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 Post subject: How did digital audio get retrofitted compatibly?
PostPosted: 27 Feb 2023, 21:47 
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Pardon if this was asked before. I am new to laserdiscs but have messed with other optical media somewhat extensively.

Did MCA reserve space for future expansion on laserdiscs?
Do we have any documentation or standards documents or other lore on this?

I've read that PAL LD's only can have digital or analogue audio, but not both. So was LD designed to support PAL from the start, and then the extra, unused 0.8MHz of bandwidth on NTSC discs used for digital audio samples?

The alternative way to implement PAL that I can think of would be to increase the rotation speed and decrease the runtime of PAL discs from ~30 minutes to ~25 minutes per side, but keeping about the same amount of unused space.

Or was digital audio already planned as a future expansion for NTSC discs and space allocated accordingly, with PAL/625 line systems as an afterthought repurposing?

Did NTSC discs just use the extra space that a PAL signal would require to store a complete video field to store digital audio? How was that done in a way that wouldn't interfere with video or analogue audio playback on existing NTSC players? Was the digital sound hidden offscreen in the NTSC setup period/porches or vertical blanking interval or something else entirely? Was it filtered out of the signal before being output from the player?

Hope I'm making sense.
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 Post subject: Re: How did digital audio get retrofitted compatibly?
PostPosted: 28 Feb 2023, 05:27 
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Remember Laserdisc was developed around the early 1960's and a format that both NTSC and PAL was compatible with was established. Digital Audio come along much later so it was not even imagined during development. Laserdisc format always struggled since you cannot record but people with home theaters loved it. When CD/Digital Audio started being the future, laserdisc had to decide how to implement Digital Audio to add CD capability to the players as a means to try to keep the format alive. Laserdisc NTSC used as a base the HiFi Stereo technique used for VHS/Beta tape by using an unused spectrum to store the digital audio (VHS/Beta Audio was stored in an analog format and laserdisc stored digital format). Sadly PAL does not have this unused space.

There are documents stored with the development of laserdisc, maybe others will point you there.
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 Post subject: Re: How did digital audio get retrofitted compatibly?
PostPosted: 01 Mar 2023, 02:34 
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LaserDisc digital audio works just like CD audio - it occupies the lowest 1.75 MHz or so of the frequency spectrum on the disc. The analogue audio channels and the video are encoded using frequency modulation onto carriers at different frequencies (just like FM radio stations), with two narrow frequency bands for the audio channels and a very wide one for the video. On PAL discs, the analogue audio carriers are within the same frequency range that CD audio uses, so you can't have both at the same time; on NTSC discs, they don't overlap so you can have both.

But why? The answer is in the paper Digital Audio Modulation in the PAL and NTSC Optical Video Disk Coding Formats - see figure 3. The video channel is placed at the highest frequencies possible on the disc, working downwards - it has to start a bit lower for PAL than for NTSC (presumably because a CAV PAL disc is spinning slower). Because of the characteristics of the video signal - it includes a strong component at the colour subcarrier frequency - the modulation process also generates a band of interference (the "second sideband" in the text) at the frequency of the video carrier minus twice the colour subcarrier frequency, which would interfere with the audio channels. So the audio channel frequencies had to be chosen not to overlap with the video signal or the unwanted sideband - and there's not a lot of space left with PAL, so they end up being below 1 Mhz there.

So they didn't reserve frequency space for future expansion in the original design - they just made the best use of the space they had, and it happened that that left the lower 2 MHz reasonably clear for NTSC.
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 Post subject: Re: How did digital audio get retrofitted compatibly?
PostPosted: 01 Mar 2023, 05:56 
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atsampson wrote:
LaserDisc digital audio works just like CD audio - it occupies the lowest 1.75 MHz or so of the frequency spectrum on the disc. The analogue audio channels and the video are encoded using frequency modulation onto carriers at different frequencies (just like FM radio stations), with two narrow frequency bands for the audio channels and a very wide one for the video. On PAL discs, the analogue audio carriers are within the same frequency range that CD audio uses, so you can't have both at the same time; on NTSC discs, they don't overlap so you can have both.

But why? The answer is in the paper Digital Audio Modulation in the PAL and NTSC Optical Video Disk Coding Formats - see figure 3. The video channel is placed at the highest frequencies possible on the disc, working downwards - it has to start a bit lower for PAL than for NTSC (presumably because a CAV PAL disc is spinning slower). Because of the characteristics of the video signal - it includes a strong component at the colour subcarrier frequency - the modulation process also generates a band of interference (the "second sideband" in the text) at the frequency of the video carrier minus twice the colour subcarrier frequency, which would interfere with the audio channels. So the audio channel frequencies had to be chosen not to overlap with the video signal or the unwanted sideband - and there's not a lot of space left with PAL, so they end up being below 1 Mhz there.

So they didn't reserve frequency space for future expansion in the original design - they just made the best use of the space they had, and it happened that that left the lower 2 MHz reasonably clear for NTSC.

So it sounds like some of my guesses were right... it was coincidence that it worked for NTSC.

Now that I think of it, Philips is a European company (at least originally). So I guess it'd make sense to design it with PAL systems in mind.

Thanks for all of the additional technical details! Interesting that the analogue audio in PAL/SECAM systems apparently had a better signal/noise ratio.

This stuff becomes even more amazing when you consider all of the additional electronics that had to be used to encode and decode these color TV formats to begin with. So many layers of abstraction!
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 Post subject: Re: How did digital audio get retrofitted compatibly?
PostPosted: 03 Mar 2023, 05:59 
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In fact, Laserdisc was the *only* consumer video format that used unaltered composite video, which skipped a good bit of processing circuitry. CED remodulated it to 1.5ish mhz, and VHS/Beta were 'color under' formats which broke out the chroma as a 600khz signal.

CD audio just happened to be smoosh-able into unused-for-video LD bandwidth, so Philips ran with it :)
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