[MPlayer-dev-eng] [REQ FOR COMMENT] HRTF headphone libaf filter

[Yue Shi Lai]

Hi,

I just wrote an experimental HRTF (head related transfer function)
filter, that generates in headphone an acoustic sound space of a
5.1 surround speaker system. Included is the patch necessary for
libaf. The patch includes data derived from KEMAR HRTF data by B.
Gardner and K. Martin, the MIT Media Lab copyright notice states,
however, that usage is permitted if a reference is kept.

The filter is faily well commented, including how to obtain the
processed filter coefficients. The filter "cheats" at several
points to enhance the 3D sound obtained:

- focus the front center by generating echos in the front
  left/right.
- making the HRTF transparent by equalizing the higher frequencies
  and compensate the bass damping
- linear stereo expand (slightly)

The filter also includes a simple _active_ matrix surround decoder
to decode 5.1+1 surround with an matrix encoded rear ceter (like
Dolby Digital EX). I think the same principle of steering matrix
could be used in libaf's "surround" filter (which in fact even
lacks an center decoding), maybe somebody could have a look at it.

I think it is best to not submit this as an patch, as people could
maybe just try it. Discussion and comments about this patch would
be in fact very useful at this stage.

How to use the af_hrtf filter:

mplayer -channels 5 -af hrtf <rest of your command line>
mplayer -channels 6 -af hrtf <rest of your command line>

It is important that the sound is decoded with at least 5
channels, or possibly 6 channels (including LFE). The filter also
only accepts 48 kHz sampling frequency, as the coefficients are
sampled at this frequency.

This patch seems to generate good results while hearing with a
Sennheiser HD 570, but while the 3D sound impression is still
present, sounds less impressive with a (cheaper) Sennheiser
PX200.

However: Do not expect to be able to distinguish 5 discrete
channels, as even in a surround speaker setup, it is virtually
impossible to do that without head movement.

Best wishes,

Yue Shi Lai
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