13 Nov
2008
13 Nov
'08
4:18 p.m.
This is an old issue that I have, it goes beyond Linux but it does
affect and include this OS.
I've a Teac "home theater" amplifier that I bought 7 years ago, complete
with 5.1 speakers. With a DVD player (and more recently a PS3) connected
to the optical input, the amplifier can do 5.1 audio. When the player is
sending 5.1 through the optical link, the Dolby Digital indicator lights
up on the amplifier, and I do get all the channels, front, center and
back. Movies with 5.1 sound, PS3 games with 5.1, they all use the rear
channels correctly and "fill the room" with sound that travels back and
forth as the sources move through the stage. All is well.
Repeatedly, I tried to replicate this result with the PCs, using
different soundcards, different OSes over these years, connected to the
digital coaxial input on the amp.
Usually what I get from a computer is - when it starts sending the
digital carrier (when it boots up), the Dolby ProLogic indicator lights
up on the amp, and I get 2.1 sound. The rear channels are mute.
Sometimes I get the same result with the DVD player or the PS3 - the
ProLogic LED turns on, but that's when the player is sending 2.1 or 2.0
signal over the optical connector, because that's how the source is,
which is the correct and desired behavior.
But with the PC, once the digital coaxial carrier comes up, it's always
ProLogic and 2.1, never Dolby Digital and 5.1
I tried recently with my latest dual-purpose PC: MythTV frontend on
Linux, games on Windows. It's a Gigabyte GA-M78SM-S2H motherboard with a
Realtek ALC888 sound chip connected over digital coax.
http://www.gigabyte.com.tw/Products/Motherboard/Products_Overview.aspx?Prod…
Even on Windows, I go into Sound Properties, I set the speakers type to
5.1, then I run the Realtek software to test the speakers, that sends
signal into every channel. The rear channels are silent. Can't seem to
send 5.1 audio from PC to amp.
I remember a number of years ago, when I was using a SoundBlaster Live
card on Linux, connected to digital coax, I was doing some experiments
with mplayer and I think I was able to turn on the Dolby Digital
indicator on the amplifier. I do not remember if it was actually doing
5.1 sound.
So there are a few possibilities here:
1. I am really dumb.
2. I just had a long string of bad luck and just used bad sound cards
3. There's something wrong with the amplifier
So how do I enable true 5.1 sound connected to the digital coaxial input
on the amp? I want to be able to play 5.1 audio tracks in Linux, and
play games on Windows with 5.1 sound, the way it works with the PS3.
--
Florin Andrei
http://florin.myip.org/
14 Nov
14 Nov
5:49 a.m.
Florin Andrei wrote:
[...]
But with the PC, once the digital coaxial carrier comes up, it's always
ProLogic and 2.1, never Dolby Digital and 5.1
So there are a few possibilities here:
1. I am really dumb.
2. I just had a long string of bad luck and just used bad sound cards
3. There's something wrong with the amplifier
4. software patents
SPDIF was designed for stereo, so it has only the bandwidth for two
channels of data at 48 kHz. (There are extensions for higher sample
rates by increasing the clock, but there are never more than two
channels.)
To transport more than two channels over SPDIF, the audio data has to
be compressed so that it fits into the bandwidth that two uncompressed
channels would use. There are two common codecs, Dolby/AC-3 and DTS;
both are heavily patented.
Any sound card that wants to encode multichannel audio for transport
over SPDIF has to include an encoder license. Very few do (the Windows
drivers for CMI8768+/8770/8788 and some X-Fi cards come with encoders).
Without encoder, 5.1 playback works only when the source data has
already been encoded previously (e.g., on a DVD).
AFAIK there is no Linux distribution that ships with an encoder,
but you can download and install an AC-3 encoder manually (see
<http://www.alsa-project.org/main/index.php/A52_plugin>).
HTH
Clemens
4:02 p.m.
Clemens Ladisch wrote:
Florin Andrei wrote:
Awesome - that clarifies some things I discovered last night when
googling around and doing experiments with the motherboard.
The consensus seems to be it's very hard to do 5.1 over S/PDIF unless
the data is already encoded like that (DVD) - you provided the
explanation for that. So, if I play a DVD it might be doable, but if I
play a game that has capabilities for 5.1 output, most likely it will
not work.
What I discovered is that the sound chip on the mobo can send 5 audio
channels over analog. I guess that should remove all limitations, isn't it?
[...]
But with the PC, once the digital coaxial carrier comes up, it's always
ProLogic and 2.1, never Dolby Digital and 5.1
SPDIF was designed for stereo, so it has only the bandwidth for two
channels of data at 48 kHz. (There are extensions for higher sample
rates by increasing the clock, but there are never more than two
channels.)
To transport more than two channels over SPDIF, the audio data has to
be compressed so that it fits into the bandwidth that two uncompressed
channels would use. There are two common codecs, Dolby/AC-3 and DTS;
both are heavily patented.
Any sound card that wants to encode multichannel audio for transport
over SPDIF has to include an encoder license. Very few do (the Windows
drivers for CMI8768+/8770/8788 and some X-Fi cards come with encoders).
Without encoder, 5.1 playback works only when the source data has
already been encoded previously (e.g., on a DVD). AFAIK there is no Linux distribution that ships with
an encoder,
but you can download and install an AC-3 encoder manually (see
<http://www.alsa-project.org/main/index.php/A52_plugin>).
Is there anything similar for Windows? Some sort of universal plugin
that works with any card?
--
Florin Andrei
http://florin.myip.org/
5856
days inactive
5857
days old
linux-audio-user@lists.linuxaudio.org
2 comments
2 participants
participants (2)
-
Clemens Ladisch -
Florin Andrei