SPDIF is only stereo?

ROBSCIX · Feb 17, 2006, 05:28 PM

Hey guys, can you clear this up for me...SPDIF if COAX or OPtical is only and always stereo correct?-meaning you will never get 5.1 surround information through SPDIF..

Maddogg6 · Feb 17, 2006, 05:35 PM

SPDIF can be encoded (AC3) which will yield 5.1 channels.
Further 'compression' can allow more channels (DTS)

I found that using SOME types of AC3 encoding doesnt work with latest KX version. This depends on modes available in the SPDIF source hardware.

Maddogg6 · Feb 17, 2006, 05:41 PM

for clarity:

If I set my set top DVD player to PCM output mode - it is stereo only.
Three is also a 'bitstream' mode - which '*should* be DD5.1 mode - but also seems to ;follow' how ever the disc is encoded.

I also have a Digital cable box with only 2 possible modes: PCM and DolbyDigital 5.1. In this case PCM sends stereo only - BUT, when I set to DD5.1 mode - ONLY 1 channel outputs (FXBuss 4)

Back to the set top DVD player - three is ALSO a 'DTS ON/OFF' - Which if I do turn this to ON - KX will decode a DD51 encoded disc properly.

So it would seen there may be differences in how 'encoding' is done.

If you want to decode such inputs on SPDIF - be sure to enable the 'SPDIF AC3 decode' on KX main mixer page. When KX AC3 decoder becomes active - a pop-up will appear showing thet its active.

ROBSCIX · Feb 17, 2006, 06:41 PM

But don't the Creative cards only output Stereo in digital only mode?

Russ · Feb 17, 2006, 06:59 PM

1) With multiple SPDIF ouputs, you can get stereo on each one.
2) As Maddogg6 stated, you can encode multiple channels in the signal (i.e. Dolby Digital), and use passthru to send it to an external decoder, and thus get multiple channels (over a single digital connection) that way.

ROBSCIX · Feb 18, 2006, 01:35 PM

wil the native creative drivers also do this? or is it a KX thing?...I am curious because I as always told that SPDIF digital only contained stereo info....It never di contain surround and never could....but you guys tell me it can and does?....thx for any info to clear it up...I am trying to help another but they are using Creative drivers..But this is for my information aswell..

Russ · Feb 18, 2006, 01:47 PM

Yes, of course the Creative drivers can do it too. Do not get confused by the term stereo. It is really just a single stream of digital data. In uncompressed form, it is normally just stereo, but it can be encoded (compressed) to contain multiple channels. It is the same thing as connecting the digital output of a DVD player to a digital surround receiver. You have one digital connection, but can have multiple channels. Read this thread from the Creative forum:
Digital Connections, SPDIF and Dolby Digital Info

Note that is says for UNCOMPRESSED sound, it is only stereo, and then read the section about Movies wth Dolby Digital (AC3) & DTS.

ROBSCIX · Feb 18, 2006, 02:21 PM

No I asm not getting confused by the term stereo I thought the SPDIF standar only carried left and right stereo information ...not AC3...But I really appreciate you guys clearing it up for me...so all in all SPDIF carries surround info

Russ · Feb 18, 2006, 02:38 PM

Quote:

Originally Posted by ROBSCIX

so all in all SPDIF carries surround info

Yes, but only for compressed signals.

i.e.
The information about the additional channels is encoded within 2 channels, and a decoder is necessary to extract that information out.

Maddogg6 · Feb 18, 2006, 02:47 PM

You can think of Ac3/Surround as like wav file compression - as opposed to RAW PCM - RAW PCM is most typical with wave files - but you can compress to smaller size/lower bitrate (but in the case of AC3 - add more channels too - in same bitrate)

Russ · Feb 18, 2006, 02:55 PM

The other point to make clear is that the signal has to have been encoded, and the driver does not do AC3 encoding, only decoding.

So for your typical game/music (upmixed to 5.1), it is only a stereo signal sent to each of the SPDIF outputs (i.e. front|rear|center/sub are sent to a different SPDIF output)). So, on your typical receiver, that has only 1 spdif input, you will get only stereo, unless you use some kind of software AC3 encoder (like Tril's), to convert it to AC3.

ROBSCIX · Feb 18, 2006, 03:14 PM

I see...but Logitech Z-5500's have a decoder built in....as Long as the signal has been encoded correct? like from a software DVD player or such is that right? but for normal games..mp3..etc..it'll be stereo..because there is nothing to compress it and send it SPDIF? do I have it figured out? sound right...

Russ · Feb 18, 2006, 03:18 PM

Yes, provided the signal has been encoded, and you use AC3 passthru.

ROBSCIX · Feb 18, 2006, 03:26 PM

I see...SO if this guy has his Z-5500's hooked up SPDIF..and he thinks he has something wrong because his games and such are only stereo...but thats normal correct?-X-fi by the way...now if he used something like Power DVD or WindVD and set it properly it would come out surround sound correct? as the software would compress the AC3 and the card would send it to the reciver..is that also correct?....I think I have it now...So all in all there is NO way to just have 5.1 using SPDIF..on all the time correct? he'd need to use analog cables for that..thast about right russ? BTW I appreciate it becuase I had it all wrong..

Russ · Feb 18, 2006, 03:36 PM

Yes, for the most part.
Basically the audio on the DVD is allready compressed, and the software just passes it unaltered to the soundcard, which then passes it to the receiver. (although some may do AC3 encoding as well).

You can however get uncompressed multi-channel sound when using all of the SPDIF outputs (i.e. with some of the Creative speaker packages). It is basically the digital equivalent of the analog connections, and can be used for multi-channel sound in games, etc.

BTW: The X-FI may do AC3 encoding, I am not sure, and would have to look it up.

<edit>
I did a quick search, and I do not think that the X-FI does AC3 encoding either.