5.1 sound with external USB soundcard (Soundblaster HD)

Question

I'm trying to make 5.1 sound work on my old Soundblaster HD. This through the toslink digital output of the card.

I set those values in the pulseaudio daemon:

 remixing-produce-lfe = yes
 remixing-consume-lfe = yes
 lfe-crossover-freq = 80

 default-sample-channels = 6

And this line in the default config file:

load-module module-combine channels=6 channel_map=front-left,front-right,rear-left,rear-right,front-center,lfe

When I test with

speaker-test -c6

front-left, center and front-right work but not the rear speakers. The rear speakers white noise go through the front ones (as if it was stereo).

I can't test -c 6 with IEC958. If I understood well the theory, this makes sense since I should have the profile

output:iec958-ac3-surround-51

But I just have stereo profiles associated with the card.

What's also bizarre, is that when I play a 5.1 Dolby track, the channel map isn't the one I set in my pulseaudio configuration; the rear speakers are replaced with side speakers (I muted the other channels to see whether the sound went through the rear speakers only - which it doesn't):

 sink input(s) available.
    index: 0
    driver: <protocol-native.c>
    flags: START_CORKED FIX_RATE 
    state: RUNNING
    sink: 2 <alsa_output.usb-Creative_Technology_USB_Sound_Blaster_HD_000000VD-00.iec958-stereo>
    volume: front-left: 0 /   0% / -inf dB,   front-right: 0 /   0% / -inf dB,   side-left: 100270 / 153% / 11.08 dB,   side-right: 100270 / 153% / 11.08 dB,   front-center: 0 /   0% / -inf dB,   lfe: 0 /   0% / -inf dB
            balance 0.00
    muted: no
    current latency: 1260.19 ms
    requested latency: 40.00 ms
    sample spec: float32le 6ch 48000Hz
    channel map: front-left,front-right,side-left,side-right,front-center,lfe
    resample method: speex-float-1
    module: 15
    client: 4 <VLC media player (LibVLC 3.0.12)>

The question now being: can I manually create or force a 5.1 sound through IEC958 with those outputs?

toslink has some weird encoding for 5.1 due to bandwidth restrictions, it's possible the drivers can't handle this. First take Pulseaudio out of the picture (`pulseaudio --kill` if it doesn't respawn, or `pasuspender`), and do a `speaker-test` directly on the iec958 output (use `aplay -L`). — dirkt, Jan 30 '21 at 19:07

score 0 · Answer 1 · answered Jan 31 '21 at 16:23

I should have the profile output:iec958-ac3-surround-51. But I just have stereo profiles associated with the card.

As I wrote: This means that the driver doesn't produce the somewhat weird AC3 5.1 encoding that TOSLINK uses.

I looked into this in more detail in another answer either here or on superuser, and I think I remember it was mostly a licensing problem. Nevertheless, there is no driver support, though I think the result was with enough time and energy one could write some.

Anyhow, you are better off using different means (either several analog channels, or a different kind of digital channel) instead of TOSLINK if you care for good quality 5.1 sound.

the channel map isn't the one I set in my pulseaudio configuration;

This is unrelated.

5.1 sound with external USB soundcard (Soundblaster HD)

1 Answers1