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From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga.tech
Subject: Re: CD-ROM vs. CD Players
Message-ID: <2313@lpami.wimsey.bc.ca>
Date: 2 Dec 90 20:56:54 GMT
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To: van-bc!rnews

In <1990Dec2.073639.22387@zorch.SF-Bay.ORG>, xanthian@zorch.SF-Bay.ORG (Kent Paul Dolan) writes:
>In article <90335.112615AXN100@psuvm.psu.edu> AXN100@psuvm.psu.edu writes:
>
>> I have a Pioneer (sp) 6pack player. On the back there are two pin-outs
>> for a receiver to take control of the player. I was wondering if there
>> was anyway for me to use these pin-outs to let my amiga take control
>> of the CD player; thereby, making it into a CD-ROM jukebox.
>
>I'm no world class expert on CD-ROM, so take this with a grain of salt.
>
>Both digital CD-ROM and audio CDs use binary data on the media, in the
>sense that it is either on or off, with no intermediate states.
>
>However, if I understand it right, in audio CDs, it is the _length_ of
>the "on" area or "off" area along the track, rather than its mere
>presence (as in the data bit on a digital CD-ROM), that is important,
>sort of like the zero crossing distance, rather than the amplitude, is
>the real signal in an FM radio signal, and it is this ability to ignore
>signal amplitude that lends the audio CD its enhanced sound reproduction
>ability.

You are perhaps thinking of what happens somewhat after the data comes off the
disk.  PCM is the encoding technique used to translate between digital data and
analog data, and NRZI encoding can be an intermediate stage, but what goes onto
the CD is essentially the same in audio or CD-ROM, when looked at in terms of
the way the individual bits are stored. In the case of CD-Audio, it is a code
representing the instantaneous level of the sound recorded. In the case of
CD-ROM, it is a code representing a byte value.

>As a result, there is a profound incompatibility between the data
>storage signal recovery methods for audio CDs versus digital CD-ROMS,
>even though the mechanics of reading with a laser and so on are very
>similar.

There is an incompatibility, but it has more to do with the format of the data,
rather than the way the bits are encoded. CD-ROM requires better error
correction and a different 'block or sector' size and format. CD-Audio is in a
format which is unsuitable for CD-ROM, but it is due to the lead-in, lead-out,
sector sizes, etc. There is just too much overhead in the CD-Audio format to
allow for good computer use.

>If this is right, you can't use a CD player to read digital CD-ROMS
>without a whole separate signal extraction "mechanism" behind the read
>laser.

This is probably correct, but mostly because the CD-Audio player interprets
what it sees on the disk and treats it in the only way it knows how, which
means that the data is treated as a continuous stream (one of only 99 streams
(in music parlance, tracks, or bands) that may exist on the disk, another
limitation of the CD-Audio format).

Interestingly enough, many current production CD-ROM drives have the ability to
recognize both the CD-ROM and CD-Audio formats, and can play sound from any
part of the disk that is recorded appropriately. Yes, the two formats can exist
on the same disk.

DAT (Digital Audio Tape) is a similar thing, and a DAT drive intended for
computer use can have audio capability as well (The Archive Python has a
digital audio output jack on the back).

>None of this says you couldn't let the Amiga work to control the CD
>player as a CD player, which might be a fun project in itself.

It would be. Sun's CD-ROM drives have audio capability, and a few of the
fellows in our office have brought in headphones, and use a little OpenLook
tool to play them. They seem to enjoy it. :-)

I'm no expert either, but I do have a book called 'The Brady Guide to CD-ROM'.
It's a fairly typical Brady book, but has more information content than most of
them.

-larry

--
The only things to survive a nuclear war will be cockroaches and IBM PCs.
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|   //   Larry Phillips                                                 |
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