[pulseaudio-tickets] [Bug 95021] LFE channel not synthesized properly

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=95021

--- Comment #24 from Tanu Kaskinen <tanuk at iki.fi> ---
Based on IRC discussions, it seems likely that we will revert the default
configuration to what it was in 6.0. That is, we won't synthesize LFE channel
at all by default.

The solution that I thought we'd do was to sum the low-pass filtered audio from
all channels, instead of averaging it like we do now. That, however, can cause
clipping in the LFE channel. The clipping can be avoided by lowering the
volume, but it has its problems too.

When converting 5.0 content to 5.1 channels, the absolutely safe thing would be
to divide the amplitude of all channels by 5. Since that's pretty big reduction
in volume, we could also do an empirical test on some collection of content and
see how much the volume has to be reduced in practice in order to avoid
clipping in 99% of cases.

If we reduce the volume on 5.0 -> 5.1 conversion but not when playing 5.1
content, that means that 5.1 content will play louder than 5.0 content, which
is not really acceptable, so we would have to reduce the volume the same amount
in all cases.

Alexander reported that Windows 10 doesn't synthesize the LFE channel (nor the
center channel) when playing stereo content to a 5.1 device. The front channels
just get copied to the rear channels, and also the amplitude is halved.

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