How do I get digital audio out to my headphones?

[Tim]

On Thu, 2010-02-11 at 17:23 -0500, Marcel Rieux wrote: 
> As I already said, the only sound I can get at the present time is
> analog sound on my computer. I suppose my HD TV doesn't accept analog
> sound, at least not through an HDMI cable.

If you mean HDMI as per one of these <http://en.wikipedia.org/wiki/Hdmi>,
then no, it will not accept analog through it.  It's an all-digital
connection.  Nor can you simply apply S/PDIF to a pair of connectors in
the plug, it's a different type of data format that HDMI carries.

If you have a graphics card that lets you pipe S/PDIF into it, to send
out a digital connection (like HDMI), that card has to adapt the data to
suit.  That *could* be done independently by the hardware on the card,
or it *could* require some software driving to make it work (in which
case, you may well need NVidia drivers to do something).

> But, if I plug headphones in my computer, then the video card and the
> S/PDIF wire are not involved.

Correct, if you're connecting ordinary headphones.  If you had
headphones with a digital interface, they'd most likely be USB, and
they'd behave as their own sound card.  I'm yet to see headphones with
their own S/PDIF input, but it'd be technically feasible.

I don't think I've noticed you say what sort of headphones you're
trying.

> Shouldn't digital audio come out on the computer if I select digital
> audio in pulseaudio, you know, right clicking the speaker in the top
> panel and selecting the digital output in "hardware"?

One would think so, but there might also be other things involved.  Your
S/PDIF hardware would have to be supported, as well. 

My laptop has a S/PDIF connector (confusingly, it's a shared
dual-purpose plug, that also is used for the headphone output - somehow
it detects which is connected, and supplies the appropriate signal), but
no options seem to show up digital audio hardware as being available for
me to select.

> The correct entry should be HDA ATI SB, as my sound card is an
> integrated Sound Blaster, chip Realtek ALC888. Is it OK if it doesn't
> show as default? Should Pulseaudio really be the default? This is, at
> best, rather confusing. If the default should be HDA ATI SB, how to I
> change this?

System generates audio (a beep, a wave file is played, etc.)...
Goes to pulseaudio to manage...  (software volume levels and multiple simultaneous streams)
Pulseaudio goes to the sound hardware...  (to generate the actual sound)
If using Alsa, it's mixer has some control over the sound hardware...  (hardware volume levels)
And it's as this end of the chain that (Alsa) drivers talk to your hardware.

So, audio software (music players, etc.), are set to use pulseaudio...
Pulseaudio's set to output to the audio hardware you want (if you have several choices).

And, as you can see, from one thing feeding into the other, both
pulseaudio and alsa volume controls (for PCM and master, at least) will
need to be raised to get any audio out.

> In "0  HDA ATI SB", I have the same "Master entry". The "Headphone" is
> enabled but set to 00 and there's no way to change this.

On some sound cards, there's separate controls for separate outputs to
the headphone, versus outputs to the speakers, and/or line outs.  On
others, there's only one main output, and they all use it.  And on
others, still, there's only one main output, but when it detects a
headphone has been plugged in, it'll flip over to using the headphone
volume level control for controlling the one main output (which is quite
useful when speakers and headphones produce radically different audio
levels).

> There are settings there for:
> 
> S/PDIF
> S/PDIF Default PCM
> 
> Both are enabled, or unmuted, if you prefer. Both are set to 0 and
> it's impossible to change this setting.

Seems like they're disabled...

I wonder if your hardware has some /smarts/ that detect when a S/PDIF
device is plugged in.

-- 
[tim at localhost ~]$ uname -r
2.6.27.25-78.2.56.fc9.i686

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