locate OPTIMOD-AM at the transmitter, and it must insteadbe located on the studio side of the link connecting the audio plant to the transmitter. (Thelink might be telephone/post lines, analog microwave radio, or various types of digitalpaths.)

This situation will yield lower performance than if OPTIMOD-AM is connected directly tothe transmitter, because artifacts that cannot be controlled by OPTIMOD-AM will beintroduced in the link to the transmitter, by transmitter peak limiters, clippers, filters, andthe transmitter itself. Even so, you should expect a noticeable improvement in sound.

In our experience, locating OPTIMOD-AM at the studio site instead of the transmitter sitewill usually result in 2-4dB lower average modulation level, and possibly reduced intelligi-bility. This deterioration will be directly related to the degree that the STL and the rest ofthe audio chain fail to meet the linear phase deviation and frequency response specificationabove.


Studio-Transmitter Link

The audio received at the transmitter site should be of as good quality as possible. To achievethe full audible benefit of OPTIMOD-AM processing, a studio-transmitter link (STL) thatis as flat as the bandwidth of OPTIMOD-AM as used in your plant (usually 4.5kHz or9.5kHz) should be used.

Because the audio processor controls peaks, it is not important that the audio link feedingOPTIMOD-AM’s input terminals be phase-linear. However, the link should have low noise,the flattest possible frequency response from 30-9500Hz, and low non-linear distortion.

OPTIMOD-AM at the Transmitter — Gain Control Before the STL

If the audio link between the studio and the transmitter is noisy, the audibility of this noisecan be minimized by performing the AGC function at the studio site. AGC applied beforethe audio link improves the signal-to-noise ratio because the average level on the link willbe greater. Compressing the signal may also be desirable if the STL has limited dynamicrange.

Further, many STLs require level control to prevent the STL from being overloaded.

To apply such level control and compression, we recommend the Orban Model 8200STCompressor/Limiter/HF Limiter/Clipper prior to the STL transmitter. The 8200ST is atwo-channel device; for mono installations, the other channel can be used for a second linkor for backup. The 8200ST performs the function of OPTIMOD-AM’s internal broad-bandautomatic gain control (AGC), while simultaneously protecting the STL. If this is done,OPTIMOD-AM’s broad-band AGC should be defeated by accessing the ST CHASSISfunction within the Setup menu and setting it to YES.

Where Access to the Transmitter Plant Is Not Possible

If the transmitter plant is not accessible, all audio processing must be done at the studio, andany damage that occurs later must be tolerated.

If a broad-band phase-linear link to the transmitter is available, one can minimize the usualperformance degradations caused by installing OPTIMOD-AM at the studio. Use OPTI-MOD-AM at the studio to directly feed the STL. The output of the STL should directly feedthe transmitter, with no intermediate processing of any kind (limiting, clipping, low- andhigh-pass filters). A broad-band phase-linear link would need to meet the same requirementsas required for the transmitter: The deviation from linear phase must be less than ±10degrees, 30-9500Hz, and frequency response must be less than 3dB down at 0.15Hz, andless than 0.1dB down at 9.5kHz.

Where only an audio link is available, feed the audio output of OPTIMOD-AM directly intothe link. If possible, the transmitter protection limiter should be adjusted so that audio isnormally just below the threshold of limiting: The transmitter protection limiter shouldrespond only to signals caused by faults or by spurious peaks introduced by imperfectionsin the link.


Where maximum quality is desired, it is wise to request that all equipment in the signal pathafter the studio be carefully measured and aligned, and qualified to meet the appropriatestandards for bandwidth, distortion, group delay and gain stability. Such equipment shouldbe measured at reasonable intervals.

Transmission from Studio to Transmitter

There are three types of studio-transmitter links (STLs) in common use in AM broadcastservice: microwave, digital, and analog land line (telephone/post line). In AM service, theaudio processor is most commonly located at the transmitter and fed unprocessed audio fromthe STL. However, sometimes this is impractical and the STL must pass processed audio,as discussed above.

Microwave STLs:

In general, a microwave STL provides high audio quality, as long as there is a line-of-sighttransmission path from studio to transmitter of less than 10 miles (16 km). If not, RFsignal-to-noise ratio, multipath distortion, and diffraction effects can cause serious qualityproblems.

Unless carefully designed, microwave STLs can introduce non-constant group delay in theaudio spectrum, distorting peak levels when used to pass processed audio. Nevertheless, ina system using a microwave STL the 9200 is sometimes located at the studio and anyovershoots induced by the link are tuned out as well as possible by the 9200’s transmitterequalizer. The 9200 can only be located at the transmitter if the signal-to-noise ratio of thelink is good enough to pass unprocessed audio. The signal-to-noise ratio of the link can beused optimally if the link is protected from overload by an Orban 8200ST Compres-sor/Limiter/HF Limiter/Clipper or an Orban Transmission Limiter.

If the 9200 is located at the transmitter and fed unprocessed audio from a microwave STL,it may be useful to use a companding-type noise reduction system (like dbx Type 2 or DolbySR) around the link. This will minimize any audible noise build-up caused by compressionwithin the 9200.

Some microwave links may be modified to meet the specification for frequency responseand phase linearity stated in “Location of OPTIMOD-AM: At the Transmitter is Best” onpage 1-4. Many such links have been designed to be easily configured at the factory forcomposite operation, where an entire FM stereo baseband is passed. The requirements formaintaining stereo separation in composite operation are similar to the requirements for highwaveform fidelity with low overshoot. Therefore, most links have the potential for excellentwaveform fidelity if they are configured for composite operation (even if a composite FMstereo signal is not actually being applied to the link).

Further, it is not unusual for a microwave STL to bounce because of a large infrasonic peakin its frequency response caused by an under-damped automatic frequency control (AFC)phase-locked loop. This bounce can increase the STL’s peak carrier deviation by as muchas 2dB, reducing average modulation. Many commercial STLs have this problem.

Some consultants presently offer modifications to minimize or eliminate this problem. Ifyour exciter or STL has this problem, you may contact Orban Customer Service for the latestinformation on such services.


Digital links:

Digital links may pass audio as straightforward PCM encoding, or they may apply lossy datareduction processing to the signal to reduce the number of bits per second required fortransmission through the digital link. Such processing will almost invariably distort peaklevels, and such links must therefore be carefully qualified before you use them to carry thepeak-controlled output of the 9200 to the transmitter or stereo encoder. For example, theMPEG-1 layer 2 algorithm can increase peak levels up to 4dB at 160kB/sec by adding largeamounts of quantization noise to the signal. While this noise may be psychoacousticallymasked by the desired program material, it is nevertheless large enough to affect peak levelsseverely. For any lossy compression system, the higher the data rate, the less the peak levelswill be corrupted by added noise, so use the highest data rate practical in your system.

Older-technology links may use straightforward PCM (pulse-code modulation) withoutlossy data reduction. These can be very transparent and can exhibit accurate pulse responseprovided that their input anti-aliasing filters and output reconstruction filters are rigorouslydesigned to achieve constant group delay over the frequency range that contains significantprogram energy. This is not particularly difficult to do with modern over-sampled convertertechnology.

Because the 9200’s output spectrum contains virtually no power above 9.5kHz, you maybypass any anti-aliasing filters in digital links driven by the 9200. This ensures the mostaccurate possible transient response.

NICAM is a sort of hybrid between PCM and lossy data reduction systems. It uses ablock-companded floating point representation of the signal with J.17 pre-emphasis.

Older technology converters (including some older NICAM encoders) may exhibit quanti-zation distortion unless they have been correctly dithered. Additionally, they can exhibitrapid changes in group delay around cut-off because their analog filters are ordinarily notgroup-delay equalized. The installing engineer should be aware of all of these potentialproblems when designing a transmission system.

Any problems can be minimized by driving a digital STL with the 9200’s optionalAES/EBU digital output, which will provide the most accurate interface to the STL. Thedigital input and output accommodate sample rates of 32kHz, 44.1kHz, and 48kHz.

Analog land line (PTT/post office line):

Analog land line quality is extremely variable, ranging from excellent to poor. Whether landlines should be used or not depends upon the quality of the lines locally available, and uponthe availability of other alternatives. Even the best land lines tend to slightly veil audioquality, due to line equalizer characteristics and phase shifts. Slight frequency responseirregularities and non-constant group delay characteristics will alter the peak-to-averageratio, and will thus reduce the effectiveness of any peak limiting performed prior to theirinputs.

Using Lossy Data Reduction in the Studio

Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 or DolbyAC2 to increase the storage time of digital playback media. In addition, source material isoften supplied through a lossy data reduction algorithm, whether from satellite or over land


lines. Sometimes, several encode/decode cycles will be cascaded before the material isfinally presented to OPTIMOD-AM’s input.

All such algorithms operate by increasing the quantization noise in discrete frequencybands. If not psychoacoustically masked by the program material, this noise may beperceived as distortion, “gurgling,” or other interference. Psychoacoustic calculations areused to ensure that the added noise is masked by the desired program material and not heard.Cascading several stages of such processing can raise the added quantization noise abovethe threshold of masking, such that it is heard. In addition, there are at least two othermechanisms peculiar to AM broadcasting that can cause the noise to become audible at theradio. First, OPTIMOD-AM’s multi-band limiter performs an “automatic equalization”function that can radically change the frequency balance of the program. This can causenoise that would otherwise have been masked to become unmasked because the psychoa-coustic masking conditions under which the masking thresholds were originally computedhave changed. Second, the radical high frequency rolloff of a typical AM radio can removeprogram material that was used to make the psychoacoustic masking calculations and thatwould otherwise have masked the added quantization noise.

Accordingly, if you use lossy data reduction in the studio, you should use the highest datarate possible. This maximizes the headroom between the added noise and the thresholdwhere it will be heard. Also, you should minimize the number of encode and decode cycles,because each cycle moves the added noise closer to the threshold where the added noise isheard.

Transmitter

The behavior of an FM station is more or less determined by the behavior of the exciter.Alas, this is not true in AM broadcast! The performance of an AM broadcast station is highlydependent upon the high-power sections of the transmitter, and upon the behavior of theantenna system.

The extremely high average power and the pre-emphasized high-frequency component ofaudio processed by OPTIMOD-AM put great demands upon the performance of thetransmitter and antenna system. While improved results can be expected from most plants,outstanding results can only be achieved by plants having transmitters that can accuratelyreproduce OPTIMOD-AM’s output without changing the shape of the waveform, andhaving wide-band, symmetrical antenna arrays.

Any AGCs, compressors, limiters, and clippers that follow OPTIMOD-AM in the circuitshould be bypassed. OPTIMOD-AM provides all of these functions itself.

Bypassing the Transmitter’s Internal Filters and Clippers

Some AM transmitters, especially those supplied to stations outside of North or SouthAmerica, contain built-in filters and clippers after their audio inputs. The filters may havevarious purposes: A low-pass filter is often included to ensure that the transmitter’s outputspectrum adheres to the occupied bandwidth specifications of the governing authority. A


high-pass filter may be present to protect the transmitter from damage. Safety clippers areoften present to prevent the modulator from being over-driven.

As discussed in earlier sections, accurate reproduction of OPTIMOD-AM’s output requiresthat the deviation from linear phase must be less than ±10 degrees, 30-9500Hz. Frequencyresponse must be less than 3dB down at 0.15Hz, and less than 0.1dB down at 9.5kHz.

The highly processed output of OPTIMOD-AM is carefully band-limited and peak-control-led. This output will often contain waveforms with flat-tops — like square waves. If thetransmitter has constant group delay above 30Hz, these difficult waveforms will be trans-mitted intact and peak modulation will be accurately controlled.

However, if low-frequency response is more than 3dB down at 0.15Hz, as would be the caseif a high-pass filter is present, the group delay above 30Hz will not be constant. For example,a typical 50Hz high-pass filter introduces significant non-constant group delay to 500Hz —ten times the cut-off frequency. This non-constant group delay will tilt the flat-tops producedby OPTIMOD-AM. The tilt increases the peak level of the audio waveform, but not theaverage level. This will force you to decrease the average modulation to prevent the spuriouspeaks from overmodulating.

Similarly, a typical EBU 4.5kHz filter will introduce significant non-constant group delaydown to 1kHz — about one-fourth the cut-off frequency. This will cause overshoot in thehighly-processed waveforms produced by OPTIMOD-AM. The overshoot increases thepeak level of the audio waveform, but not the average level. This will force you to decreaseaverage modulation even more.

Alternatively, if you do not decrease the average modulation to accommodate the spuriouspeaks introduced by the filters, the peaks will be clipped by the transmitter’s safety clipper.This will introduce out-of-band energy that will almost certainly violate the limits onoccupied bandwidth specified by the governing authority, and will greatly degrade thespectral control provided by OPTIMOD-AM.

To achieve the full performance capability built into OPTIMOD-AM, any filters in thetransmitter must be bypassed. This is essential!

OPTIMOD-AM contains low-pass and high-pass filters that are fullycapable of protecting the transmitter and controlling occupied bandwidth.Because of their location within OPTIMOD-AM, the internal filters do notintroduce spurious modulation peaks.

Any built-in peak clippers in the transmitter should be defeated.

OPTIMOD-AM contains a safety clipper that is fully capable of control-ling transmitter modulation without introducing out-of-band energy. If thedrive level to the transmitter is even slightly excessive, the transmitterclipper will be driven hard enough to create excessive spurious spectrum.To prevent this possibility, defeat any clipper within the transmitter.

This problem will be even worse if OPTIMOD-AM’s transmitter equalizeris in use. OPTIMOD-AM’s output level will frequently exceed 100%modulation because it is pre-distorted to complement the transmitter’spulse response. The transmitter’s built-in safety clipper will surely clip thispre-distorted waveform.


Power Supplies

An AM transmitter is required to provide 150% of equivalent unmodulated carrier powerwhen it is modulating 100%. High-voltage power supplies are subject to two major prob-lems: sag and resonance.

Sag is a result of inadequate steady-state regulation. It causes the conventional “carrier shift”that is seen on a modulation monitor. Good transmitter engineering practice usually limitsthis shift to −5% (which corresponds to about 0.5dB — not a highly significant loudnessloss).

A more serious problem is dynamic carrier shift, or “bounce.” This has been known to causeup to 3dB loudness loss. It is usually caused by resonances in the power supply’s LC filternetwork. Any LC network has a resonant frequency. In order to achieve reasonableefficiency, the power supply filter network must be under-damped. Therefore, this reso-nance is excited by high modulation, and can cause overmodulation on the low-voltagepeaks of the resonance.

Curing bounce is not at all straightforward because of the requirement that the power supplyfilter smooth the DC sufficiently to achieve low hum. One approach that has been employedis use of a 12-phase power supply. Upon rectification, the ripple component of the DC isdown about −40dB without filtering. A single-capacitor filter can thus be used, eliminatingthe filter inductor as a potential source of resonance with the capacitor.

Other sources of resonance include the modulation reactor and modulation transformer inconventional plate-modulated transmitters. Such transmitters will not greatly benefit from a12-phase power supply.

A new generation of transmitters employ switching modulation techniques to control“bounce” far better than do older plate-modulated designs. The latest transmitters using“digital” modulation techniques have even better performance.

Pre-1965 Transmitters

Some older transmitters were under-designed by today’s standards, because modern audioprocessing techniques to increase average modulation had not yet been developed, andbecause the designers of those transmitters assumed that average power demands on themodulator would be relatively small. If you have a transmitter designed before 1965, itshould be carefully monitored to make sure that OPTIMOD-AM processing is not overheat-ing the modulation transformer, the modulation reactor, or the power supply. The high-fre-quency boost performed by OPTIMOD-AM can cause unusually high voltages in the finalamplifier, which could cause arcing and/or component breakdown (although the latter isvery rare). There are no simple cures for such problems. Pre-1965 transmitters almostalways require substantial modification, including the addition of heavier-duty componentsand perhaps of a whole new power supply for the modulator alone. Because of dramaticimprovements in transmitter design since these transmitters were built, we recommend thatsuch transmitters be replaced. The latest solid-state transmitters not only sound audiblybetter on-air, but their higher efficiency results in substantially reduced operating powercosts.


Slew-induced Distortion

The audio community has given much attention to “transient intermodulation distortion” or,more correctly, “slew-induced distortion” (SID) in feedback amplifiers. This distortion iscaused by an amplifier having an insufficient available rate of change for its outputwaveform. Such an amplifier exhibits SID when program waveforms try to force its outputto change faster than this “slew-rate limit.”

The cause of this distortion is usually an open loop response that starts to roll off at a verylow frequency. Since this rolloff is usually introduced after the input stage, the output of theinput stage is forced by feedback to be highly pre-emphasized (to overcome the rolloff inthe following stage). This pre-emphasis makes it easy for the input stage to overload —which is the direct cause of SID.

It is not widely realized that certain transmitters are extremely prone to this type of distortionif they employ overall audio feedback around the transmitter (using rectified RF to supplythe feedback signal). Because of the delays involved in the transmitter, it is necessary to rolloff the open loop response at a very low frequency to make the feedback loop stable. Thisfeedback improves the steady-state distortion measurements (total harmonic distortion andSMPTE intermodulation tests). However, the conditions for severe SID still exist. Suchdistortion is highly offensive to the ear; it is a plausible explanation for why some transmit-ters may measure good, but sound bad.

Because the output of OPTIMOD-AM is pre-emphasized, it can cause severe audibledistortion in a transmitter that has considerable SID. SID is therefore of substantial concernin an OPTIMOD-AM installation. SID can be detected with the ITU-R difference-frequencyintermodulation test, which involves introducing a pair of high-frequency tones (closelyspaced in frequency) to the input of the system being evaluated. The level of the differencefrequency that results from passing these tones through the system is then measured.

Curing SID in a transmitter is not simple. The only way to do it is to reduce the amount ofoverall feedback employed, then to try to compensate for the decreased feedback bydecreasing the amount of high-frequency rolloff and, finally, to linearize the modulator stageby stage. THD and/or SMPTE IM test results may well be worse as a result of this process.But the transmitter will probably sound better.

Caution: Some of the procedures suggested in the preceding paragraphinvolve modifications to the transmitter that may cause it to violate appli-cable government regulations regarding occupied bandwidth. Check themodified transmitter with a spectrum analyzer to ensure that it still meetswhatever emission rules may be current.

Asymmetry

While any AM modulation system is limited by the physics of carrier pinch-off to anabsolute negative modulation limit of 100%, it is possible to modulate positive peaks as highas desired.

In the United States, the FCC permits positive peaks of up to 125% modulation. Othercountries have similar restrictions.

However, many transmitters cannot achieve such modulation without substantial distortion,if they can achieve it at all. The transmitter’s power supply can sometimes be strengthened


to correct this. Sometimes, RF drive capability to the final power amplifier must beincreased.

Voice, by its nature, is substantially asymmetrical. So asymmetrical modulation becamepopular in an attempt to increase the loudness of voice. Traditionally, this was achieved bypreserving the natural asymmetry of the voice signal. An asymmetry detector reversed thepolarity of the signal to maintain greater positive modulation. The peaks were then clippedto a level of −100%, +125%.

OPTIMOD-AM takes a different approach: OPTIMOD-AM’s input conditioning filtercontains a time dispersion circuit (phase scrambler) that makes asymmetrical input material,like voice, substantially symmetrical.

OPTIMOD-AM permits symmetrical or asymmetrical operation of both the safety clipperand multi-band distortion-canceling clipper. Asymmetrical clipping slightly increases loud-ness and brightness, and will produce dense positive peaks up to 125% if this is desired.However, such asymmetrical processing by its very nature produces both odd and even-or-der harmonic and IM distortion. While even-order harmonic distortion may sound pleas-ingly bright, IM distortion of any order sounds nasty.

There is really nothing lost by not modulating asymmetrically: Listening tests easilydemonstrate that modulating symmetrically (provided that time dispersion has been appliedto the audio) produces a considerably louder and cleaner sound than does asymmetricalmodulation that retains the natural asymmetry of its program material.

Some of the newer transmitters of the pulse-width modulation type have circuitry forholding the carrier shift constant with modulation. Since artificial asymmetry can introduceshort-term DC components (corresponding to dynamic upward carrier shift), such carriershift cancellation circuitry can become confused, resulting in further distortion.

Transmitter Equalization

OPTIMOD-AM’s transmitter equalizer can cure linear problems caused by the transmitteror antenna system.

The transmitter equalizer cannot cure non-linear problems, particularly those caused byinadequate power supplies, modulation transformers, or reactors. If any of these componentssaturate or otherwise fail to perform under heavy power demands, no amount of small-signalequalization will solve their problems.

OPTIMOD-AM was designed with the assumption that one audio processor would bedevoted to two transmitters, usually called main and standby (or alternate). Each transmittermay be called upon to change power at night, or to drive a different antenna array. Only onetransmitter would be on the air at a given time.

Thus, OPTIMOD-AM provides two analog outputs (called ANALOG OUT-PUT 1 and ANALOG OUTPUT 2), and one (optional) AES/EBU digitaloutput (DIGITAL OUTPUT). In its normal operating mode, OPTIMOD-AMprovides a single set of transmitter equalizer controls for each output.These controls include LF Breakpoint and LF Gain for the LF tilt equalizer,HF Shelf equalization, and HF Delay equalization

A special “NITE MODE” (mostly for use in North America), provides the ability to haveseparate sets of day and night transmitter equalization, low-pass filter, and high-pass filter


settings for day and night operation. To switch between the DAY/NIGHT settings, you mustfirst enable NITE MODE (in the Setup screen), in order to choose day and nite mode from thefront panel or by remote control.

For example, in countries observing NRSC standards you might want totransmit the full 9.5kHz bandwidth during the day, and, in cooperation withother stations on first-adjacent channels, reduce audio bandwidth to 5kHzat night. This will eliminate any skywave-induced “monkey-chatter” inter-ference between first-adjacent channels. Or your nighttime directionalantenna array might have poor VSWR performance at high modulatingfrequencies, and you might find that your transmitter works better andproduces less distortion if you limit the audio bandwidth to those frequen-cies where the antenna is well-behaved. Further, if you operate a talkformat during certain parts of the day, you will probably find that you canoperate the processing for a louder on-air sound if you restrict the transmit-ted bandwidth below the maximum permitted by government regulation.(Bear in mind that most AM radios have an audio bandwidth of 2.5-3kHzand changing transmission bandwidth from 5kHz to 9.5kHz will produceno audible difference on these radios.)

Antenna System

AM antenna systems, whether directional or non-directional, frequently exhibit inadequatebandwidth or asymmetrical impedance. Often, a system will exhibit both problems simulta-neously.

An antenna with inadequate bandwidth couples RF energy into space with less and lessefficiency at higher sideband frequencies (corresponding to higher modulation frequencies).It reflects these higher-frequency sideband components back into the transmitter, or dissi-pates them in the tuning networks. This not only results in dull sound on the air (and defeatsOPTIMOD-AM’s principal advantage: its ability to create a highly pre-emphasized signalwithout undesirable side effects); it also wastes energy, can cause distortion, and can shortenthe life of transmitter components.

Asymmetrical impedance is simply the common point impedance’s not being symmetricalon either side of the carrier frequency. This problem can cause transmitter misbehavior andsideband asymmetry, resulting in on-air distortion in receivers with envelope detectors.

Neither problem is easily solved. Unless the radio station engineer is a knowledgeableantenna specialist, a reputable outside antenna consultant should be employed to designcorrection networks for the system.

It should be noted that many antenna systems are perfectly adequate. However, if thetransmitter sounds significantly brighter and/or cleaner into a dummy load than it does intoyour antenna, the antenna system should be evaluated and corrected if necessary.

As noted above, if your circumstances or budget preclude correcting yourantenna’s bandwidth and/or symmetry, you will often get lower on-airdistortion if you set OPTIMOD-AM’s low-pass filter to a lower frequencythan the maximum permitted by the government. Because OPTIMOD-AM’s output bandwidth is easily adjustable in real time, it is very easy toexperiment to see which bandwidth gives the best audio quality on anaverage AM radio, given the quality of your transmitter and antenna.


Monitoring

Modulation Monitors and Their RF Amplifiers

Many AM modulation monitors (particularly older ones) indicate dynamic modulationinaccurately — even though they may accurately measure sine-wave modulation. Thisoccurs when the audio filter after the demodulator diode is not phase-linear, and producesovershoot and ringing. An incorrectly designed modulation monitor may indicate thatmodulation is as much as 3dB higher than it actually is.

When modulation monitors are used at locations distant from the transmitter, they are drivenfrom highly selective RF amplifiers. These have been known to suffer from similar prob-lems. They can overshoot and ring if the passband filters are too sharp, causing the monitorto falsely indicate high modulation.

If your modulation monitor does not agree with an oscilloscope monitoring the RF envelopeat the common point, do not assume that the monitor is indicating fast peaks that your eyecannot see. A probable cause of the disparity is overshoot in the modulation monitor or itsRF amplifier. If you observe this problem, we recommend that you assume that what yousee on the oscilloscope is correct — oscilloscopes are designed to display pulse waveformsaccurately. (Make sure the oscilloscope is switched to DC coupling.)

Note also that modulation percentages will vary depending on where in the transmissionsystem the RF sample is taken. Depending on the location observed, actual modulation canbe either lower or higher than modulation observed at the common point. What is crucial iswhether the carrier has in fact been pinched-off at the final amplifier, because carrierpinch-off is what causes splatter. If the carrier appears slightly suppressed because of apeculiar choice of monitoring point within the system, negative peaks will “fold around”zero instead of cutting off. This causes no problem with out-of-band radiation, and far-fieldradiation is likely to show normal AM modulation envelopes. We therefore recommend thatyou use an RF sample from the final amplifier.

Monitoring on Loudspeakers and Headphones

The output of a loudspeaker fed from the modulation monitor typically sounds shrill andstrident because, unlike virtually all real AM radios, the modulation monitor and loud-speaker have a flat response. Rolloff filtering can be used to supply monitors with audio thatmore closely resembles that heard over a typical receiver.

A Monitor Rolloff Filter is provided with the OPTIMOD-AM processor. The filter is asmall, separate unit designed to be installed between the modulation monitor and themonitor amplifier. (See page 2-10 for installation instructions). It provides complementaryde-emphasis and a 10kHz notch for off-air monitoring of NRSC standard audio — theoutput of the rolloff filter accurately simulates the sound of a “standard” NRSC receiver.Alternately, for use in non-NRSC countries, an adjustable 18dB/octave rolloff to comple-ment the 9200’s HF GAIN control can be selected with jumpers.

If a different tonal balance is desired for off-the-air monitoring, install a simple programequalizer after the Monitor Rolloff Filter, and boost the 5kHz region to taste.


Note: Normal 9200 processing may not allow the full modulation level asrequired by EAS standards. It is therefore necessary to temporarily defeatthe 9200’s processing during the broadcast of EAS tones and data. Defeat-ing the processing can be done by placing the 9200 in BYPASS mode. TheBYPASS GAIN control allows a fixed gain trim through the 9200. See page2-53 for more information.

To place the 9200 in BYPASS mode locally:

a) Press Setup button.

b) Press TEST soft key button.

c) Hold down the MODE soft key button, turn the control knob to displaybypass, then release the button.

d) Begin EAS broadcast.

After the EAS broadcast, resume normal processing:


b) Press TEST soft key button.

c) Hold down the MODE soft key button, turn the control knob to displayoperate, then release the button. This will restore the processing presetin use prior to the test mode.

d) Alternately, you may press Recall button to exit bypass test.

•Place the 9200 in BYPASS mode by remote control. To do this, you must programany two REMOTE INTERFACE inputs for “test” and “exit test,” respectively:


b) Press REMOTE soft key button.

c) Press REMOTE INTERFACE soft key button.

d) Select the desired REMOTE INTERFACE inputs (1 - 8) using Next andPrev keys to display additional pages.

e) Hold down either soft key button below the desired REMOTE INTER-FACE input and turn the control knob to display “test: bypass,” thenrelease the button.

f) With a different REMOTE INTERFACE input, hold down either softkey button below the desired input and turn the control knob to display“exit test,” then release the button.

g) Connect two outputs from your station remote control system to theREMOTE INTERFACE connector on the rear panel of the 9200, ac-cording to the wiring diagram on page 2-9.

To place the 9200 in BYPASS mode by remote control:

a) A momentary command from your station’s remote control to the inputprogrammed as “test: bypass” will switch the 9200 into BYPASSmode.

b) Begin EAS broadcast.

After the EAS broadcast, resume normal processing by remote control:

a) A momentary command from your station’s remote control to the inputprogrammed as “exit test” will restore the processing preset in useprior to the test mode.

•You may also choose to insert EAS broadcast tones and data directly into thetransmitter for the duration of the EAS broadcast.


Why the North American NRSC Standard?

Over the years, as the North American air waves has become more crowded, interferencefrom first and second adjacent stations has become more and more of a problem. Receivermanufacturers responded by producing receivers with decreased audio bandwidth, so thatan adjacent station’s modulation extremes would not be audible as interference. This cuttingof the bandwidth had the effect of reducing the receiver’s high-frequency response, but itwas felt that lower fidelity would be less obnoxious than interference. As long ago as 1978,Orban proposed and implemented pre-emphasis and low-pass filtering for AM broadcast toprovide brighter sound at the receiver while minimizing interference. This approach hasbecome widely accepted. The NRSC-formalized standard is acceptable to all industrysegments, and when implemented, can result in a vast improvement in AM radio.

AM Stereo Introduces a Pre-emphasis Dilemma

Certain AM receivers manufactured since 1984 for sale in North America, particularly thosedesigned for domestic AM stereo reception, have a frequency response that is substantiallywider than that of the typical mono AM receiver. The frequency response was widenedlargely to enhance the sales potential of AM stereo by presenting a dramatic, audibleimprovement in fidelity in the showroom. As these new receivers became more prevalent,broadcasters had to choose whether the station’s pre-emphasis would be optimized for thenew AM stereo receivers or for the existing conventional receivers that form the vastmajority of the market. If the choice was for conventional receivers (which implies arelatively extreme pre-emphasis), the newer receivers might sound strident or exceptionallybright. If the choice favored the newer receivers (less pre-emphasis and probably lessprocessing), the majority of receivers would be deprived of much high-end energy andwould sound both quieter and duller.

NRSC Standard Pre-emphasis and Low-pass Filtering

In response to this dilemma, the National Radio Systems Committee (NRSC) undertook thedifficult task of defining a voluntary recommended pre-emphasis curve for AM radio thatwould be acceptable to broadcasters (who want the highest quality sound on the majority oftheir listeners’ radios) and to receiver manufacturers (who are primarily concerned withinterference from first- and second-adjacent stations).

After a year of deliberation, a “modified 75-microsecond” pre-emphasis/de-emphasis stand-ard was approved (See Figure 1-2). This provides a moderate amount of improvement forexisting narrowband radios, while optimizing the sound of wideband radios. Most impor-tantly, it generates substantially less first-adjacent interference than do steeper pre-emphasiscurves. The second part of the NRSC standard calls for a sharp upper limit of 10kHz (at−15dB) for the audio presented to the transmitter. OPTIMOD-AM’s “NRSC” low-passsetting is essentially flat to 9.5kHz and substantially exceeds the NRSC standards above thatfrequency. This essentially eliminates interference to second and higher adjacencies. Whilesome have protested that this is inadequate and that 15kHz audio should be permitted, theunfortunate fact is that interference-free 15kHz audio could only be achieved by a completere-allocation of the AM band! The practical effect of widespread implementation of the


A front panel soft key, labeled END PC CONTROL can be pressed to end remote control ofthe 9200; this feature effectively prevents simultaneous remote and local control.

Warranty, Feedback

Warranty

The warranty, which can be enjoyed only by the first end-user of record, is located on theinside back cover of this manual. Save it for future reference. Details on obtaining factoryservice are provided in Section 5.

User Feedback Form

We are very interested in your comments about this product. Your suggestions for improve-ments to either the product or the manual will be carefully reviewed. A postpaid UserFeedback Form is provided in the back of the complete manual for your convenience. If itis missing, please write us at the address printed in the front of the manual, or call or fax ouroffices at the number listed. We will be happy to hear from you.


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