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ac3jack is a tool for creating an AC-3 (Dolby Digital) multichannel stream from its JACK input ports. Using this tool, an AC-3 stream (up to 5.1 channels) is created in real time and either written to a file or streamed to standard output.

When streamed to stdout and piped through the ALSA tool ac3dec -C, the AC-3 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 outputs and inputs.

AC-3 is a compressed audio stream, so quality will suffer somewhat, but 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. The AC-3 encoder used here comes from the FFMPEG project, and I have not done any quality comparisons with commercial encoders.

News
Version 0.1.2 released -- 12 February 2004
*** fixed compilation bug (the code should be pure C now, honest :)
Version 0.1.1 released -- 1 January 2004
*** Right and Center channels were swapped, fixed. *** Initial channel port connections can now be specified on command line. *** warning added if streaming to stdout and not using 48kHz sample rate.
Version 0.1.0 released -- 29 December 2003

Download
ac3jack written by Jesse Chappell. It is licensed under the GPL and comes with NO WARRANTY of any kind.

Get the source code to the latest release and build it yourself -- ac3jack-0.1.2.tar.gz

Requirements
  • JACK -- version >= 0.80.0
    providing real-time low-latency audio interconnection and delivery. If you haven't used JACK before please study the Linux Audio User Guide and make sure to read the section on JACK.
  • FFMPEG -- version >= 0.4.6
    Note that you'll need to make sure the header files are installed, some packages do not install the headers for some reason. A make install from the source code will do this.

Usage
ac3jack -o <str> [options...] [portname ...]
Options:
-o , --output=<str> specify file to write output AC-3 stream, 'stdout' will write to standard output
-c <num> , --channels=<num> number of AC-3 output channels (2-6). default is 6 (5.1)
-b <num> , --bitrate=<num> bitrate of AC-3 stream (kbits/s). default is 448
-d <num> , --duration=<num> duration of output stream (secs). default is 0 (forever)
-t , --jack-transport use jack transport state (rolling/stopped) to control stream output
-j <str> , --jack-name=<str> jack client name, default is ac3jack
-B <num> , --bufsize=<num> internal buffer size (samples), if overrun problems occur try a large value
-q , --quiet do not output status to stderr
-h , --help this usage output
-V , --version show version only
If portname arguments are added, they are initially connected in channel order, if possible.

Notes
The allowable bitrates for AC-3 encoding are nominally variable from 32-640 kbits/s, but there are common conventions and recommendations for bitrate based on channel count. Only certain bitrates are likely to be accepted by the codec as well.
2 ch 192
5.1 (6 channel) 448

The sample rates allowable are 32, 44.1, and 48kHz. Note that for use as part of DVD soundtracks multiplexed with MPEG2 files, only 48kHz is allowed. Currently the sample rate is selected by the native rate of the active JACK server.

The channels options will make the following input ports with each given channel count:

1 - Left
2 - Left, Right
3 - Left, Center, Right
4 - Left, Right, LeftSurround, RightSurround
5 - Left, Center, Right, LeftSurround, RightSurround
6 - Left, Center, Right, LeftSurround, RightSurround, LFE

If the -t (--jack-transport) option is used, ac3jack will only write to the output when the JACK Transport state is Rolling. This is useful in order to do port connection before the streaming begins, or to synchronize AC-3 stream capture to disk. If extra arguments are added to the end of the command line, they are treated as JACK ports to connect to the inputs (in channel orders shown above).

OUTPUT TO FILE

ac3jack -o file.ac3
This will create the ac3jack client with 6 inputs, (L,R,C,LS,RS,and LFE) to which you can connect with your favorite JACK patchbay (eg. qjackctl). It will create an AC-3 stream and the output will be written to the file specified. As mentioned above, the -t option can be useful here.

REAL-TIME OUTPUT TO SURROUND RECIEVER VIA SPDIF

It appears that SPDIF AC-3 passthrough is only done at a 48kHz sample rate. Therefore, you currently must run JACK at 48kHz in order to maintain an uninterrupted audio stream.

The following command line can be used to send the AC-3 stream through your audio interface (sound card) SPDIF output. You must have the tool 'ac3dec' installed from the alsa-tools package available from http://alsa-project.org. Chances are you have it already if you are using ALSA.

ac3jack -o stdout | ac3dec -C
This will create the ac3jack client with 6 inputs, (L,R,C,LS,RS,and LFE) to which you can connect with your favorite JACK patchbay (eg. qjackctl). It will create an AC-3 stream and the output will be piped to the ac3dec command. The -C option to ac3dec causes it to try to open the SPDIF portion of your sound card for raw output and send the stream through it. You should see something like the following on the terminal when successful:

ac3jack encoding with 6 channels, 48kHz, 448 kbit/s bitrate, output to stdout
Using PCM device 'plug:iec958:{AES0 0x2 AES1 0x82 AES2 0x0 AES3 0x2}'
AC3 Stream 48 KHz 448 kbps

Some sound cards may not allow the SPDIF to be accessed when JACK is using it. If you get no output from the SPDIF, or you never see the line including something like "AC3 Stream 48 KHz 448 kbps" you are one of the unlucky ones. In this case, to do real-time streaming you must have two sound cards, with JACK running on one, and the -c (lowercase) option used with ac3dec to choose the other card for the SPDIF output (clock drift is likely going to be a problem here). At some point in the future, JACK may provide a capability to stream raw SPDIF internally.


Copyright 2004 Jesse Chappell <jesse at essej dot net>

Last modified: Thu Jan 1 23:47:53 EST 2004