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ac3jack

NAME

ac3jack - realtime AC3 stream encoder for JACK

SYNOPSIS

ac3jack [options]

DESCRIPTION

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 created 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 outputs.

Of course AC3 is a compressed audio stream, so quality suffers 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.

OPTIONS

ac3jack accepts the following options:

-h, --help

Show summary of options.

-V, --version

Show version of program.

-o <str>, --output=<str>

specify file to write output AC3 stream, 'stdout' will write to standard output

-c <num>, --channels=<num>

number of AC3 output channels (2-6). The default is 6 (5.1)

-b <num>, --bitrate=<num>

bitrate of AC3 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

NOTES

The allowable bitrates for AC3 encoding are nominally variable from 32-640 kbits/s, but there are common conventions and recommendations for bitrate based on channel count.
  2ch             = 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, Right, Center
 4 - Left, Right, LeftSurround, RightSurround
 5 - Left, Right, Center, LeftSurround, RightSurround
 6 - Left, Right, Center, 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.

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 AC3 stream and the output will be written to the file specified.

REALTIME OUTPUT TO SURROUND RECIEVER VIA SPDIF

The following command line can be used to send the AC3 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.

  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 AC3 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 on successful use:

  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 somthing like "AC3 Stream 48 KHz 448 kbps" you are one of the unlucky ones. In this case, to do realtime 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.

SEE ALSO

jackd(1) ac3dec(1)

AUTHOR

Jesse Chappell <jesse@essej.net>.