2

Generation loss occurs when a VHS tape is copied, the copy is copied again, and so on. I have seen two different patterns in how video and sound degrade.

In one type, horizontal bleeding starts in early generations. Colour bleeds, then is lost. Horizontal tracking becomes very poor. Timing of both image and sound remains good for many generations. Example

In the other type, ringing artefacts appear. Colour forms horizontal bands in the primary colours, and takes more generations to disappear completely. The video speeds up, slows down, sometimes repeats, and the audio track desyncs. Horizontal tracking takes much longer to degrade. Example

What causes each of these two patterns, and why do they differ so much?

  • I work more at the systems level than I ever spent time on analog decks, but I suspect that the different problems result from variations in tape stock, the different tape speeds (SP, LP, & EP), the VCR's drum size and head configuration, and the interaction with the color-under subcarrier arrangement. – Michael Liebman Apr 15 '17 at 21:57
  • I'd say it was mostly the latter. Basically you can think of colour-under as being an analogue form of compression. Just like the colour information in digital video is sub-sampled, the colour information going to tape was heterodyne-d to a much lower frequency, which inevitably involved information loss, as well as analogue noise. Each successive generation involved more loss. en.wikipedia.org/wiki/Heterodyne – stib Apr 18 '17 at 13:23
2

This is a difficult question to accurately answer; because you haven't detailed out the exact setup of how the tapes are being duplicated.

First, VHS is an Analog format, and the video signal is typically output via a composite connection. Some higher end VHS consumer units did have component out, either by RGB or via S-Video Out. This connector would yield a higher quality transmission from your feed deck vs your record deck.

So signal loss plays a big factor, especially if the decks are connected via composite (co-axial) connection, as all color information, along with luminance information, is on the same channel.

Using S-Video at least separates out the Blue and Red Values; and pairs luminance with Green.

Quality of the decks comes into play. Are we talking consumer decks; or professional Sony Editing/Duplicating decks. In their hey-day, The Sony Editing Decks, which were the size of about 3 normal VCRs tall, and 1.5 times as wide; were highly accurate in terms of carriage speed for the tape. The highest end decks were nearly frame accurate. They allowed pre-roll, where the decks would back up the tape, get the tape up to speed, and then insert edit in the record at the correct designated frame.

Lastly; if the decks are not synced, there will be slippage regardless... It's analog not digital.

The only way to maintain that both decks sync frame for frame is via genlock. The unit most popular during the Pro-VHS days was called a "Kitchen Sync", it was basically a black burst generator that had a bunch of outputs that all of your decks (they made different size syncs); would connect to, and all the decks would keep in sync with the master clock.

Without that; it's impossible.

Finally; VHS over time stretches, becomes brittle, there's different quality tape, it's magnetic, so it's susceptible to loss of information based off magnetic contact.

Cheers!

|improve this answer|||||
  • 1
    Just a point of order: S-video is not color-difference video. It doesn't separate red & blue from luminance (Y, R-Y, B-Y), it separates the narrow-band chrominance from luminance (Y/C). It's a good match with VHS because it doesn't require encoding/decoding steps when dubbing. – Jim Mack Apr 27 '17 at 21:01
0

The earliest VHS cassettes were often using existing tape stock that was produced for open reel recorders.As the format developed, a number of tape manufacturers produced tape with differing formulations in the coating, which produced a very noticeable difference, particularly in reproduction of the chrominance signal.

These differing formulations also varied in terms of head wear, with the coarsest head grinding tapes tending towards the best signal reproduction, whilst the gentlest generally produced inferior quality image reproduction.

|improve this answer|||||

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.