[pulseaudio-tickets] [Bug 78879] New: Analog stereo matrix-encoded surround discrete channel upmix

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=78879

          Priority: medium
            Bug ID: 78879
                CC: lennart at poettering.net
          Assignee: pulseaudio-bugs at lists.freedesktop.org
           Summary: Analog stereo matrix-encoded surround discrete channel
                    upmix
        QA Contact: pulseaudio-bugs at lists.freedesktop.org
          Severity: enhancement
    Classification: Unclassified
                OS: All
          Reporter: quequotion at mailtothis.com
          Hardware: All
            Status: NEW
           Version: unspecified
         Component: alsa
           Product: PulseAudio

This is a feature request.

Dolby Surround (the old, analog stereo matrix-encoded surround codec, not Dolby
Digital) encoded audio ouptut could be greatly improved by a plugin that could
detected the matrix encoding, upmixed and output the stream discrete channels
for systems that support output to four or more channels.

LabDV has a description of the technology (and audio samples):
http://www.labdv.com/guide/audio/dolby_surround-en.html

There is debate about whether Dolby Surround encoded three or four channels. It
encoded four, but not discretely, and the Center channel (a simple blend of
Left & Right) seems to be the one not everyone counts.

As it is, if a user has an entirely analog setup, with an analog stereo output
and a Dolby Surround (ProLogic I/II) receiver, then analog stereo
matrix-encoded surround may already be working (so long as the encoding isn't
resampled out by pulseaudio, the receiver should be able to decode it). 

However, recently more users have digital setups with either HDMI or iec958
output to digital receivers.

I have such a setup and I've enabled the a52 plugin for AC3 digital 5.1
surround output, which creates a virtual device with five discrete channels.
With this setup Dolby Digital, DTS, or discrete channel audio streams are
mapped as is and stereo signals are stretched around the 5.1 channels.

If pulseaudio is configured for 5.1 (digital) output (a52 plugin, etc), the
analog stereo matrix-encoding seems to be lost. The stereo signal is stretched
around the 5.1 speakers, resulting in an artificial surround effect where the
rear channels do not sound independently of the front

If pulseaudio is configured for stereo (digital) output, and the receiver is
set to decode Dolby Surround (ProLogic (II)), it may be working but the quality
of surround seems actually less than stretching the stereo signal around 5.1
speakers.

I haven't been able audibly to distinguish that the surround, center, or LFE
channels are in use independently of the front in either case.

What I have in mind is this:

something streams a stereo signal to pulseaudio ->
an (alsa?) plugin detects analog stereo matrix encoding - >
the plugin upmixes the signal into discrete channels ->
Surround channel encoded audio is sent to Side and Rear speakers (as
available),
Center channel encoded audio is sent to Center,
Left channel encoded audio is sent to Front Left speaker,
Right channel encoded audio is sent to Front Right speaker.
For ProLogic II, LFE channel encoded audio is sent to LFE.

Much like the a52 plugin, this one may face patent restrictions.
Steve Harris's GPL "Surround Matrix Encoder" LADSPA plugin implements most of
the codec without violating Dolby's intellectual property:
http://plugin.org.uk/ladspa-swh/docs/ladspa-swh.html#tth_sEc2.102

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