Listen Headphone seminar 2017€¦ · Complete audio test in a single software package Meas urement hardware to complete your test system Listen Int’l Sales

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Listen Int’l Sales Meeting 2015 1Confidential © Listen, Inc. 2015© Listen, Inc. 2016 1


Practical Headphone/Headset MeasurementsFor R&D + QC

Agenda1. About SoundCheck Software2. Interface and Measurement Hardware3. Typical Headphone Measurements4. Practical Examples:

a) Analog Headphonesb) Hi‐Res Headphonesc) ANC Headphonesd) Bluetooth Headsetse) Lightning Connected Headphones

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Complete audio test in a single software package Measurement hardware to complete your test system


One System, Many Configurations

R&D QCTests

Results(additional analysis)

Fully‐loaded, laboratory‐class system• Accuracy• Power• Flexibility

Basic, low cost production line system• Speed• Noise immunity• Automation

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SoundCheckTM Software

Interactive Troubleshooting

Automated Test Sequences

Data Analysis & Reporting


Listen Hardware

Provides the bridge between the acoustic interface and the computer:

Audio interfaces

Microphone power supplies

Audio test amplifiers

DC current monitors

Bluetooth test interfaces

MEMS microphone interfaces

Digital Serial Data Audio Interfaces

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AmpConnect ISCTM

All‐in‐one hardware interface simplifies measurement

Single USB interface to computer

70W power amplifier with built‐in current sensor

Headphone amplifier

2 microphone pre‐amplifiers & power supplies

Fully controlled by SoundCheck

Automatic calibration and setup

Automatic signal routing

Automatic input ranging

The only interface you need to realize a complete loudspeaker, headphone, headset or microphone test system.


AudioConnectTM

2 Channel USB Audio Interface

Selectable 0/20 dB gain

Electret/Condenser microphone power supply (SCM Mic power)

Automatic configuration and calibration by SoundCheck

All the features you need for audio test; none of the features that you don’t need.

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SoundConnect 2TM

Universal 2‐channel measurement microphone pre‐amp and power supply

Compatible with all types measurement microphones.– Electret/Condenser Microphones (Listen SCM)

– IEPE/CCLD/CCP Constant Current Pre‐Polarized Capsules

– 200V Externally Polarized LEMO Capsules

Wide frequency response for a range of applications– Bandwidth > 100kHz

– gain control from ‐20dB to +40dB in 10dB increments

– user‐selectable low and high pass filters

Low Self‐Noise; excellent noise immunity and ground loop immunity.

– 1 uVrms EIN in +30 dB gain range Cost‐effective, 2‐ channel USB controlled microphone power supply.


Bluetooth Interfaces

• Test Bluetooth sink and Bluetooth source devices (e.g. headsets, speakers, car‐kits)

• Comprehensive suite of codecs including A2DP SBC and APT‐X codecs, Hands Free Protocol (HFP) and Headset Protocol (HSP) CVSD and mSBC codecs

• Full control over bluetooth protocol settings, codec choice and transmitter power

• Control via front panel or from within SoundCheck

• Test Bluetooth sink devices (e.g. headsets, speakers, car‐kits)

• Simple setup with just one USB connection to the computer for power and control.

• Includes A2DP SBC and Headset Protocol (HSP) CVSD and mSBC codecs

• Control over codec choice, and transmitter power

• Control directly within SoundCheck• Rugged, compact and low cost

BTC‐4148 for R&D BQC‐4148 for QC

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Typical Analog Headphone Test SetupWith HATS for R&D


Typical Analog Headphone Test Setup with Couplers for QC

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Typical Headphone Measurements Frequency Response

– Sometimes with correction curve applied

Sensitivity (in dBSPL per stimulus level)

Distortion– THD– Rub& Buzz– Non‐Coherent Distortion

Impedance

Channel Balance (Left response vs. Right)

Max SPL (European standard for max loudness)– Provided as optional SoundCheck module


Practical Example: Analog Headphones

• Demonstration

• Walk Through Sequence

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“Hi‐Res” Audio circa 1986

Recording guitar with Sony PCM F1 and B&K microphones– Digital Audio format: 44.1kHz @ 16 bits!


What is High‐Res(olution) Audio (HRA)?• “The Digital Entertainment Group, Consumer Electronics

Association and The Recording Academy have, together with record labels, come up with a formal definition for high‐res audio.” – www.whathifi.com

• Audio that has a higher sampling frequency and bit depth than CD, which is 16‐bit/44.1kHz– Typically 96kHz or 192kHz at 24‐bit, but you can also have

88.2kHz and 176.4kHz too

• Most popular audio file formats– FLAC (Free Lossless Audio Codec)

– ALAC (Apple Lossless Audio Codec)

• But there’s no universal standard for high‐res audio!

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Practical Translation of Hi‐Res Audio Requirements

For Triangular Probability Distribution Dithered PCM Audio (Standard digital audio encoding format)

– 1‐bit = 6.02 dB of dynamic range (Ratio of max signal to noise floor)

– 16‐bit (CD audio) = > 96 dB dynamic range

– 24‐bit = > 144 dB dynamic range

– Human threshold of pain = 120 dBSPL

(Max legal exposure level for 1 hour = 94 dBSPL)

– Typical office space has ambient noise = 60‐70 dBSPL

Even 16‐bits of dynamic range is beyond the ability of almost all test environments to measure


Practical Translation of Hi‐Res Audio RequirementsBased on Sampling Theory

– Bandwidth = ½ Sampling Frequency, Nyquist limit

– Some bandwidth is lost due to low‐pass filtering prior to sampling in order to avoid alias products

– > 20 kHz bandwidth for 44.1~48 kHz sample rates

– > 80 kHz bandwidth for 192 kHz sample rates

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Electrical Interface

AudioConnect 4x4 & SoundConnect 2

‐ 192 kHz sample rate

‐ 24‐bit word depth

‐ 1 uVrms Equivalent Input Noise

Can match bandwidth and dynamic range requirements of theoretical high‐res audio.


Frequency Analysis In The Ear

70 Hz1 kHz 250 Hz4 kHz

20502005001k2k5k20k 10010k

Frequency [Hz]

mm0 5 10 15 20 25 30

Basilar Membrane

Cochlea(unwrapped)

Stapes

Ossicles

© Copyright CJS Labs

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Why Exceed 20kHz?!

• In order for a transducer to have a flat phase response out to 20kHz requires a flat magnitude out to 40kHz

– By definition, at resonance, there is a 90deg phase shift

• But inaudible high frequency components, e.g. 30&33kHz resonances, can produce intermodulation products that “beat” down into the audio band of frequencies



Limitations of couplers and ear simulators

– Conventional 711 ear simulator is only defined to 8 kHz

– No currently available ear simulator or coupler has defined behavior above 20 kHz

– Only ¼” or 1/8” microphones have flat frequency and phase response above

– Repeatable solution – ¼” microphone in a “flat plate” coupler

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How to Measure HRA?

• For acoustic measurements:

– First make sure the audio interface has a high sampling rate e.g. 192kHz or higher sampling rate and 24 bits or better

– Make sure the measurement microphone and preamplifier can measure accurately beyond the frequency range of interest e.g. at least above 40kHz

• 1/4 & 1/8th inch measurement mics can measure up to 100kHz & 160kHz respectively


Phase Response of Measurement Mics

• From Microphone Handbook

– Even a ¼ inch microphone only has a flat phase response to 1kHz!

Phase of frequency responses (pressure) for :a) 1 inch microphoneb) ½ inch micc) ¼ inch mic

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Phase Response Matters

“The phase relationships of harmonics from a complex tone contain more information about the sound source than the fundamentals.” – David Griesinger

If the harmonics are not in phase, the peak level will decrease


Stax SR‐009 Headphones

• Frequency Response: 5 – 42kHz

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Stax SR‐009 Headphones

• Spectrum resulting from 30k & 35kHz tones

– Intermodulation distortion products @ 5k, 10k, 15kHz, etc.


Sony i.hear

1/12th Octave Frequency and Phase Responseon Flat Plate Coupler with ¼” Microphone

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Practical Example: Hi‐Res Headphone

• Demonstration



What is Non‐Coherent Distortion?

Non‐Coherent Distortion is a normalized cross‐correlation measurement that determines the degree to which the system output is linearly related to the system input (AES 121st preprint 6877)

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AES 137th Convention, Los Angeles, CA, Oct. 10, 2014 31

• Measure the headphone frequency response and sensitivity

• Create equalization curves for each headphone that will result in all headphones having the same target response curve

NCD Measurement Setup

AES 137th Convention, Los Angeles, CA, Oct. 10, 2014 32

• Non‐Coherent Distortion measured using Music @ 80dB SPL average (B‐weighted)– Headphone A(red), B(pink), C(blue), D(orange) & Ref(green)

– Headphone D had considerably more NCD in the midrange than the others

– Program material did not seem to have a significant affect on measured NCD

– For more information on NCD, please read our AES paper from 2006 (preprint 6877)

NCD Distortion Measurements

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Listen Int’l Sales Meeting 2015 33Confidential © Listen, Inc. 2015© Listen, Inc. 2016 33© 2010 Listen, Inc.

Noise Cancellation• Three Measurements:

1. Un‐occluded ear

– Baseline of noise at open ear

2. Passive attenuation

– Measured with headphones in place but active cancellation off

3. Active cancellation switched on

• Measurements are relative:

– Passive isolation = #2 ‐ #1

– Active component = #3 ‐ #2

– Total noise cancellation = #3 ‐ #1


Noise Canceling Headphone Test Setup

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Practical Example: Noise Cancelling Headphone

• Demonstration



Practical Bluetooth & Open Loop Headphone Measurements

Agenda

1. What does “Open Loop” mean?2. Practical Example – Bluetooth Headset3. Practical Example – Lightning Headphone

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Closed Loop vs Open Loop

What we are used to.

What’s becoming more common.

VS.


Closed Loop vs Open LoopClose Loop Audio Test: Uninterrupted signal path from Output to Input

E.G. Audio Interface (Sound Card) ‐> Amplifier ‐> Speaker ‐> Microphone ‐> Audio Interface

Properties: Input and Output are in the same domain (analog) Input and Output are synchronous (identical sample rate)

Open Loop Audio Test: Input and Output are not directly connected

E.G. Audio Interface (Sound Card) ‐> Amplifier ‐> Speaker ‐> Microphone ‐> .Wav file

Properties: Input and Output may be in different domains (Output=Analog, Input=Digital) Input and Output are asynchronous (different sample rates) Variable delay between input and output

VS.

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Measuring Open Loop Devices with SoundCheck

Input and Output are in different domains (Analog vs Digital)

Use Calibration, Signal Path, to create equivalent calibration or digital signal path AES17 requires 1.414 FS/FS for Output, 0.707 FS/FS for Input

Input and Output are asynchronous (different sample rates and phase)

Post‐Processing – Re‐Sampling corrects for large difference in SR, e.g. 44.1 vs 48 kHz. Post‐Processing – Frequency Shift corrects for fine difference in SR, e.g. 47.999 vs 48.01

Variable delay between input and output

Analysis – AutoDelay automatically compensates for delay. AutoDelay may will not work if delay is greater than 2.5 seconds.


SoundCheck Audio Analysis

Frequency Response

THD

Perceptual Rub & Buzz

Stimulus Response

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Playback Only

Playback Only is Easy!

1. Prepend trigger tone to stimulus.Uncheck “Analyze” checkbox in stimulus editor

1. Save Stimulus as .WAV file. (Convert to native device format, if necessary.)Use normalized Peak‐to‐Peak

2. Use Triggered, Record Only Acquisition Step

3. Re‐Sample and Frequency Shift (Only Frequency Shift may be required)

4. All other steps are normal.


Record Only

Record Only is a little harder

1. A stimulus step is still required. Ideally, make sure the output channel has the same nominal sample rate as the playback device.

2. Use a Play only acquisition to generate the stimulus signal

3. Use a Recall step to pull the recorded device response in to memory.

4. Re‐Sample and Frequency Shift.

5. Use Intersection Post‐Processing step to locate the response waveform inside the recording.

6. All other steps are normal.

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Re‐sampling & Frequency Shift

One or both steps are required any time the output path and input path are not the same device!

Re‐sample IF:

Stimulus & Response waveforms do not have the same sample rate.

SoundCheck will generate an error if stimulus and acquisition waveforms do not have matching sample rates.

Frequency Shift IF:

If output path and input path are not the same device.

SoundCheck will not provide an error but results may be incorrect in proportion to sample rate mismatch.

Re‐sample, then Frequency Shift – That’s my advice.


Watch Out For: Use Auto‐Delay in Analysis Steps

This locates the response signal inside the recording.

Manually trim or use post‐processing ‐ windowing, to remove excessive silence at the beginning of the recorded time waveform.

“Goofy” frequency response is indicative that frequency shift is requiredIf the acquisition is not long enough, frequency response will be completely cut‐off above or below a certain frequency.

Make sure that acquisition step is long enough to account for operator delay

You can combine stimuli and responses for multiple different measurements:Cut acquisitions apart using P‐P Windowing – Playback OnlyMultiple record only acquisitions – Record Only

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Summary All the measurements that SoundCheck can make, it can make on a closed loop,

playback only, or record only device.

ALL the measurements, all the stimulus types, no restrictions!

Many devices are effectively open loop devices:

SmartphonesBluetooth and wireless audio (asynchronous sample rates)Network audio devicesMP3, portable audio playersDigital voice recordersEtc…


Practical Example: Bluetooth Headset

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Brief Overview of Bluetooth

A low data rate, low power, short range digital wireless network communication system.

Provides for many different types of devices from heart rate monitors, keyboards and mice to audio devices to be interconnected.

Different types of services are divided in to “profiles”.

For audio there two relevant profiles:– A2DP – Unidirectional, 16‐48 kHz, Stereo – Musical Playback

– HFP/HSP – Bidirectional, 8‐16 kHz, Mono – Voice Communication

– Audio profiles have been unchanged since Bluetooth 2.0


Practical Example: Bluetooth HeadsetPairingBefore two Bluetooth devices can exchange data (audio) they must first pair, this usually has three steps:

1. Inquiry – Device broadcasts an identification beacon and waits for all devices to respond. This can take a long time.

2. Authentication and exchange of profile capabilities – Devices exchange the features they support and authenticate, usually by Simple Secure Pairing protocol.

3. Exchange of link keys – Link keys are encrypted tokens that serve to keep Bluetooth connections private

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Practical Example: Bluetooth HeadsetAutomating Inquiry

1. Pair by Name – Perform inquiry and automatically pair with the first device that matches the specified name.– Problem, unless every test station is RF shielded process will be

confused by multiple devices.

– Problem, Inquiry is very slow.

2. Pair by Address – If you know the Bluetooth address of the device, you can skip device discovery and go directly to link key exchange. – Save 30~60 seconds per test!



• Demonstration


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Bluetooth is an asynchronous transmission system. There is no “master clock”.

Different devices have different clock crystals

Will never produce EXACTLY the same frequency

Uncorrected, this will lead to measurement errors

Post‐processing, Frequency Shift re‐aligns the waveforms in the time domain to correct for this error

Bluetooth – Why Frequency Shift


Post‐Processing – Frequency ShiftHow does it work?

Calculates the centroid or center of gravity of a stationary tone:

Can resolve frequencies below the FFT resolution.

Only works with sine waves!

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Provides a solid signal for Frequency Shift to lock on to

Provides a trigger tone for triggered record or Intersection

By pre‐pending a fixed tone, you can Frequency Shift logTSR, noise, speech and other non‐sinusoidal waveforms.

Post‐Processing – Frequency Shift

Prepend a 1 kHz, 250ms tone to your stimulus when using frequency shift.


Frequency Shift

When do you need to use Frequency Shift?

1. Anytime you use Resampling step!

2. Anytime you use triggered record or recall waveform based measurement, e.g. open loop!

3. Any time you use two audio interfaces and they are not synchronized, e.g. Bluetooth!

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Practical Example: Lightning Headphones


Practical Example: Lightning HeadphonesOpen Loop test

Any device that does not provide an analog or digital input and output is an “open loop” device, from a test perspective.

A headphone that uses the Apple Lightning port for connection is open loop because Apple does not provide lightning audio adapters.

We can still measure these headphones but now an iPhone must be used to store and play back the test signal.

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Playback Only

Playback Only is Easy!

1. Exported SoundCheck stimulus and copied to iPhone via iTunes(Used LogTSR (Farina’s exponential, continuous sine sweep with pilot tone)

2. Set Trigger Threshold in Record Only acquisition step

3. Start sequence, playback test signal

4. Sequence runs automatically and displays results


Open Loop Measurement

Setting the trigger threshold

Finding an appropriate trigger level can be difficult:

– Too low and ambient noise will false trigger the acquisition

– Too high and it will never trigger

– Ideal is above the ambient noise and below DUT output level

To find ideal trigger level, use multimeter and spectrum analyzer VI in Max mode. Remember, trigger is in Peak value (must multiply RMS result by 1.414 to find peak).

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Open Loop Measurement

Setting the trigger thresholdPeak Level – Sweep Playback

Peak Level – Ambient Noise


Record Only

Record Only is a little harder

1. External tools including faad (http://www.rarewares.org/aac‐decoders.php) and sox (http://sox.sourceforge.net/) can be used to automatically convert unconventional file formats into SoundCheck compatible PCM, two’s complement .wav files. These can be invoked by a System step.

2. A Recall step is used to automatically pull the recorded waveform into SoundCheck.

3. Use Post‐Processing Intersection to locate the waveform inside the recording and then P‐P Windowing to isolate just the response waveform.

4. Frequency Shift is, of course, required.

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Using Post‐Processing Intersection

Auto‐Delay can only compensate for a small amount of offset between the beginning of the waveform and the start of the response signal.

Previous solutions include editing waveforms outside of SoundCheck or using prompts inside SoundCheck.

Instead, you can use an Intersection step as a kind of trigger to automatically find the beginning of the response waveform inside a long recording. Windowing is then used to select out just the response waveform.

Automating open loop microphone measurement



1. “Trigger” value stored in a memory list item.

2. An appropriate trigger value still needs to be determined.

3. Intersection result is then returned to memory list and passed to next step, Windowing.


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1. Waveform start time is passed directly to Windowing step via memory list.

2. Stimulus length is added to start time to determine stop time.

3. Windowing then neatly cuts out just the portion of the recording that is interesting to the measurement.



Automating the processing of recalled waveforms

System, Intersection, and Windowing steps can be used to fully automate testing of open loop devices.

Use System steps to call external tools such as adb, sox, faad, etc… to control the device under test and pre‐process waveforms as required.

Use Intersection and Windowing to select out the response waveform from the recording

Frequency Shift is always required for open loop measurements

Resampling can be used if audio interface and DUT sample rates are not the same

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Practical Example: Lightning Headphones

• Demonstration

• Walk through Sequence


We Can Measure It!

1. SoundCheck is completely and perfectly agnostic about where the stimulus and response waveform are generated and captured.

2. Closed Loop tests are generally simpler, but as long as there is a path to playback the stimulus waveform and capture the response waveform, SoundCheck can test it.

3. Open Loop tests is the solution to many otherwise difficult test challenges involving new audio interfaces.

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Questions?

Thank you

Documents

Listen Headphone seminar 2017€¦ · Complete audio test in a single software package Meas urement hardware to complete your test system Listen Int’l Sales