Most soundcards these days are capable of more than plain stereo output. To take advantage of this, HSTREAM and HMUSIC channels can be assigned to specific speakers. For example, channels can be played on the front or rear speakers to effectively have 2 separate stereo outputs from the one device. A 3rd stereo output is available on 5.1 cards, and a 4th on 7.1 cards.
To use speaker assignment, the output device obviously needs to support more than 2 speakers - the speakers member of the BASS_INFO structure can be used to check that. The device's speaker config (in it's control panel) will need to be set accordingly too. In Windows, it also requires the output device to have WDM drivers installed. For reference... Windows XP/2000 use only WDM drivers. Windows Me/98SE can use either WDM or the older VxD driver model. Windows 98/95 do not use WDM drivers.
Stereo speaker assignment flags
BASS_SPEAKER_FRONT The front speakers.
BASS_SPEAKER_REAR The rear/side speakers.
BASS_SPEAKER_CENLFE The center and LFE (subwoofer) speakers in a 5.1 setup.
BASS_SPEAKER_REAR2 The rear center speakers in a 7.1 setup.
Mono speaker assignment flags
BASS_SPEAKER_FRONTLEFT The left-front speaker.
BASS_SPEAKER_FRONTRIGHT The right-front speaker.
BASS_SPEAKER_REARLEFT The left-rear/side speaker.
BASS_SPEAKER_REARRIGHT The right-rear/side speaker.
BASS_SPEAKER_CENTER The center speaker in a 5.1 speaker setup.
BASS_SPEAKER_LFE The LFE (subwoofer) speaker in a 5.1 setup.
BASS_SPEAKER_REAR2LEFT The left-rear center speaker in a 7.1 setup.
BASS_SPEAKER_REAR2RIGHT The right-rear center speaker in a 7.1 setup.
As well as these defined speaker location flags, there is the BASS_SPEAKER_N(n) macro that can be used to
access the extra speakers of soundcards that have more than 8 speakers, where n is the n'th pair of speakers (up to a maximum of 15). For example, BASS_SPEAKER_N(1) is equivalent to BASS_SPEAKER_FRONT. To use a speaker in mono, add the BASS_SPEAKER_LEFT or BASS_SPEAKER_RIGHT flags.
The stereo speaker assignment flags can also be used with mono channels, so that, for example, a mono channel can be played on both the front speakers. But mono speaker assignment flags can not be used with stereo channels, so, for example, it's not possible to play a stereo channel on just the center speaker.