30 Dec
2003
30 Dec
'03
1:54 a.m.
Introducing the initial release of ac3jack, bringing you realtime
AC3 stream encoding of any JACK audio.
http://essej.net/ac3jack/
ac3jack is a tool for creating an AC3 (Dolby Digital) multichannel
stream from its JACK input ports. Using this tool, an AC3 stream (up
to 5.1 channels) is encoded in realtime and either written to a file or
streamed to standard output.
When streamed to stdout and piped through the ALSA tool 'ac3dec -C',
the AC3 stream can be passed out the SPDIF port on your audio interface
for connection to a multichannel surround receiver. In this way,
you can achieve full 5.1 surround mixing and monitoring of your JACK
applications with a single digital cable, and no need for hardware
supporting discrete inputs and outputs.
AC3 is a compressed audio stream, so quality is somewhat compromised.
It is the price you pay for easy surround sound. After all, if it is
good enough for DVD and film soundtracks, it must be OK.
Please try it out, read the usage notes, and let me know of any
build or runtime problems you find.....
jlc
30 Dec
30 Dec
10:41 a.m.
OMG this is awesome!
Thank you very much for making this!
Ivica Ico Bukvic, composer & multimedia sculptor
http://meowing.ccm.uc.edu/~ico
-----Original Message-----
From: linux-audio-dev-bounces(a)music.columbia.edu
[mailto:linux-audio-dev-
bounces(a)music.columbia.edu] On Behalf Of Jesse
Chappell
Sent: Tuesday, December 30, 2003 12:50 AM
To: linux-audio-dev(a)music.columbia.edu; linux-audio-
announce(a)music.columbia.edu
Cc: jackit-devel(a)lists.sourceforge.net
Subject: [linux-audio-dev] ac3jack - realtime AC3 stream encoder for
JACK
Introducing the initial release of ac3jack, bringing you realtime
AC3 stream encoding of any JACK audio.
http://essej.net/ac3jack/
ac3jack is a tool for creating an AC3 (Dolby Digital) multichannel
stream from its JACK input ports. Using this tool, an AC3 stream (up
to 5.1 channels) is encoded in realtime and either written to a file
or
streamed to standard output.
When streamed to stdout and piped through the ALSA tool 'ac3dec -C',
the AC3 stream can be passed out the SPDIF port on your audio
interface
for connection to a multichannel surround receiver.
In this way,
you can achieve full 5.1 surround mixing and monitoring of your JACK
applications with a single digital cable, and no need for hardware
supporting discrete inputs and outputs.
AC3 is a compressed audio stream, so quality is somewhat compromised.
It is the price you pay for easy surround sound. After all, if it is
good enough for DVD and film soundtracks, it must be OK.
Please try it out, read the usage notes, and let me know of any
build or runtime problems you find.....
jlc
4:21 p.m.
Seems very cool !
(don't have the time nor the gear to test it but I'm making some
comments anyway)
A few questions:
1)
on your page you say that on some soundcards jack blocks the SPDIF out
thus you need
to have two cards to use jack and AC3 out.
Would it make sense to add AC3 support directly to jack ?
Basically integrating ac3dec's SPDIF out within jack.
That way the user could use the capure audio ports and AC3 at the same time
(with jack exposing the 5.1 ports).
2)
is the quality of the encoder you are using at par/comparable with
commercial
windows AC3 encoders ?
how much CPU is used for full realtime 5.1 encoding -> AC3 spdif out on
let's
say a 2GHz box (or give figures on your own box please).
3)
Any idea what the latency figures look like when you pipe in a realtime
audio signal into
jack-ac3 and the audio gettting out via SPDIF ?
Does ac3 encoding similar framesizes to mp3 (which means a minimum of
50msec delay).
In short, with these virtual "AC3 jack ports", would it be possible to
attach a jack-capable softsynth
and play it lively ? (I guess not).
cheers,
Benno
Jesse Chappell wrote:
Introducing the initial release of ac3jack, bringing
you realtime
AC3 stream encoding of any JACK audio.
http://essej.net/ac3jack/
5:11 p.m.
Benno Senoner wrote on Wed, 31-Dec-2003:
1)
on your page you say that on some soundcards jack blocks the SPDIF out
thus you need
to have two cards to use jack and AC3 out.
Would it make sense to add AC3 support directly to jack ?
Basically integrating ac3dec's SPDIF out within jack.
That way the user could use the capure audio ports and AC3 at the same time
(with jack exposing the 5.1 ports).
Well, it might be a good idea to add some features to JACK's alsa
driver to optionally access the SPDIF for passthrough mode. It
might involve a new port type to be created so clients can connect
and send raw data through it. I am skeptical about integrating
any AC3 specific code in JACK...
2)
is the quality of the encoder you are using at par/comparable with
commercial
windows AC3 encoders ?
how much CPU is used for full realtime 5.1 encoding -> AC3 spdif out on
let's
say a 2GHz box (or give figures on your own box please).
The encoder is supplied by FFMPEG's libavcodec. I have not done
any real quality tests at all. According to some documents from
Dolby, encoding with decent quality is not too hard/complex at
the higher bitrates, but can get difficult at lower bitrates.
I looked briefly at the source for libavcodecs' ac3 encoding but
haven't tried to analyze it. The one thing that does suck, is
that libavcodec is limited to 16bit audio input (an API limitation)
although in theory the AC3 encoding can be done from higher bit-depth
source. Right now, I am not dithering the jack input, but that
is easy to add.
On my 1.8G P4, a 448kbit/s 5.1 stream encoding takes around 10%
CPU. I'll note that the blocksize/buffersize i'm using could
be better optimized to match the JACK buffersize. Also, i'm doing
the encoding/streamin in another thread (not the realtime process thread),
which might become a realtime performance problem on heavily loaded systems.
Turn up the buffering (-B) if upon exit it tells you overruns
occurred.
3)
Any idea what the latency figures look like when you pipe in a realtime
audio signal into
jack-ac3 and the audio gettting out via SPDIF ?
Does ac3 encoding similar framesizes to mp3 (which means a minimum of
50msec delay).
In short, with these virtual "AC3 jack ports", would it be possible to
attach a jack-capable softsynth
and play it lively ? (I guess not).
I haven't quantified them, but there is buffering going on in
my app and in ac3dec, so i don't think that live instrument playing
is going to be very useful at all. The framesize of AC3 represents
a time interval of 1536 PCM samples (31ms at 48k), and i'm sure
the latency of ac3jack | ac3dec is a good bit more than this :)
I haven't done much realtime testing of the streaming yet, so
I'm sure some software updates might be in order once I get some
feedback from people (and myself).
jlc
31 Dec
31 Dec
3:31 p.m.
Jesse Chappell wrote on Tue, 30-Dec-2003:
I haven't done much realtime testing of the
streaming yet, so
I'm sure some software updates might be in order once I get some
feedback from people (and myself).
Well, having done some testing last night, I can say that it really
doesn't work smoothly at all yet! There are some buffering
issues I need to work out to assure that the 'ac3dec -C' has all
the data it needs to keep the stream undisturbed to the SPDIF.
Alternatively, some work might need to be done to get the writing
to the SPDIF done within JACK itself.....
Just a warning to anyone trying the example given on the ac3jack
website.....
jlc
7433
days inactive
7434
days old
linux-audio-dev@lists.linuxaudio.org
4 comments
3 participants
participants (3)
-
Benno Senoner -
Ivica Bukvic -
Jesse Chappell