Audio interference from illegal white video levels
 

Hello all,

I am working on a video restoration project, which involves some audio issues. I cannot for the life of me remember the technical term to describe the unwanted audio buzz that can occur when, say, the white text in white-on-black credits is too bright. Can anyone help? Also, is this something that may be inherent to a videotape (as in, the sound occurs independent of any monitor it may be played back on, including a plasma or other flatscreen) or is it dependent on the monitor, and a CRT-only issue?

Any advice is welcome.

Thanks very much,

Jeff Martin
Chicago, IL

Depends where the buzz came from. It could be from the machine that recorded it, the tape deck or your connections or monitor.

The phenomenon you attribute it to should only be present in NTSC RF. 1st step eliminate ALL RF connections in your set up ( they degrade the video making bad copies and hiding recording flaws in the picture on your monitor) and switch to composite video and audio...if the baseband audio out of your deck is buzzy it is either a tracking issue, a bad deck, or a flaw in the original recording.

What Tom said^^^.

If the buzz is really caused by whites being illegally high, it is "overmodulation."
BUT: it could be overmodulation in the source (unlikely) or overmodulation in the receiver itself due to IF alignment putting the video carrier more than 50% down from the high frequencies at the audio takeoff point.

Due to use of negative video modulation, the whites produce the minimum amplitude of the carrier wave. In intercarrier sound detection/reception, this also instantaneously produces the minimum level of 4.5 MHz intercarrier signal.

On an RF connection, excessive white will shut off the carrier. And most TV sets use the intercarrier sound method, where the sound carrier ends up looking like a 4.5MHz FM modulated signal. This disappears while the peak white of the video is too high. On the picture carrier of a TV channel, they used AM modulation, and the peak of modulated carrier is on sync tips, and minimum RF carrier is when there is white in the video image. If the white is too much, the modulated carrier drops to zero, and the video detector stops producing the 4.5MHz sound IF. You then get "holes" in the detected audio, which sounds like buzz (as the picture repeats at the vertical rate, and the excessive white repeats).