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Re: Technical question for the Pros using yesterdays gear with todays (long)

208.158.191.6

Posted by SSL Tech on March 12, 2002 at 06:49:40

In Reply to: Technical question for the Pros using yesterdays gear with todays (long) posted by Organbuff64' on March 11, 2002 at 23:01:10:

...I think I remember those... 1984 APRS show in London I saw one, some ridiculous dynamic range... -The first delta-error prediction system, if I rcall correctly. The sample rate was through the roof, but the data rate was comparable to PCM, since it only recorded the differences from the previous sample (which considering the sampling rate, thus the super-short time elapsed between consecutive samples) should only be very small!

The only trouble this thing had was with square waves... -If I remember correctly, it used the logic that if it had got the last few guesses close -or correct- than it used to 'zoom-in' to the next level of resolution. each error larger than the current 'target window' used to cause it to 'zoom out'. -Problem was that with square waves, it used to keep zooming in on the flat part of the waveform, until the sudden, massive leading/trailing edge used to cause colossal overload. -For this reason, it's transient response was always lamentable, but the steady-state sine wave response was out-of-this-world!

Well, if you attempt to compress the video data, (MPEG-2)You're gonna end up with absolute unlistenable crap. -I don't mean bad-sounding audio, I mean it just plain old isn't going to work.

Unfortunately, the compression systems for picture result changes that have been developed to not be too offensive to the eye... unfortunately, the effect on a data stream would almost certainly be catastrophic!

Most of the video-based digital encode/decode units used to make a 'picture' comprising a set of black vertical bars on a white background, and the width of each black bar changed at each of the horizontal line intersections, to represent a one or a zero in the data stream recorded to the VCR. This means that vertically adjacent pixels on a complete image could easily -and frequently did- alternate black/white/black... this is probably going to be re-written by an MPEG encoder as a block of grey... losing all vertical distinction, and presenting the decoder with an interesting puzzle to take a guess at!!!

Take a look at the NTSC video stream coming out of an encoder... I've never looked at one from the DBX series, but the lower-resolution (16-bit, 44.1/44.056/48/47.whatever) Sony PCM-F1 and PCM-501/701, PCM-1610/1630 etc. all worked as described above. The front end was radically different on the DBX's, but it would stand to reason that the video back end would use similar methods.

Seems to me to be a few steps too many to convert audio to bits, then convert the bits to a picture, then convert the picture to bits again for storage... Naturally a manufacturer wouldn't do this, but back in the mid-eighties, video tape was cheap (still is) but affordable large-volume archive data storage just wasn't available.. hence the various video-evolved systems. -The precursor to DAT (and all the problems that that brought!) but not even as robust, since DAT at least uses saturation recording, so stands a better chance of survival over time.

Keith

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