CED VideoDisc Development

[andy]

Does anyone know any technical specs for CED such as how the video and audio are modulated? Whar carrier frequencies are used and how do they compare to LD, or VHS? How is the color encoded? What's the resolution?

[wa2ise]

Originally Posted by andy

Does anyone know any technical specs for CED such as how the video and audio are modulated? Whar carrier frequencies are used ? How is the color encoded? What's the resolution?

They used FM to encode the video onto the disc, very similar to methods used on videotape. The audio was FM modulated onto another carrier, and for stereo, they used a 2nd FM modulated carrier for L-R (I don't remember what the frequencies were). They put the chroma subcarrier at around 1.5Mhz and mixed it in with the video luma, and the player upconverted it up to 3.58MHz after using IIRC a line comb filter. The luma resolution was about 3Mhz, better than VHS tape, but worse than laserdisc (4MHz).

[andy]

Originally Posted by wa2ise

They used FM to encode the video onto the disc, very similar to methods used on videotape. The audio was FM modulated onto another carrier, and for stereo, they used a 2nd FM modulated carrier for L-R (I don't remember what the frequencies were). They put the chroma subcarrier at around 1.5Mhz and mixed it in with the video luma, and the player upconverted it up to 3.58MHz after using IIRC a line comb filter. The luma resolution was about 3Mhz, better than VHS tape, but worse than laserdisc (4MHz).

So, it's not composite like LD. I wonder if an S-video output could be added?

[wa2ise]

Originally Posted by andy

So, it's not composite like LD. I wonder if an S-video output could be added?

It is a strange kind of composite video, where the chroma subcarrier is at around 1.5MHz instead of 3.58 MHz. They did that partly because they didn't have enough bandwidth to have it at the usual 3.58MHz. This also allowed them to remove the effects of wow and flutter (from the turntable spinning at 60/8 times a second) by varying the beat frequency used to shift the chroma subcarrier from the 1.5MHz to 3.58Mhz. The resulting 3.58Mhz would be compared to a local reference crystal and the error would then shift the beat frequency up or down to correct it. A little like a PLL (phased lock loop). The beat frequency was about 5.11MHz, beat that against the 1.5MHz and you get 3.58MHz.

In a sense, there's no overlap in the luma and 3.58MHz chroma, as the luma bandwidth cut off around 3MHz, and the chroma bandwidth was about 600KHz. You can still sometimes see cross luma into the 1.5MHz chroma, though. Nothing you can do about that, as it's that way on the disc.

[old_tv_nut]

As wa2ise says, it used a comb filter. It had to, or the low frequency color carrier dot pattern would ruin the picture on anyting except gray objects. I believe the source was also combed before encoding to (mostly) prevent luma from getting into the color. The comb filtering attenuated diagonal details. They had a demo disk including a drive across a suspension bridge, and some of the diagonal wires got pretty indistinct.

[ha1156w]

Another pitfall of the CED format was the silicon lubrication that they misted the disk with when manufactured to reduce friction and increase lifetime. Over time it evaporates/deteriorates/rubs out in the caddy/whatever. Has anyone in the enthusiast circles figured out a way of renewing it?

The stylii were a wierd multi-angle affair that really skated across the surface, and using an electrode on the leading face, read the varying capacitence of the disk. Word has it that one of the original lapping machines is still stored somewhere in one piece but with the recent sale of Thomson's consumer electronics division to various sources, they may have finally let go of it. Once the stylus supply dries up, there's no more. Ever.

[bgadow]

Seems as though somewhere on the CED Magic site I saw a link to a place that rebuilds the stylus. A good idea to have spares, either way.