CED VideoDisc and Player Discussion

[RikDL]

Hi there,

I have some questions:
- What are the dimensions of the caddy of the CED? (mm x mm x mm)
- How thick is the LaserDisc?
- What are the dimensions of the caddy of the VHD?

Thanks

[RT9342]

I can get the measurements for the caddy and the disc, but I don't have a VHD to measure. Just remember, this is not a laserdisc (maybe you already knew that, but I just wanted to make sure you were aware of that, as many people seem to think both formats are the same). If you're interested, I can see if there's a difference in the thickness of a CED disc and a laserdisc, as I have many discs of both formats.

[RT9342]

Okay, here's what I measured. Please keep in mind that these measurements may not be 100% exact, and that some of the measurements may be off by up to 1mm.
CED caddy width: 325mm
CED caddy height: 355mm
CED caddy thichkness: 6mm
CED disc diameter: 300mm
CED hole diameter: 33mm
CED disc thickness*: 2mm
*NOTE: the thickness was measured at the rim and the center of the disc. As with LP records, CED discs are thicker at the rim and center than at the recorded surface. The recorded surface appears to be sunk in about 1/3mm on each side, so the thickness of the recorded section of the disc is probably about 1.3mm more or less.

If you're interested in how these figures compare to LaserDiscs, here's what I measured with LaserDiscs. Please keep in mind that there are variances in the jackets, so I listed a range of the ones that I measured.
LD jacket width: 312-313mm
LD jacket height: 310-315mm
LD disc diameter: 300mm
LD hole diameter: 35mm
LD disc thickness: 3mm (uniform throughout entire disc)
It may be interesting to note that the hole is slightly smaller on a CED than on a LaserDisc. If you've ever tried loading a CED in a top-loading LaserDisc player, you were probably already aware of this, seeing that the CED doesn't fit the spindle properly. (By the way, for those who haven't tried that, a LaserDisc player cannot even focus the beam on a CED, so even a player like the LD-660 with no microprocessor won't even try to play a CED, and even if it did, it wouldn't be able to decode the signal. I was hoping I could put CED circuits in a player to make it play CEDs with a laser, but if the laser can't even focus on the grooves, it's pretty hopeless.)

[Spirantho]

There are sooo many reasons you can't play CEDs in LD players... that's like taking the insides of a cassette tape and covering a CD in the tape, then expecting a CD player to be able to play the music!
_________________
Ian Gledhill
IanG@--remove this--amiganet.org (remove -- to --)

[RT9342]

Absolutely. I know what you mean - different groove structure, FM carrier frequencies, color carrier frequency, RPM, direction, etc. Still, it would be cool to be able to modify some kind of laser pickup to be able to at least focus on and track a CED groove. I've heard that a red laser isn't sutiable for such purpose and that a green laser would be necessary, but I don't know if that's only for trying to build a CED recording device (using a laser instead of the cutter that RCA and CBS used) only, or if it includes playback as well. But of course, even if a laserdisc pickup could be modified to scan a CED, ALL of the playback circuitry would have to be replaced. I wanted to try something like that at one time.....but I don't have much time to mess with it now. Besides, how many people would really be interested in buying a laser CED player? Just the few of us who chat on this forum maybe? Still, it would be cool if someone (or myself) could come up with a laser pickup that fits in the stylus slot of CED players. It couldn't just be a drop-in part, though - you'd need to connect it to power somehow. Anyone else have the time and patience to try to build such a device?

[SRSanford10]

I found two engineering papers from RCA Review, Sept. 1978 and March of 1982 which describe in detail the theory and construction of two versions of a CED optical reader for playing back master recordings. These would be available at a large university library or by an inter-library loan. They used a helium-neon red laser and a microscope objective lens of 0.9 numerical aperature. The reflective surface was either copper for the electromechanical recordings or gold (coated on top of the modulated photoresist) in the case of electron beam or laser recordings. This would probably mean that the black plastic of the consumer discs would not be as good a reflective surface. The effect would most likely be a poorer carrier to noise ratio than that which was obtained from the master substrates. Another problem that was covered by the researchers was that the sound carrier (being 20 db down from the video carrier) had a quite poor carrier to noise ratio compared to the sound CNR obtained by the capacitance playback system and replicated discs.
_________________
Scotty

[RT9342]

Interesting - I remember reading about how the copper masters were tested with a laser, but I didn't realize they were using a helium-neon laser. That's the same laser that the older laserdisc players use. But as you mentioned, I'm pretty sure that the black plastic discs may not be reflective enough, unless you made the pickup assembly very sensitive, but there would probably be a lot of noise. When I tried to see if my LD-660 would focus on a CED, it wouldn't even obtain focus, even though it obtains focus on almost any reflective object you put in front of the lens (the LD-660 has no CPU, so it just looks for a reflection and starts spinning the spindle, trying to synchronize to something without ever realizing there's no laserdisc there). So apparently the CED is absorbing too much of the light. But still it would be intersting to find a way to make the pickup more sensitive, and drive it into CED decoder circuitry.

[SRSanford10]

The sensitivity of a pick-up could be improved using avalanche photodiodes. All of this effort would probably only make sense if all sources of stylii were to dry up(perhaps a hundred years from now?).
Chemists in the forum might be able to tell us if the discs will last that long.
_________________
Scotty

[RT9342]

True, it probably wouldn't be practical while NOS styli and stylus rebuild services are still available. I forgot about avalanche photodiodes. I remember there used to be some company that was selling turntables on the internet that used a laser to play the vinyl LPs instead of a stylus - I wonder if they used something like that.

[Red October]

I just heard about laser turntables a few weeks ago. They can be used to play pressing masters and other odd types of records. At any rate, the technology exists, so, many years down the line, when the suply of styli is exhausted, it would probably be cheaper to have such machines fabricated rather than have new styli fashioned... Although there's a fundamental problem there, as I'm sure most of use use CED not because it's fundamentally superior or attractive as a video format (it only manages to outclass VHS in terms of picture quality and it has a few desirable features that come with it being a disc format), but because of nostalgia or love of all things obscure, retro, or technologically "neat" as opposed to outright superiority. And if there were no nifty, retro-looking, built-like-a-tank, American-made CED players to play them on, there would be little point in playing CEDs.

[SRSanford10]

Red October talked about some interesting reasons why people would still be playing and maintaining CED players, etc. One other reason I can think of in the longer term future is that it may turn out that a particular piece of software is only available in the CED format. Others , I'm sure know more about the rarity of certain programs and artists of the early 1980's than I do.
The other thing that I find unique about the CED system is the huge amount of documentation available about it. The "RCA Review" technical papers from the period of 1978 to 1985 cover the system in great detail. Further papers are to be found in "RCA Engineer" in the early 1980's. These papers are readily available to anyone at large university libraries.Tom Howe has done a great job of cataloging all the CED related patents. These are another source of detailed information available on the internet at the U.S. Patent office web site.
_________________
Scotty

[RT9342]

Yeah, I guess I've been kind of shooting in the wrong direction, defeating the purpose of using the CED format. I think it's because having grown up with literally thousands of dollars worth of CEDs and two players that wouldn't play worth a darn, and not knowing where to find replacement styli (with very little help from RCA), I was always more interested in trying to find a reliable way to play those discs. But now all of those discs were destroyed due to a natural disaster where I lived, and I sort of started buying players and CEDs afterwards, but always had that desire to find a better method of playback. But now I don't have as much desire. And to be honest, I'm thinking about just selling off all of my CED stuff. I really don't have any discs that weren't also released on laserdisc, DiscoVision, or DVD - just old movies from before my time (you gotta remember, I come from a generation that thinks that the first James Bond movie was GoldenEye). But of course, even if I get rid of all my CED stuff, I'll always be more than happy to discuss or help anyone who needs info or help with their CED players. The technology behind it still fascinates me - I guess that's why I hung onto the format for so long.
Funny, how this topic got off track - didn't it start with someone wanting to know the dimensions of the caddies?

[SRSanford10]

RT9342 mentioned in a post of early April about his attempt to get a CED disc to be focused by the laser reader of his LD-660. There are some papers in "RCA Review" from 1978 and 1982 that cover the optical readers used to "read" master CED's. With laser discs, CD's, and DVD's I believe that often a multiple beam system is used in order to identify the signal depressions (or pits) and it uses the unrecorded surface of the record (or "land" as it is called ) as the reference plane for focus. The CED recording has no such "land" because the tracks butt up against each other. The earliest RCA optical players used an air bearing puck system to maintain a fixed distance between the laser beam focus point and the disc surface. I believe later they designed a focus tracking servo with a fairly wide beam which covered several tracks. This was done so that the signal vertical undulations would not confuse the servo.
_________________
Scotty