dolby digital output

dolby digital output

[Hod McWuff]

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What would be involved in enabling 5.1-channel spdif output? 


I'd imagine the first step would be to add an ac3 encoder, and then
(possibly) the 2->5.1 channel demuxing for stereo streams. This would
result in the ability to mix dolby digital streams (DVD's) with more
mundane PCM streams, such as IM or telephony apps.


I'm willing to tackle some moderate coding, but I haven't attacked alsa
internals at all yet... where's the best docs for that?

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Re: dolby digital output

[Andy Lo-A-Foe]

On Sun, Feb 17, 2002 at 01:33:32AM -0500, Hod McWuff wrote:

> I'd imagine the first step would be to add an ac3 encoder, and then
> (possibly) the 2->5.1 channel demuxing for stereo streams. This would
> result in the ability to mix dolby digital streams (DVD's) with more
> mundane PCM streams, such as IM or telephony apps.

None of the available open source software AC3 encoders are fast enough to
do realtime 5.1 encoding I think. The main task would be to try and
optimize this. Outputting compressed AC3 frames through alsa is probably
the easiest part (see http://www.alsaplayer.org for ac3play)

Andy

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Re: dolby digital output

[Markus Plail]

Hi there!

On Sun, 17 Feb 2002 18:15:55 +0100 Andy Lo-A-Foe wrote:

> On Sun, Feb 17, 2002 at 01:33:32AM -0500, Hod McWuff wrote:
> 
> > I'd imagine the first step would be to add an ac3 encoder, and then
> > (possibly) the 2->5.1 channel demuxing for stereo streams. This would
> > result in the ability to mix dolby digital streams (DVD's) with more
> > mundane PCM streams, such as IM or telephony apps.
> 
> None of the available open source software AC3 encoders are fast enough
> to do realtime 5.1 encoding I think. The main task would be to try and
> optimize this. Outputting compressed AC3 frames through alsa is probably
> the easiest part (see http://www.alsaplayer.org for ac3play)

Are there really any AC3 ENcoders? I thought only quite expensive
(windows) software would be able to do that, because of Dolby's licensing.
If there are any what would those be?

Regards
Markus Plail

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RE: dolby digital output

[James Courtier-Dutton]

>From what I understand, we can do ac3 "like" encoders in linux for free, and
they might actually make sounds come out on Commercial AC3 decoders, but
unless we pay a license fee, we cannot actually call it AC3.
This is why we now have the "liba52" decoder which just happens to work
quite nicely on ac3 input.

I don't see why we could not also do a liba52 encoder.
I think that if anyone started making a lot of money from liba52, then Dolby
would probably start asking for some royalties.
But there is no point in them making a fuss yet because there is no money
involved.

Cheers
James


> -----Original Message-----
> From: alsa-devel-admin@lists.sourceforge.net
> [mailto:alsa-devel-admin@lists.sourceforge.net]On Behalf Of Markus Plail
> Sent: 17 February 2002 17:48
> To: alsa-devel@lists.sourceforge.net
> Subject: Re: [Alsa-devel] dolby digital output
>
>
> Hi there!
>
> On Sun, 17 Feb 2002 18:15:55 +0100 Andy Lo-A-Foe wrote:
>
> > On Sun, Feb 17, 2002 at 01:33:32AM -0500, Hod McWuff wrote:
> >
> > > I'd imagine the first step would be to add an ac3 encoder, and then
> > > (possibly) the 2->5.1 channel demuxing for stereo streams. This would
> > > result in the ability to mix dolby digital streams (DVD's) with more
> > > mundane PCM streams, such as IM or telephony apps.
> >
> > None of the available open source software AC3 encoders are fast enough
> > to do realtime 5.1 encoding I think. The main task would be to try and
> > optimize this. Outputting compressed AC3 frames through alsa is probably
> > the easiest part (see http://www.alsaplayer.org for ac3play)
>
> Are there really any AC3 ENcoders? I thought only quite expensive
> (windows) software would be able to do that, because of Dolby's licensing.
> If there are any what would those be?
>
> Regards
> Markus Plail
>
> _______________________________________________
> Alsa-devel mailing list
> Alsa-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/alsa-devel


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Re: dolby digital output

[Steve Harris]

On Sun, Feb 17, 2002 at 06:09:02 -0000, James Courtier-Dutton wrote:
> >From what I understand, we can do ac3 "like" encoders in linux for free, and
> they might actually make sounds come out on Commercial AC3 decoders, but
> unless we pay a license fee, we cannot actually call it AC3.
> This is why we now have the "liba52" decoder which just happens to work
> quite nicely on ac3 input.

I'm a little dubious about that. It's possible to write a Free pro-logic
compatible encoder (I have), as most of the technology predates dobly, but
it can't, legally be 100% compatible for licencing reasons. AC3 is much
more complicated, and you couldn't feasibly reproduce the encoded output
without having followed the spec (unless I'm missing something).

Not that Free software with dubious legality is anything new ;)

- Steve

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Re: dolby digital output

[Steve Harris]

On Mon, Feb 18, 2002 at 02:08:12 +0000, Steve Harris wrote:
> I'm a little dubious about that. It's possible to write a Free pro-logic
> compatible encoder (I have), as most of the technology predates dobly, but
> it can't, legally be 100% compatible for licencing reasons. AC3 is much
> more complicated, and you couldn't feasibly reproduce the encoded output
> without having followed the spec (unless I'm missing something).

Sorry, I was wrong. The situation is very simmilar. It could be entirly
legal to produce a stream that is ATSC52 compliant, and therefore AC3
compatible, you just can't call it Dolby Digital, etc.

The standards document is not very clear on the patent situation.

PC's should just about be able to do AC3 encoding in realtime too, last
time I tried it was way off, but that was on a 400MHz PII or similar.

- Steve

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Re: dolby digital output

[Bob Ham]

On Mon, 2002-02-18 at 14:08, Steve Harris wrote:

> AC3 is much
> more complicated, and you couldn't feasibly reproduce the encoded output
> without having followed the spec (unless I'm missing something).

Assuming you haven't signed (or clicked "yes" on) any agreement in order
to get the spec, I can't see what they can legally do.  Unless there's
software patents, but those are US-specific anyhow (for the moment) so
they don't really matter.

Bob

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Re: dolby digital output

[Steve Harris]

On Mon, Feb 18, 2002 at 03:00:56 +0000, Bob Ham wrote:
> > AC3 is much
> > more complicated, and you couldn't feasibly reproduce the encoded output
> > without having followed the spec (unless I'm missing something).
> 
> Assuming you haven't signed (or clicked "yes" on) any agreement in order
> to get the spec, I can't see what they can legally do.  Unless there's
> software patents, but those are US-specific anyhow (for the moment) so
> they don't really matter.

Well, it might be illegal to distribure in the US, and the US has been
keen to arrest people for breaking US laws in other parts of the world
before... especially when large corporation are involved.

- Steve

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Re: dolby digital output

[Takashi Iwai]

At Sun, 17 Feb 2002 18:47:35 +0100,
Markus Plail wrote:
> 
> Hi there!
> 
> On Sun, 17 Feb 2002 18:15:55 +0100 Andy Lo-A-Foe wrote:
> 
> > On Sun, Feb 17, 2002 at 01:33:32AM -0500, Hod McWuff wrote:
> > 
> > > I'd imagine the first step would be to add an ac3 encoder, and then
> > > (possibly) the 2->5.1 channel demuxing for stereo streams. This would
> > > result in the ability to mix dolby digital streams (DVD's) with more
> > > mundane PCM streams, such as IM or telephony apps.
> > 
> > None of the available open source software AC3 encoders are fast enough
> > to do realtime 5.1 encoding I think. The main task would be to try and
> > optimize this. Outputting compressed AC3 frames through alsa is probably
> > the easiest part (see http://www.alsaplayer.org for ac3play)
> 
> Are there really any AC3 ENcoders? I thought only quite expensive
> (windows) software would be able to do that, because of Dolby's licensing.
> If there are any what would those be?

ffmpeg can encode ac3, too.
i once made a patch to allow 5.1 encoding.  it seemed working, at
least for tests.
not sure how they handle this license problem, though..


Takashi

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RE: dolby digital output

[James Courtier-Dutton]

Any AC3 or DTS takes some time to decode even with an external decoder.
The nice thing about them is that the standard tell us that they are fixed
latency decoders.
So, AC3 decoder will take a fixed amount of time to take a digital SPDIF in
and convert it to 6 analogue out's for your speakers.

The delay is long enough to make it not suitable for low latency
applications like games and telephony.
Some applications which don't care about the latency, but only care that it
is predictable work ok with AC3 audio.
E.G. DVD playing. The latency of decode does not matter, what matters is if
we can predict the latency so we can keep audio and video in sync.

If you then add an encoder, so you can mix streams, you add to the latency
again.
PCM is the audio amplitude sampled at fixed time periods.
These samples can translated into samples in the frequency domain using the
Fast Fourier Transform, thus giving the amplitude of certain frequencies at
certain times.
These samples in the frequency domain is what is contained in AC3.
If you could mix the channels in the frequency domain instead of the PCM
domain, you could save a lot of CPU time.

Before: -
1) AC3 stream from DVD
2) decode to 6 PCM channel
3) Mix with other PCM source
4) 6 PCM to AC3 encoding
5) SPDIF out.
6) External decoder.
7) speakers.

After: -
1) encode other PCM source into AC3.
2) mix DVD's AC3 with other sources AC3 using a frequency domain mixer.
3) SPDIF out.
4) External decoder
5) speakers.

Notice that alsa just provides the SPDIF out bit, nothing else.
ALSA knows nothing of AC3 or DTS. It just outputs the digital stream without
modifying it.

I think it would be best for your application to do the mixing.

Cheers
James




> -----Original Message-----
> From: alsa-devel-admin@lists.sourceforge.net
> [mailto:alsa-devel-admin@lists.sourceforge.net]On Behalf Of Hod McWuff
> Sent: 17 February 2002 06:34
> To: alsa-devel@lists.sourceforge.net
> Subject: [Alsa-devel] dolby digital output
>
>
>
> What would be involved in enabling 5.1-channel spdif output?
>
>
> I'd imagine the first step would be to add an ac3 encoder, and then
> (possibly) the 2->5.1 channel demuxing for stereo streams. This would
> result in the ability to mix dolby digital streams (DVD's) with more
> mundane PCM streams, such as IM or telephony apps.
>
>
> I'm willing to tackle some moderate coding, but I haven't attacked alsa
> internals at all yet... where's the best docs for that?
>


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RE: dolby digital output

[James Courtier-Dutton]

> -----Original Message-----
> From: Hod McWuff [mailto:hod@wuff.dhs.org]
> Sent: 17 February 2002 18:21
> To: James Courtier-Dutton
> Subject: RE: [Alsa-devel] dolby digital output
>
>
>
> So, the upshot is that thanks to the AC3 latency spec, even with a
> lightning-fast encoder, it wouldn't work.
>
> >
> > I think it would be best for your application to do the mixing.
> >
>
> I think you may be right... but that still doesn't solve the additional
> latency, and digital output is still (unhappily) limited to two channels
> unless you're playing a canned stream, such as a dvd.
>
> Any other ideas on how to deal with that, or would an extension to
> S/PDIF be required?
>
I think some high speed S/PDIF is available which can do 8 Channel 96Khz 24
bit audio but very few PC sound cards can do that.

I would have preferred everything to be converted to packet technologies,
and then we could link all consumer equipment together with Ethernet cables!
Circuit based technologies are quite complicated because one has to worry
about clocking and synchronisation all the time.
With packet, each packet would just have a small header, with Stream ID,
duration of packet, Packet number, with maybe a timestamp if one needs to
synchronise two or more different streams together. Also circuit based
technologies have no error correction.

A simple 100 Megs Full duplex point to point Ethernet cable can handle just
about anything S/PDIF, USB, SCSI, IDE can throw at it.
Maybe that is why there is an IP version of SCSI now.

Cheers
James


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RE: dolby digital output

[Bob Ham]

On Sun, 2002-02-17 at 18:44, James Courtier-Dutton wrote:

> A simple 100 Megs Full duplex point to point Ethernet cable can handle just
> about anything S/PDIF, USB, SCSI, IDE can throw at it.

I assume you've seen http://magic.gibson.com/?

Bob

-- 
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RE: dolby digital output

[James Courtier-Dutton]

So all I need now, is an Digital Amp with an Ethernet port for all 6 Channel
PCM, AC3 and DTS output.
I already have an ethernet card for my PC and a HUB.

magic.gibson.com must be stupid not to realise that what we really want is
an AMP with an Ethernet port, not yet another sound card.

We could then just connect the AMP to the ethernet port on any PC, and have
perfect digital quality output.
Alsa would then not be needed, because once the audio is in packets, it
would be better to keep it that way in the application.

One could even use Radio Lan technologies, and thus remove all the cabling
as well.

Cheers
James


> -----Original Message-----
> From: alsa-devel-admin@lists.sourceforge.net
> [mailto:alsa-devel-admin@lists.sourceforge.net]On Behalf Of Bob Ham
> Sent: 18 February 2002 14:51
> To: James Courtier-Dutton
> Cc: alsa-devel@lists.sourceforge.net
> Subject: RE: [Alsa-devel] dolby digital output
>
>
> On Sun, 2002-02-17 at 18:44, James Courtier-Dutton wrote:
>
> > A simple 100 Megs Full duplex point to point Ethernet cable can
> handle just
> > about anything S/PDIF, USB, SCSI, IDE can throw at it.
>
> I assume you've seen http://magic.gibson.com/?
>
> Bob
>
> --
> Bob Ham: bob@ham.org
>
> My music: http://mp3.com/obelisk_uk
> GNU Hurd: http://hurd.gnu.org/
> Do your bit for UK drugs law reform: http://www.angeldeclaration.com/
>
> A pessimist expects the weather to get worse.
> An optimist hopes it will improve.
> A realist adjusts the sails.
>
>
> _______________________________________________
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RE: dolby digital output

[Bob Ham]

On Mon, 2002-02-18 at 15:46, James Courtier-Dutton wrote:

> magic.gibson.com must be stupid not to realise that what we really want is
> an AMP with an Ethernet port

Umm.. this *is* what magic can provide.  A guitar with a an ethernet
port, an amp with an ethernet port, a mixer with an ethernet port, a
computer with an ethernet port.. you name it.  (And when I say ethernet
here, I mean RJ45.)  Has the following specs:

32 channels @ 44.1 kHz x 32 bits
32 channels @ 48 kHz x 32 bits
16 channels @ 96 kHz x 32 bits
8 channels @ 192 kHz x 32 bits

This is over one cable; it's not a network in the sense of ethernet. 
You can't plug all your instruments into a 100baseTX hub, unfortunately,
but you can plug all your instruments into NICs in the back of your
computer.

-- 
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My music: http://mp3.com/obelisk_uk
GNU Hurd: http://hurd.gnu.org/
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RE: dolby digital output

[Bob Ham]

On Mon, 2002-02-18 at 16:26, Bob Ham wrote:
> Has the following specs:
> 
> 32 channels @ 44.1 kHz x 32 bits
> 32 channels @ 48 kHz x 32 bits
> 16 channels @ 96 kHz x 32 bits
> 8 channels @ 192 kHz x 32 bits

I should also note that this is full duplex.

-- 
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My music: http://mp3.com/obelisk_uk
GNU Hurd: http://hurd.gnu.org/
Do your bit for UK drugs law reform: http://www.angeldeclaration.com/

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RE: dolby digital output

[James Courtier-Dutton]

The magic web site did not mention anything about an AMP.


> -----Original Message-----
> From: alsa-devel-admin@lists.sourceforge.net
> [mailto:alsa-devel-admin@lists.sourceforge.net]On Behalf Of Bob Ham
> Sent: 18 February 2002 16:27
> To: James Courtier-Dutton
> Cc: ALSA Development Mailing List
> Subject: RE: [Alsa-devel] dolby digital output
> 
> 
> On Mon, 2002-02-18 at 15:46, James Courtier-Dutton wrote:
> 
> > magic.gibson.com must be stupid not to realise that what we 
> really want is
> > an AMP with an Ethernet port
> 
> Umm.. this *is* what magic can provide.  A guitar with a an ethernet
> port, an amp with an ethernet port, a mixer with an ethernet port, a
> computer with an ethernet port.. you name it.  (And when I say ethernet
> here, I mean RJ45.)  Has the following specs:
> 
> 32 channels @ 44.1 kHz x 32 bits
> 32 channels @ 48 kHz x 32 bits
> 16 channels @ 96 kHz x 32 bits
> 8 channels @ 192 kHz x 32 bits
> 
> This is over one cable; it's not a network in the sense of ethernet. 
> You can't plug all your instruments into a 100baseTX hub, unfortunately,
> but you can plug all your instruments into NICs in the back of your
> computer.
> 
> -- 
> Bob Ham: bob@ham.org
> 
> My music: http://mp3.com/obelisk_uk
> GNU Hurd: http://hurd.gnu.org/
> Do your bit for UK drugs law reform: http://www.angeldeclaration.com/
> 
> A pessimist expects the weather to get worse.
> An optimist hopes it will improve.
> A realist adjusts the sails.
> 
> 
> _______________________________________________
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RE: dolby digital output

[Bob Ham]

On Mon, 2002-02-18 at 17:05, James Courtier-Dutton wrote:
> The magic web site did not mention anything about an AMP.

Magic is just a way to ferry audio data around.. that can be between any
audio devices.  The specification document gives a lot more details than
the site and has a few setup examples (including one with an amp :)

-- 
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RE: dolby digital output

[Dan Hollis]

On Mon, 18 Feb 2002, James Courtier-Dutton wrote:
> The magic web site did not mention anything about an AMP.

The specs do.

-Dan
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RE: dolby digital output

[Dan Hollis]

On 18 Feb 2002, Bob Ham wrote:
> On Sun, 2002-02-17 at 18:44, James Courtier-Dutton wrote:
> > A simple 100 Megs Full duplex point to point Ethernet cable can handle just
> > about anything S/PDIF, USB, SCSI, IDE can throw at it.
> I assume you've seen http://magic.gibson.com/?

sadly, magic is patent-encumbered.

-Dan
-- 
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