Subject:	S/PDIF limit - 20 or 24 bits?
From:	"Martin Galway" <mgalway@digitalanvil.com>
Date:	27 Oct 1997 23:09:02 GMT

I used to think it was 20 bits, for some reason, but lately I have read
about products claiming to squeeze 24 bits down the S/PDIF line.  (E.g.
Layla marketing BS)  Does anyone know the standard's capability?
Also can it go up higher than 48000s/sec, for example 96000s/sec?

From:	jay@ccrma.Stanford.EDU (Jay Kadis):
S/PDIF dedicates 20 bits for audio but provides 4 extra bits for auxiliary  
audio use.

From:	Marc <marc@bowery.com>:
Actually it's AES that has 20 bits of audio with 4 aux bits, which can
be an aux audio channel or an extra 4 bits for the main channel.  SPDIF
doesn't allow for using those bits for the main channel.  But, alot of
companies are transmitting AES formatted data over SPDIF electrical or
optical links -- the formats are so similar that it doesnt matter, and
you can then use the 4 bits to extend the main audio channel.

From:	"Mike Suchoff" <mikes@in-tune.com>:
The difference between AES and S/PDIF has largely become a difference in the
electrical connection between devices. AES is balanced using XLR connectors,
S/PDIF is single ended using RCA plug/jacks.
The Crystal Semiconductor chips that are pretty much the standard for
digital audio interfaces produce both the AES and S/PDIF formats and can be
wired for either electrical interface.
The important thing here is that the # of bits and bit rate (at a given
selected sample rate) that is transmitted/received is the same for both AES
& S/PDIF.
<snip> (explaining the structure of a subframe)
<snip> (explaining the structure of a block)
In terms of how many bits are used, the channel status programs the chip's
interfance to the D/A (for a receiver chip) or A/D (for a transmitter) such
that it issues the correct number of bits and data strobes.
So all that is required to make a 24 bit interface is to setup the channel
status correctly so the bits are strobed into/out of the chip.
As far as 96000 sample rate, this isn't possible.  As I mentioned above, the
# bits per sample is the same regardless of sample rate.  When sample rate
is changed, the bits are sped up/slowed down accordingly.  The receive chips
have a clock recovery circuit that only works up to 48,000 samples/sec.
Why would you want to go faster?

Bret Costin <costin@mail.com>:
There are also transmitter and receiver versions that support up 
to 96k samp/sec.  Crystal makes 'em and they are drop-in replacements
for the 48k parts (according to their lit).
> Why would you want to go faster?
For 96k sample rate!


Subject:	Re: s/pdif cable:maximum length?
From:	kludge@netcom.com (Scott Dorsey)
Date:	Thu, 6 Nov 1997 21:38:12 GMT

In article <63t984$pe@bgtnsc03.worldnet.att.net> RLHaile@worldnet.att.net writes:
>Typically, what is the maximum length run for an s/pdif cable before
>problems arise, assuming good quality cable, properly terminated?

If everything follows the S-PDIF spec, you can run several hundred feet
without any real error problems.

If you have Panasonic equipment, you can often get as much as twenty feet.
--scott
