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 Post subject: Why does the CD audio section of a PDO CDV always play ok?
PostPosted: 11 Aug 2017, 12:47 
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We all know PDO gold CDV discs were terrible quality, but one thing that has bugged me is why do the CD audio only tracks almost always play fine when the digital audio and analog video portion are nearly always degraded on the same disc?

I thought digital audio was stored on LDs in much the same way as CDs but there must be a difference otherwise why the drop in quality on the exact same disc?
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 15:05 
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Publius explains here how the digital audio track fits on NTSC LD (it isn't exactly like a CD):

http://forum.lddb.com/viewtopic.php?p=4888#p4888
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 15:19 
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elahrairrah wrote:
Publius explains here how the digital audio track fits on NTSC LD (it isn't exactly like a CD)


I think laserdisc_fan was referring to 5" CD Video where few tracks are standard CD audio (red book) and one track is LD.

Naively I would say that both audio and video areas suffer from laser rot but the CD audio data are fully (or partially) corrected by checksum/algorithm that can reconstruct some of the damaged bit?

Julien
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 15:30 
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admin wrote:

I think laserdisc_fan was referring to 5" CD Video where few tracks are standard CD audio (red book) and one track is LD.

Julien


Hi Julien - yes it's a 5" CD Video I was referring to.
I received another today - All About Eve and the 4 CD audio only tracks played perfectly, while the audio accompanying the video was just hum and noise. Interestingly the video was actually not bad on this occasion but certainly well below what laserdisc should deliver.
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 15:52 
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Digital data streams are way more resistant to rot than analog waveforms. The video section contains audio, but that audio is a small portion of the overall FM signal and it has to be in tact in order to get the audio from it.

The CD section is all squarewave. Error correction works better on it because the player only needs to see High and Low to make sense of it, whereas with the video section there is an infinite degree of subtlety. This is one reason why everything "went digital" in the first place. CD players also have clever oversampling tricks and whatever to do a sort of error correction so the few read errors you get are inaudible.
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 17:51 
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admin wrote:
I think laserdisc_fan was referring to 5" CD Video where few tracks are standard CD audio (red book) and one track is LD.

Naively I would say that both audio and video areas suffer from laser rot but the CD audio data are fully (or partially) corrected by checksum/algorithm that can reconstruct some of the damaged bit?

Julien

Indeed, but since the video portion is burnt on the disc like an LD, the storage of the audio and video would be the same as how publius talked about. Not really stored the same way as a CD, ie: buried in the FM subcarrier.

And yeah, like you said, more prone to rot than the redbook audio portion.

Now that I think about it, would the polycarbonate nature of the CD covering have anything to do with it? LD covering is acryilic, right?
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 11 Aug 2017, 18:52 
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I don't see how the plastic would have anything to do with anything.
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 12 Aug 2017, 00:05 
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It's interesting that if they had stuck with the original analogue audio for laserdiscs, it would not have been so prone to rot degradation as this hybrid signal comprising digital audio wrapped inside an analogue FM subcarrier.
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 Post subject: Re: Why does the CD audio section of a PDO CDV always play o
PostPosted: 12 Aug 2017, 17:08 
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I've got some where there is also noticable distortion on the audio only parts, sounding like a record with a lot of cracks and dust, so at some point the error correction on the cd audio part can't hold up anymore with some bad rotters.
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