Originally Posted by Animephile
Okay, okay. I think we've established that you're probably an even bigger video geek than I.
Hey now, that's a bit of a stretch. We were talking about TBCs and chroma modulation schemes, remember? Once you go digital, the chroma modulation scheme has nothing to do with TBCs.
Anyway, let's get back to the important stuff, shall we? Is this an area you're actively investigating? Has anyone just tried calling shops experienced in servicing LD players to ask whether they're capable of such mods?
Also, are the PWM decoders analog or digital?
Do you know whether the SDH phase is always fixed, on LDs?
Some clarifications here:
All commercial, industrial, and consumer VTRs used FM modulation of the video signal. The reason was to compress the frequency span into less octaves.
The chroma is AM Phase modulated in the camera. When it got to the VTR several techniques were used.
1) Direct Recording
. Here the entire composite signal was sent to the FM modulator. In order for this to work, you need a sync tip frequency of at least 5mhz. So this was limited to 2inch quad machines and the higher end industrial machines that had head to tape speeds of 1000ips.
2)Various SECAM Systems
. These involved demodulating the chroma to RY/BY and putting each on it's own FM carrier. That meant three FM carriers in a limited spectrum dictated by the head to tape speed, often less than 700ips. This system was only used on early industrial machines due to the somewhat severe morie problems. It was quickly replaced by the Color Under system.
. Here the chroma is separated and frequency converted to around 700khz. The first industrial machines used 767khz, 3/4in and Betamax used 688khz, and VHS used 629khz. The down converted chroma allowed it to occupy unused spectrum below the luminance FM carrier. Additional tricks inverting the chroma phase were needed on VHS and Betamax due to the azimuth recording.
. This was done on Betacam and M formats. Here the chroma is demodulated to RY/BY (Betacam) or I/Q(M format), analog time compressed with CCD delay lines, and put on tape with a seperate track. The luminance had it's own track. Expensive system thus only used on broadcast machines.
5)Full Bandwidth Digital
. Two primary approaches, composite and component. Highly complex due to the error correction systems needed.
. Using digital compression such as JPEG and MPEG to reduce bandwidth thus relaxing the stringent requirements of full bandwidth digital recording.
All these analog techniques require some form of time base correction at least for the chroma path. The easiest is the hetrodyne system but that results in unlocked chroma. Another system was the demod/remod. The chroma was demodulated using a subcarrier derived from the off tape H sync and then remodulated with a stable 3.58mhz carrier - still unlocked though. In broadcast applications, the entire direct record signal was time base corrected, first with switched analog delay lines, later with digital memory. The digital systems were very easy to add a TBC as the signal was already digitized and by that time, 1990s, high speed memory was getting cheap.
Color under machines used two techniques with TBCs. The best was to feed a jittered 3.58 from the TBC back to the VTR to re-lock the chroma to the luminance. Of course the signal is now totally uncorrected but once again coherent. The other method was to demod the chroma in the TBC and re-mod it with again the jittery 3.58. This system did not require a 3.58 feedback to the VTR but resulted in reduced chroma bandwidth. Of course a vertical sync signal was always fed back to the VTR to keep the video centered in the TBC memory. This also meant you needed a VTR with a capstan servo. Cheap machines did not have capstan servos. Full frame TBCs eliminated this requirement.
SC/H phasing was critical in editing. One would hope the SC/H phase on a laser disk was correct, it really doesn't matter much if it's not. The editing problem, was the 140ns H shift at an edit that was not color framed. In a match cut it was quite visible. In a scene cut you would never see it. Still good NTSC engineering practice demanded consistent SC/H and most Laser disks are.
I was the person that did the composite digital mod to a CLD95. The 8 bit video, 14mhz clock and H/V sync are tapped after the TBC memory. They go through an FPGA to put the EAV/SAV codes into the stream per SMPTE 125. I also had to widen the H sync in the digital domain. Next the signal is sent to a digital decoder in RP125 parallel form. The biggest hurdle is formatting the raw digital signal to be SMPTE RP125 compliant. This requires and FPGA these days. A also had to modify th eplayer to use an external sync source. The internal generator was not stable enough to feed a broadcast grade decoder.
As for just taking the FM RF from the laser diode and sending that into a PC to demodulate it and further process? Sure it could be done. You will still need to fabricate the interface hardware and do extensive modifications to the LD player. A typical I5 desktop has enough horse power. But the software would take an individual years to perfect. And the results are probably not going to be worth the effort.