SPDIF - Why can't lossless multichannel audio be transmitted?

[ontherocks]

AFAIK the reason stated as to why lossless multichannel audio cannot be sent through SPDIF is due its upper limit on bandwitdh (which is essentially bitrate)
SPDIF specifies 2 channel PCM, up to 20 bits per sample, at any sample rate.
But then "at any sample rate" means that there is no upper limit on bitrate. Why is then that lossless multichannel audio cannot be transmitted?

[Midzuki]

http://www.epanorama.net/documents/audio/spdif.html

[ontherocks]

Yes, seen that and lots of other references before.
Unfortunately none of them answer my question.

[Midzuki]

SPDIF specifies 2 channel PCM, up to 20 bits per sample, at any sample rate.
But then "at any sample rate" means that there is no upper limit on bitrate.

That's the "theory", so to speak. However the devices and the medium that the SPDIFed
signals travel through do impose practical limits to the achievable data transfer rate.
For a change, such "practical limits" are defined more by the industrial standards and
less by the laws of Physics.

[aedipuss]

just how high a bitrate are you talking about? i have no problems with DD 448kbps, DTS 640kbps or pcm @1536kbps over spdif. what's your source audio bitrate?

[ontherocks]

@Midzuki
Any idea what is the industrial limit?

@aedipuss
Try any lossless multichannel audio (compressed or uncompressed) and see if you can transmit it as it is through SPDIF (multichannel flac, MLP, Dolby TrueHD, dts-HDMA, etc)

[aedipuss]

i'd probably need a different receiver/amplifier on the other end of my spdif optical cable. it only does pcm, dolby prologic, dolby digital, and dts. i have a feeling it would only output hiss with anything else. but if you have a sample you can upload somewhere, i'd download it and try. i don't have any to test it with.

[ontherocks]

@aedipuss
You can get samples here http://2l.no/hires/index.html
You don't need a different receiver. What you have is perfectly fine. Use foobar.

[levick]

the standard toslink and coax digital connection does not have the bandwidth capacity to carry the new HD audio formats...and it appears that most blue ray or HD DVD machines are designed to transmit the HD audio signals down the HDMI lead..the Samsung I use scales back the audio to SD version for transmission down the toshlink or coax leads.

[aedipuss]

i tried the 2L50SACD_tr01_multi_48.flac and the corresponding 5.1 dts wav files. foobar will play the first but not the dts. powerdvd will play the dts. they both appear to be sent over the spdif as 2 ch. pcm. all that is output is stereo, and the dts file doesn't light the dts display on the receiver.

[ontherocks]

Yep, thats precisely the thing I am talking about.
Lossless multichannel is sent over as 2 channel over SPDIF. The reason people state is what I mentioned in my first post (not enough bandwidth)

(BTW what was the problem with dts.wav file in foobar? Did you get static noise or some error? FYI dolby digital a.k.a ac3 and dts are lossy multichannel and hence transmits fine over SPDIF)

[aedipuss]

foobar just output the 5.1 dts wav as hiss, no error.

[edDV]

If S/PDIF doesn't get where you want, implememt Pro AES equipment for uncompressed multichannel (4 at a time) over coax or XLR twisted pair. S/PDIF is the reduced cost consumer version of AES. The cost reduction comes from the connectors and chips used. S/PDIF is intended for AC3, DTS and PCM stereo.
http://en.wikipedia.org/wiki/S/PDIF
http://en.wikipedia.org/wiki/AES
http://www.tech-faq.com/aes-ebu.shtml

[ontherocks]

foobar just output the 5.1 dts wav as hiss, no error.

You have to turn up the volume to the maximum everywhere in Windows Audio Controller (double click on speaker icon on taskbar) and Foobar. Control the volume through your A/V Receiver only.
Look here http://staraphd.blogspot.com/2007/10/get-51-or-71-multichannel-audio-from.html

[aedipuss]

interesting. installing the foobar spdif passthrough .dll allows the 5.1 dts file to play, after renaming it from .wav to .dts. full 6 channel dts, and it lights the receiver dts display led.

but now foobar can't output the 5.1 ch. flac at all. no audio period. may be because it's 48khz and the dts is 41khz? i'm downloading a 96khz 5.1ch flac file to try.

[aedipuss]

turns out the 5.1 flac was still playable but after playing the dts the receiver won't play anything else but dts until it's reset. downloaded the 96khz flac, 96khz wav, and 48khz 5.1 wav to test also. they all play, but it seems they all get converted to 2ch. pcm before being sent to the receiver.

something's not quite right as my sound card is set to output 96khz 24bit out the spdif, but it doesn't appear to do it. the flac should be just hiss, as the receiver wouldn't know what to do with it anyway, as it only decodes pcm, dolby, dolby digital, and dts, but the high bitrate wavs should work.