CED VideoDisc and Player Discussion

[disaacs]

Hello,

Curious if anyone out there knows why light stripes might be sweeping across the screen (sometimes vertically, sometimes diagonally)during playback. Often gives the picture an annoying "flickering" appearance. Doubt it is the disc, as this condition appears on every one I watch. Is this something inherent? Also the picture is somewhat dark. Can this be adjusted internally, and if so, where?

[RT9342]

Well, Disaacs, since this is an old post, I don't even know if you'll be reading this reply or not, but it sounds to me like there's a problem with the VIDEO LEVEL adjustment setting, and maybe even the DELAYED VIDEO adjustment. ALL discs do have some video distrubance, usually caused by dirt, dust, small scratches, or even imperfections in the master disc or the molds. These disturbances are masked by the delayed video circuit, which takes the video image right above the disturbance and "duplicates" it, making the picture appear to be intact. VCRs and laserdisc players also use this. However, the delayed video must be at the same brightness as the main video. If the VIDEO LEVEL control gets too low, you'll get a dark picture, and the delayed video used to "patch up" the picture will look bright. Try adjusting these controls to see if you get a better picture. The DELAYED VIDEO adjustment is behind a hole in a metal covering on the board in the bottom of the player, and I think the main VIDEO adjustment is near the RF modulator. The delayed picture will go crazy everytime you mess with the control, but after a couple of seconds it will stabilize, letting you see if it's better or worse. The main thing is just adjust the video to the level you want, then adjust the delayed video to match it. I think that should fix the problems you're having.

[tak1610]

Is there any documentation with details on this Delayed Video circuit? I am interested in learning more about how it works. It sounds like some sort of video buffer that the analog video runs through. Sounds similar to 3D Noise Reduction circuit in today's DVD recorders/players and PC capture devices, used to clean up the video signal.

[RT9342]

If you contact CEDatum, they might be able to sell you a schematic. I ordered schematics for several different models from them a while back. I don't have them on me right now, but just off the top of my head, I can tell you that the circuit uses an analog delay, which delays the signal by one scanline. There's an IC in the video delay circuit, and I can't remember if that chip does the delay, but in most equipment (VCRs, laserdisc players), the delay is done by a "delay line", which is an electromechanical device which works similar to a spring reverb box like the ones in guitar amps, but it's much smaller (and doesn't reverberate, of course). The output from these delay lines is not real clean, so they run the RF signal from the disc (or tape) through it, before the demodulation stage.
Normally, the RF signal straight from the disc or tape is fed into the demodulator, but this signal is monitored for "dropouts", and if a dropout occurs, an electronic switch switches the demodulator input from the raw RF signal to the delayed signal, thus displaying whatever was on the previous line on the next line, patching the image. In many designs, including the CED players, this signal goes back into the delay line so that if the dropout repeats on more than one line, then the last good line will be repeated as often as necessary.
Commercial and digital equipment typically use digital technology to produce better results, but the analog equivalent, like what you see in the CED players, is not too bad, as long as it's calibrated correctly. The delayed video has its own level adjustment, and if it is off, then the dropouts become very noticable, appearing as bright lines & specks, or dark lines & specks, depending on if the signal's too strong or too weak.