I have been inputting search terms into google for a hot minute and can not seem to find an answer to this. Apologies if this is something stupid and simple but every search result I get is for OUTPUT of 5.1 surround on SPDIF, not capture/input.
The setup:
I have two sound cards:
Card 1 (output) is on the motherboard and is currently connected via analog cables to a 5.1 speaker system. It is properly configured for 5.1 and works how you would expect it to.
Card 2 is a Sound Blaster X-Fi Xtreme (CA0110 if that matters). There is an SPDIF (aka optical) cable running from my TV to the ‘Optical In’ port of the card. Using Helvum, I am able to link capture FL and FR to Card 1 Playback FL and FR and listen to it but only in stereo.
Important note: The Computer in question is NOT connected to the TV other than the optical cable. There is no HDMI cable going from the computer to the TV. There is no video or audio connection other than the audio in.
The problem/issue:
I would like to listen to a 5.1 signal coming from my TV into Card 2 on the 5.1 speakers connected to Card 1. I can currently listen to stereo but not 5.1. Is there a way to enable 5.1 input on SPDIF?
Have you tried this one. My understanding is SPDIF is only 2 channel. It requires some other things. Not sure whether the alsa-plugins automatically supply them? I’m not an audio wizard with this stuff but i see lots of information about it. Would have to do by trial and error as I don’t just have an answer.
Yes. That one only enables the output to be 5.1. The input is still only stereo.
When I use the term SPDIF, I am referring only to the hardware connection and the cable type and not any data format. Whatever is needed to transmit over optical for a 5.1 signal, I should have because switching the profile on both cards to digital 5.1 allows them to output that signal. It is just the input where I am hitting a wall. I am not even sure where to start, hence the question.
If I am not mistaken, the 5.1 for SP-DIF is only possible with a compressed stream like AC-3, DD or DTS, the physical properties allow only two channels with 24 Bit at 192 kHz max uncompressed.
The raw signal therefore will only be two channels. Everything else, you need some from of conversion or reencode.
If your TV puts out a compressed stream like the above (AC-3, DD, DTS), it would be possible to decode and map it. The generic signal is stereo only, because of these restrictions.
IEC 61937 defines a way to transmit compressed, multi-channel data over S/PDIF.
The control word bit 0:1 is set to indicate the presence of non-linear-PCM data.
The sample rate is set to maintain the needed symbol (data) rate. The symbol rate is usually 64 times the sample rate.
Data is packed into blocks. Each data block is given a IEC 61937 preamble, containing two 16-bit sync words and indicating the state and identity (type, validity, bitstream number, length) of encapsulated data present. Padding is added to match full block size as required by timing.
As you can see, multi-channel requires encapsulation. It is possible that this encapsulation is not supported for this input, only for the output of this particular card. Without driver and hardware support, you are out of luck.
Apologies. The holidays have been hectic here and it has been a bit since I have had the time to return to this.
I am also under the impression that the input is not capable of encapsulation. This was a side project anyway - just see how far I could take it. I think the final solution will be a new sound system to replace the old 5.1 computer speakers.