To trouble shoot means to figure out what is wrong with a piece of equipment or system. In computers most of the time this means figuring out what piece of software is not working nicely with others. You disable one program after another until the computer is working correctly.

But in television the role of the engineer is to fully understand the entire plant including the transmitter and know where to start looking when an on air problem is noted. Back in the analog days it was somewhat simpler as the type of visual or audio impairment would provide a good clue as to where a particular problem resided. Now with digital being the underlying transport medium it can be much tougher to know where a problem originates when the picture goes black.

The first step in troubleshooting modern audio and video issues is knowing what your looking at. As the vast majority of program material originated in an analog form, knowing what to look for is an important first step. The good news is that the analog side of a modern television plant is pretty small these days, usually consisting of just the output of a VTR or satellite receiver which is directly connected to the input of a Analog to Digital converter. Therefore the problem will be with either the output circuit of the analog device or the video passing through it. When devices like DAs, Proc Amps, video switchers, Frame Syncs and more enter into the picture it becomes more difficult to trace down the source of the problem.

Below is a list of several different types of analog picture impairments and their probable source. Once analog video has been encoded to digital the only way to remove picture impairments is to re-encode the program material with a clean copy. If your station is still playing back from analog VTRs and you encounter problems, then you need to check over the tape and VTR.

The point of this is to know where the error you see is coming from so you can address the issue and understand if it’s coming from your system if it’s just been encoded that way.

Drop out - a typical analog tape error caused by a loss of oxide on a video tape, seen as one or more horizontal lines being repeated within the picture.

Drop outs occur when small sections of pieces of the magnetic oxide are missing from the backing of a video tape. Without the magnetic oxide there is no magnetic information available at that place on the tape and hence no video. As tapes age the oxide tends to flake off the backing and drop outs occur more often. When the VTR’s time base corrector (TBC) encounters the missing signal it just repeats the last good horizontal line until the input is restored. Sometimes with large enough drop outs this can go on for several lines. This damage is irreversible and it best to playback the tape as little as possible.

Head Switching– This is very similar to drop outs but is normal in any video that originates on ¾” U-Matic video tape. At the bottom of the picture once again several lines (anywhere from 10 to 20) will be repeated over and over.

This was done to cover over the distortion and loss of information caused when the two playback heads switched from one to the other during playback. There is a gap between where the playback ended for one head and where it started for the second head. This loss of signal (picture) is inherent in the U-Matic system.

Freeze Frame – Caused by a Frame Sync loosing it’s analog input signal and holding the last good frame for output. This would happen when microwave feeds would end and the signal was cut, the frame sync would freeze the last image.

Picture Skewing – This is another ¾” U-Matic error that caused the top of the picture to bend or skew to one side. It’s caused by incorrect mechanical setup of either the record of playback VCR.

Picture Jumping – When the picture jumps vertically or horizontally it shows the analog video was not synchronized to the Analog to Digital encoder. Since the encoder derives it’s sync from the incoming analog video signal this shows that there was some problem with the incoming analog video signal. The sync level could have been too low or there could have been noise on the sync.

Either way the analog video needs to be cleaned up before being fed to the AtoD converter again. One way is with an analog processing amp (Proc Amp), this will remove the original sync from the video and replace it with clean sync of the correct level and format. If the video is directly from an analog VTR or VCR then it needs to pass through a TBC or Frame Sync first to stabilize the video for proper encoding.

Noise – noise is definitely an analog issue as any noise induced into a digital signal can only cause a complete loss of the picture. From sparkles throughout the picture to full blown noise blocking out most of the image. Noise is random energy induced into an analog video signal.

The majority of the time noise is found in analog RF transmission systems, either microwave or broadcast. During a live shot from a helicopter noise would rise and fall in the picture telling you that the station was having a hard time tracking the helicopter with it’s receive antenna. In heavy rain a television station’s 23 GHz Studio-Transmitter Link could fade as the rain drops approached the wavelength of the RF Signal and noise would appear in the picture. If the noise cleared up during commercials then you knew the problem was with the helicopter link and if the commercials also had noise then it was the STL.

Black only sparkles many times came from satellite downlinks where a digital carrier interfered with the main analog video feed.

Interference – When a second picture fades in over the main image it’s a sign that a second analog signal is interfering with the desired one. This can happen with microwave transmissions where two different transmitters are trying to share the same frequency and polarity as can happen during ENG feeds. Most people have experienced this type of interference in the past with analog broadcasting when two stations, on the same channel, could be received in the same place.

This type of picture impairment would be part of the original video and can not be corrected. Another source of the program will be needed.

Another type of interference is pattern noise, this can be in the form of a 60 Hz hum bar rolling vertically through the picture caused by a Ground Loop. Or a higher frequency signal can create a pattern of on screen, this is typical of interference in an analog RF transmission and would be part of the video with no way to remove it.

Tearing – Tearing or horizontal shifting of a section, or sections, of the frame is caused by miss-triggering of the sync circuits in an analog video system. These false pulses come from video that extends below blanking, in the area reserved for sync pulses. This can be caused by excessive enhancement of detail, usually associated with on screen text.

Detail enhancement is created by finding sharp transitions in the video and extending both the positive (white) and the negative (black) parts of the transition. Sometimes the negative transitions can extend down below blanking and miss-trigger sync circuits looking for horizontal sync.

The only way to remove this type of picture impairment, in analog source, is with the use of a Black Clipper the suppress any excursions below blanking between the actual horizontal sync pulses.

Sync Compression – We once aired a music video that had a large light being turned off and on and when this happened the video would breakup or tear. It looked like there was something wrong with the video but it played fine when checked at the output of the VTR. It was discovered that a video DA had a power supply problem that lead to the sync being compressed or reduced in amplitude with a sudden high APL (Average Picture Level).

When the image on screen when near full white the sync was compressed and the system down stream lost lock on the sync as it was too low to detect properly.

By the way we found that problem DA by using the Bounce test signal from our test generator.

Remember that analog composite sync still plays a major role in today’s digital video plant. Composite analog sync is the synchronizing signal that locks most all of todays digital equipment to a common reference clock.

I hope this gives you some idea of the types of analog video picture impairment that might be encountered as we transfer the vast stores of analog video to digital. Once in the digital world video is either present or it’s gone, there’ s no real in between. Once the picture is compressed, either for storage or transmission there is a whole other world of picture impairments that can be observed. But that’s for another article.

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