[mythtv-users] [OT] Digital Audio Formats

Steven Adeff adeffs.mythtv at gmail.com
Sat Jan 5 17:52:14 UTC 2008

On Jan 3, 2008 6:45 PM,  <clemens at dwf.com> wrote:
> Im confused here.

Your not the only one when it comes to this!  Sadly its an issue the
electronics industry faces right now, too much confusion amongst

> When I see references to PC Audio Cards I see S/PDIF as what appears to
> be a 2 channel digital audio format (both datalink and physical layer).

S/PDIF (Sony/Philips Digital Interconnect Format) is not so much a
digital audio format as it is a method of transferring digital audio
formats. The physical medium is either Optical (fibre) or Coaxial
(basically an RCA to RCA coaxial cable). The formats it has a limited
data transfer rate as it was initially designed for Stereo PCM 24bit
at 48Khz for DAT and later CD audio data. But later it was co-opted
for DVD's to transfer Dolby Digital and DTS up to 640 kbit/s

> When I see references to Home Theater products (ok HD products), I see
> references to HDMI which seems to be a multichannel (video) and audio
> format (in some cases it seems to be up to 9.1 channels?)

HDMI as well is a connection medium. It was designed to be a very high
bandwidth (10+Gbit/s, iirc) medium and so allows for both high quality
video and high quality audio to be transfered over it. Other things
like copy protection, etc. that are part of the HDMI standard have
more to do with the electronics at both end than the HDMI cable system

> There are other references to just 'digital audio', with who knows what format?

digital audio is then a whole beast in and of itself. Generically it
refers to any audio converted into a digital signal. but of course
things like MP3, PCM, WAV, WMV, AAC, OGG, etc are all containers and
compression algorithms for digital audio (and possibly video as well).

> There appear to be amplifiers that take HDMI in, but I haven't seen anything
> that takes S/PDIF in.

Pretty much any Pre/Pro and receiver made after 1990 has S/PDIF input,
but its usually just called digital in and has either a RCA connector
or the optical TOSLINK connector.

> Anyone have a reference that explains all this (plus coax vs optical versions
> of each, if applicable...)

Wikipedia. Google.

as for the difference between Coax and Optical S/PDIF, there isn't any
in definition. But of course practice is different. Optical is nice
because there is no ground connection. But both transfer the exact
same data. They both can be prone to data loss depending on the
quality of the cable and so one is not necessarily a better cable than
the other. But in the end, the quality of the signal has more to do
with the quality and design of the sending and receiving electronics
than the cable itself. So which "sounds" better (ie has less data loss
in the digital game of 'telephone' is up to the specific components
being used).

> Sometime this coming year I will be springing for a new HD tv, and would like
> to know WHAT I need to process the audio.
> And WHAT ADDITIONAL I need to have specified if I want to get audio
> from my PC, from something like MythTV.

If you plan on buying all new hardware with your HDTV (ie a new
Preamp/Processor or Receiver) I would HIGHLY suggest getting something
with HDMI. Newer models are now coming with 4 inputs and 1 or 2
outputs. Older models only have 2. Depending on how much you want to
spend of course... But all of these will also have a large number of
S/PDIF inputs as well. For instance, my 2yo Denon 3806 has two HDMI
inputs and something like 8 or 10 S/PDIF inputs.
As for from your PC, as of now only two options exist, using analog
cable or S/PDIF. Some video cards are supporting passing the S/PDIF
signal over the HDMI connection but this isn't very well supported and
not necesarily all that special since a single S/PDIF cable isn't that
much extra cabling (as compared to 6+ analog RCA cables...). Hopefully
in the future video cards will basically come with their own
"soundcard" so that not only can S/PDIF signals be sent but also
things like the lossless audio that can be had on BluRay and

> And just for good measure, I never see anything mentioned about bit rates,
> which from the Audio cards it is possible to change.

the bit rate is really more up to the transfer medium since they have
max bitrates. all audio cards can support pass through of digital
signals at these bitrates. Some also support higher bitrate/frequency
response internally but require analog output to take advantage since
the signals just can't be passed over standard S/PDIF output.

hope this helps!
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