Understanding S/PDIF
 

Does SPDIF completely define the format of an audio souce so that all S/PDIF
audio outputs coax or optical are compatible with corresponding inputs?

If so then presumable DVB-S and DVB-T receivers that have S/PDIF ouputs must
do some sort of conversion from MP2 or AC3 and is this a loss less
conversion?

If I have an amplifier with a good quality DAC am I likely to get better
sound quality if I use an S/PDIF connection rather than a coax analogue
connection?


--
Michael Chare

Understanding S/PDIF
 

On Mon, 5 Oct 2009 12:35:24 +0100, "Michael Chare"
wrote:

Does SPDIF completely define the format of an audio souce so that all S/PDIF
audio outputs coax or optical are compatible with corresponding inputs?

If so then presumable DVB-S and DVB-T receivers that have S/PDIF ouputs must
do some sort of conversion from MP2 or AC3 and is this a loss less
conversion?

If I have an amplifier with a good quality DAC am I likely to get better
sound quality if I use an S/PDIF connection rather than a coax analogue
connection?

Have a read here, it is a short article, but I think it explains it
all.

http://en.wikipedia.org/wiki/S/PDIF

The short answer to the real question, though, is that if you have a
good DAC, then use that with its analogue connection. If you go
S/PDIF, you will be using a DAC of unknown quality in your amplifier.
Having said that, you are extremely unlikely to be able to hear a
difference.

d

Understanding S/PDIF
 

In article , Michael
Chare
wrote:
Does SPDIF completely define the format of an audio souce so that all
S/PDIF audio outputs coax or optical are compatible with corresponding
inputs?

From the orginal Sony/Philips spec you can give the usual engineering
answer.... erm, 'yes' except when 'no'. :-)

By that I mean that spdif is supposed to define the format for a set of
LPCM bitrates and bits per sample. Originally that was essentially
32/44.1/48 ksample/sec with up to 20 bits per sample, stereo.

However that has since been 'extended' - e.g. to allow 24 bits per sample.

The practical problem is that there is no absolute gurantee that all spdif
receivers will be able to cope with all of the above combinations. They may
fail to recognise, or resample, some of them. Although the original spec is
likely to be fine.

If so then presumable DVB-S and DVB-T receivers that have S/PDIF ouputs
must do some sort of conversion from MP2 or AC3 and is this a loss less
conversion?

The *physical* arrangements for serial electric (coax) and optical fibre
allow the use of other formats that are not spdif. e.g. various kinds of
what I think have been called 'bitstream' that allow non-LPCM, more
channels, etc. However all of the sources I've had like DVD players allow
you to set the source to convert MP2/AC3/etc into spdif LPCM. Thus they do
the conversion for you if your chosen receiver can't. So this should be an
option. But again, I don't think there is a universal guarantee, so you'd
need to check in a specific case. Seems likely, though, for modern kit.

Not sure what you mean by 'loss less' conversion since MP2 and AC3 have
*already* lost info before the data reached your receiver/player. if you
mean 'not all decoders give identical results' then once again you have the
standard engineering answer. In theory they'd all turn the same MP2 or AC3
into the same LPCM. But in practice, practice may not agree with the
theory. Yer pays yer money and yer takes yer chance... :-)

I have certainly measured differences decoding mp3 between different
methods. I assume the same will be true in other cases with other lossy
encoding systems like MP2 and AC3.

WRT DVB: Certainly the semi-ancient Nokia 221T DVB-T box I use does output
48ksample/sec spdif that even my more ancient Meridian DAC can accept, and
gives good results. I can also record this if I wish.

If I have an amplifier with a good quality DAC am I likely to get better
sound quality if I use an S/PDIF connection rather than a coax analogue
connection?

No idea. Depends on the specifics of your case. Might sound just the same,
or better one way or t'other. However an advantage of *optical* spdif is
that it helps avoid ground loops. And coax spdif might minimise their
effects. if that matters in your specific case I can't say.

FWIW These days I tend to favour optical spdif as the systems I use seem to
gain extra boxes all the time, now including computers. So avoiding loops
is helpful and avoids some fuss. But YMMV.

Slainte,

Jim

--
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Understanding S/PDIF
 

"Michael Chare" wrote in
message

Does SPDIF completely define the format of an audio source
so that all S/PDIF audio outputs coax or optical are
compatible with corresponding inputs?

No. The source defines itself. SP/DIF is commonly used to send digital
signals that are incompatible with the digital inputs on some equipment.

If so then presumable DVB-S and DVB-T receivers that have
S/PDIF ouputs must do some sort of conversion from MP2 or
AC3 and is this a loss less conversion?

A conversion from lossy-compressed sources like MP2 is never lossless.
However, it is a fairly well standardized process as I understand it.

If I have an amplifier with a good quality DAC am I
likely to get better sound quality if I use an S/PDIF
connection rather than a coax analogue connection?

That is very much a "it depends" situation. Different equipment is designed
to do different things and the internal design and components have varying
quality and performance levels.

You seem to be asking for a generalization that probably shouldn't be made.

Just to complexify things, there is a fair amount of equipment that converts
all or most of its analog inputs to digital, processes it, and converts it
back to analog. So, if you use an external analog converter, its basically a
waste.

Understanding S/PDIF
 

"Jim Lesurf" wrote in message
...
In article , Michael
Chare
wrote:
Does SPDIF completely define the format of an audio souce so that all
S/PDIF audio outputs coax or optical are compatible with corresponding
inputs?

From the orginal Sony/Philips spec you can give the usual engineering
answer.... erm, 'yes' except when 'no'. :-)

However all of the sources I've had like DVD players allow
you to set the source to convert MP2/AC3/etc into spdif LPCM. Thus they do
the conversion for you if your chosen receiver can't. So this should be an
option. But again, I don't think there is a universal guarantee, so you'd
need to check in a specific case. Seems likely, though, for modern kit.


Thanks for all the replies.

Neither my Humax HDR PVR or my Topfield PVR allow me to set the format of
the digital output, but interestingly my Philips DVD player (which I had
overlooked) does.


--
Michael Chare