Surround on IT files is 'buggy'

Hi,

I've done a little test case for surround commands on IT files. https://drive.google.com/file/d/0B6WljCvFQYQkMHZyUTYxZVJLcXM/view?usp=sharing o Note: for download, just press that blue button.

These are the used commands:

S9B - quad surround (it's a global sound control command, it affects to all channels like S9A)
S9A - center surround (resets S9B)
S91 - surround on
S90 - surround off

The test case consist:

Position 0: mono sound [p32]
Position 1: left panning [p00]
Position 2: right panning [p64]
Position 3: center surround (S9A) (or mono surround) [p32]
Position 4: left panning surround (S9B) (p20, due to inverted-phase limitations)
Position 5: right panning surround (S9B) (p50, same as above)
Position 6: resets surround commands
From 7 to 10: other tests, using Oxx, S9x, pxx commands.

The stereo separation of both players (xmp and openmpt) is 100.

I recorded in stereo @ 48kHz the sound output with both players, then I converted it to mono.

This the result from openmpt:

As it can see:

The first three waves are ok
Then the 4th have lesser volume *
Later, the 5th and 6th have a ascendant volume *
The last three waves have differents behavour *

* Due to the nature of the surround, converting from stereo to mono sound, may have volume loss on inverted-phase waves.

And this other is from xmp:

In this case, surround sound is not working properly, it ignores these commands.

Best regards, Honguito98.

libxmp / libxmp