A PRIMER IN THE USE OF WAVEFORM MONITORS PART 3

TELEPRODUCTION TESTVOLUME 1 NUMBER 3

A B

This cleanup issue ends the first series onwaveform monitors. It shows the use of facilitiesnot covered in Parts 1 and 2.

DC Restorer

The DC Restorer switch activates a signal clampwhen ON that nails down blanking, actually thatpart of blanking that occupies the horizontal backporch, to whatever DC level is set by the verticalposition control. This keeps blanking at the zeroIRE level when vertical position is set properlyeven when the average picture level (APL) of thesignal changes radically.

Switch OFF the DC Restorer and the waveformdrifts up and down depending on APL. Thereason is that AC coupling, a single couplingcapacitor anywhere in the signal chain, removesthe DC component of the original signal. It thentends to resolve itself above and below anaverage value where the total signal area is equalabove and below the average. The clamp, ineffect, “restores” the DC component lost throughAC coupling.

To illustrate, look at Figure 3-1A. It shows theblack burst signal. The DC Restorer has beenswitched OFF and vertical position set to putblanking where it should be, at zero IRE. In B of

the figure, conditions are the same but the signalhas been switched to a 75% white raster. Thismakes the average value of the signal rise towardpeak white. This average tends to remain at zeroIRE so the black parts of the signal sink, blankingis below -50 IRE and sync is out of sight. Had theDC Restorer been ON, blanking would haveremained firmly nailed to zero IRE.

So if the DC clamp is so good, why provide thefacility to switch it off? The reason is that theclamp may cover up signal faults in some cases.Low frequency (field rate) tilt, for example, showsup as a tilt or sag in the flat parts of the waveformwhen viewed with the 2V time base. But theclamp “fixes” the tilt on a line-by-line basis andthe waveform wil l look normal with the DCRESTORER ON. Moral: to look for low frequencytilt, switch the DC RESTORER off.

Other low frequency distortions, such as thecommon power line hum that seems to creep intoevery new system hook-up, may be obscured bythe action of the DC clamp. Figure 3-2 showswhat 60 Hz hum looks like with the DCR E S TORER OFF. With the restorer on, thingslook almost normal.

Figure 3-1. DC Restorer switched off. A—Black burst;B—75% white. Note the change in vertical position.

Figure 3-2. Power-frequency hum, a problem that the DCRESTORER might hide.

Figure 3-3. The chroma filter takes out the luminance“framework”.

Chroma and Differential Gain

The CHROMA switch inserts a bandpass filtercentered at 3.58 MHz into the signal processingchain of the waveform monitor. It takes out mostof the low frequency luminance components andleaves the chroma signal at its original amplitudeintact . Refer to Figure 3-3. Burst has theexpected peak-to-peak amplitude of 40 IRE.Sync, blanking, setup and the “steps” of theluminance signal have disappeared leaving the3.58 MHz chroma signal swinging above andbelow blanking at zero IRE.

Punching the DIFF GAIN button on the waveformmonitor keeps the same chroma filter in placebut boosts gain a wee dab so that we canexpand the chroma signal to determined i fferential gain. Differential gain is a change inchroma ampli tude that depends upon theluminance level on which it rides. It’s a measureof amplifier linearity throughout the normal rangeof luminance values.

To measure differential gain, we use themodulated stairstep signal shown in Figure 3-4.This signal is a staircase of equal amplituderisers with 40 IRE units of chroma subcarrierriding on each step. Differential gain occurs whenan amplifier acts to squeeze the subcarrier onone or more steps (commonly the top step).When the DIFF GAIN button is pushed, the 3.58MHz bandpass f i l ter removes the Y signalleaving just chroma as shown in Figure 3-5. Thesteps disappear and only the equal amplitudebursts of the 3.58 MHz chroma remain. Theyshould be equal in amplitude, that is, if there isno differential gain problem.

To measure differential gain, we take the verticalamplifier out of CAL with the VARIABLE controland set gain so that the largest envelope of

subcarrier occupies 100 IRE. See Figure 3-5A.N o w, reset horizontal posit ion to place thesmallest subcarr ier envelope at the minorgraduation marks near 100 IRE. See B of Figure3-5. Look at the flat parts of the waveform onlyand ignore the spikes that occur at the “risers” inthe waveform. The smallest envelope has anamplitude of 98 IRE and differential gain is 2minor units or 4 IRE below 100 or about 4% inthis case. A consumer VCR in the E-to-E modewas used for this example. The only videoprocessing that took place was video AGC.

Differential Step

Model 5860C inserts a special bandpass filterinto the vertical amplifier when both FLAT andIRE are depressed simultaneously. This filteryields differential step (DIF’D STEP) operationwhich is used to gauge the linearity distortion inthe Y signal. The signal used is the unmodulated

Figure 3-4. Modulated staircase, the signal used to gaugedifferential gain and phase.

stairstep (actually the modulated stairstep ofFigure 3-4 may be used because the filter alsosucks out color subcarrier). What you are lookingfor here is squashed steps in the staircase signal,that is, step risers that are not equal.

The differential step filter makes it easy becauseit removes both low frequency and high frequencycomponents. This causes “flats” of the steps tocollapse to zero and the r i s e r s of the steps toappear as spikes that are lined up at zero at theirbases. Look at Figure 3-6. If the signal wereperfectly linear all the spikes would be of equalamplitude. They aren’t and the top steps (far rightspikes) are decidedly shorter. This photo wastaken after signal processing from a VCR. TheFM modulator/demodulator process in such amachine is notorious for luminance linearityproblems. To make this observation vertical gainis taken out of the CAL setting and adjusted to

make the tallest spike (usually the first followingblanking) span 100 IRE units.

System TimingThe last button to be covered on the waveformmonitor is the REF EXT/INT button. When theINT setting is chosen, the horizontal sweep istriggered internally from a sample of the signal (Aor B) that is selected for observation. When EXTsync is selected, the sweep is triggered from thesignal applied to the REF jacks on the rear panel.The signal chosen as the timing reference for twoor more cameras or VTRs should be applied tothese jacks. Tr a d i t i o n a l l y, the sync signal hasbeen composite sync (4V p-p, pulse negative)from a master sync generator. All cameras shouldbe genlocked to that generator. However, thegeneral trend is to use composite video or morecommonly black burst from the generator or anSEG that acts as the timing reference. Leader’snewer waveform monitors are set to accept blackburst, composite video or composite sync forexternal sync.

A typical setup where two or more sources are tobe genlocked to a designated timing referenceroutes black burst from a test generator to theEXT REF jacks of the waveform monitor and toeach of the genlocked sources. In a simple two-camera setup one of the cameras may bedesignated as the t iming reference and itscomposite video looped through the EXT REFjacks to the A input (and terminated).

System timing is set using the waveform monitorto see that all genlocked video sources havehorizontal sync in time at the SEG or programswitcher. An error in time will cause the picture tojump sideways when switching from one sourceto another.

Figure 3-6. Staircase signal with DIF’D STEP filteractivated shows compression of top steps.

A B

Figure 3-5. Differential gain check. (A) Set WFM gain for100 IRE. (B) Set smallest envelope under the small scaledivisions near 100 IRE.

To set system timing, select one source (thegenerator or a camera) to be the timing reference.Select EXT sync on the waveform monitor and setvertical position to put sync tip at -20 IRE. Thisputs the 50% point at the leading edge of sync onthe time-marked zero IRE graticule line. Select 1µs/div and adjust horizontal position to put theleading edge of sync on a convenient majorgraticule mark. See Figure 3-7. Make a note ofthat mark (second from the left in the figure). Now,look at the second camera. Leave the waveformmonitor controls as previously set and adjust HD E L AY on the camera under observation to putthe leading edge of sync on the same graticulemark. Repeat the process for all other videosources in the hookup. The remaining job is to setcolor subcarrier phase on all sources so that theyare in phase at the input to the SEG or switcher.We’ll take this up in the series on basicvectorscope operation.

Video Output

One additional facility in most waveform monitorsis the VIDEO OUT jack on the rear panel. Thejack provides a separately-buffered output videosignal from the source selected by the front panelA/B switch. This feed can be used to drive apicture monitor, vectorscope or any other videocomponent where it is desired to accomplishsource selection with the A/B switch on thewaveform monitor. Output is 1V p-p for 1V p-pcomposite inputs and remains 1V p-p for 4V p-pcomposite sync feeds if the A/B switch is set toA4 or B4.

Rotation

One last front panel control to be mentioned is the

R O TATION control, a screwdriver adjustmentfound on most CRT-based instruments. It controlscurrent flow in a coil mounted on the CRT to offsetthe effects of the local magnetic field. It is set sothat the horizontal trace is paral le l to thehorizontal lines in the graticule. Adjust under no-signal conditions and in conjunction with Vposition so that the trace lies on a convenient linenear center screen. No further attention is neededunless the instrument is relocated.

Terminations

All video feeds are designed to drive 75 Ω loads.Where cable runs loop-through one or morepieces of equipment in daisy-chain fashion, the 75Ω terminator must be placed at the very end ofthe run, the last piece in the chain. Failure toterminate properly causes an open line andrefections from the end that wreak havoc withfrequency response and can cause “ghosts” onlong cable runs. But the most obvious result of amissing termination is a doubling of signalamplitude. The reason is that video sourcesoperate as 2V p-p generators with an internalsource impedance of 75 Ω. When a proper 75 Ωload is in place, the signal divides equallybetween source and load resulting in 1V p-pacross the load. Open that load by removing theterminator and the full 2V p-p appears across thecable feed.

The moral: Waveforms that look too “hot” withburst and sync twice as big as they should be aresure signs of a missing terminator.

Terminations are added to the last loop-throughconnector by means of a physical terminator.Good ones don’t come cheap. They are typically±0.1% accuracy and purely resistive to work like75 Ω over a wide frequency range. Someequipment, like VTRs, are self-terminating at theinput connector. Others have switches to switchthe internal terminating resistors in or out.

Bad terminations can also be the result of doubleterminating. (Placing a terminator on the loop-through connector and switching in the internalterminator at the same time.) This also sets upreflections but results in lower signal amplitude(0.67 V p-p).

Figure 3-7. Reference the leading edge of sync to a majortic mark at zero IRE.