Hi.
 >   :m  Sorry... but I think you're wrong. Sb Pro DOESN'T have more than 1
 > digi-
 >   tal channel. What you probably read was that there were 4 diferent
 > sound sour-
 >   ces, i suppouse, you have the FM sintetiser, the digital channel, the
 > line
 >   in and the mic in. But SB Pro DOESN'T hardware mixing. The only SB
 > wich does
 >   such a thing is 32 and up.
Yes, this is more-or-less correct. A lot of people mistake sound channels for 
DMA channels, or just plain don't get it. Here is how a SB 16 is set up:
INPUT CHANNELS
External Sound Sources
    PC-Speaker   (Mono)
    Microphone   (Mono)
    Line-In      (Stereo)
    CD-ROM Audio (Stereo)
    Midi-in      (Stereo?)
On-Card Sound Sources
    Digital Out (Stereo)
    FM Out      (Stereo?)
OUTPUT CHANNELS
Speaker out (Stereo)
Line Out    (Stereo)
Midi-out    (stereo)
Digital-in  (stereo)
Now, the hard bits to understand:
The INPUT channels are ALL mixed together on the card IN HARDWARE!!!! For 
example, you can record a WAV file of a musical CD-ROM track, but if you 
happen to have the microphone on at the time the resulting WAV will be a 
mixture of the music track and whatever your microphone's hearing.
The input channels are then DIRECTLY linked to the output channel, and can be 
head at any time. For example, enable the microphone and you'll hear it come 
thru the speakers immedialte, wether you're recording or not. Even whilst the 
computer is rebooting this will work because it's done on the card. As long 
as the card has power and the mixer registers aren't altered, it works.
DMA Channels
The 'normal' way to play back & record DIGITAL sounds is with the DMA 
hannel.
The SB 16 only has ONE DMA Channel for this purpose. (Actually, it has two -
one 8-bit one and one 16-bit one, but you can only use ONE of them at a time,
never BOTH. You use the 16-bit one for stereo transfers since it's twice as 
quick).
Since the DMA can only do one thing at once, you can only record OR play 
back, never both at once. There are other cards (ie sound galaxy) with two 
independant DMA channels which do allow playback and record at the same time, 
but these are rare.
The FM/MIDI *IS* a seperatly useable channel, but is usually considered 
limited since it cannot handle digital sound, it can only handle simple FM 
tunes (FM is really Adlib) or MIDI data. FM is often used in games for the 
simple 'background' music, whilst the digital adds the effects. The card's 
mixer mixes both together and you hear both sounds at the same time.
Mixer
The mixer controls the colume of each sound source and also enables or 
disables completely any sound source. It also takes care of switching between 
FM and MIDI. MIDI is a 'special case' and is hard to include, but here goes:
Either card-generated FM or the external midi-port can be a MIDI sound 
ource,
but not BOTH. The midi-out port is conected ONLY to which either of these is
selected. The midi-in channel readable by the PC is also which ever source is 
selected.  The PC can only output midi using the FM out, so you can't play 
midi and FM at the same time. you also can't record midi at the same time 
you're playing it. This is why the SB's midi port is often referred to as 
'crippled'. A proper midi port follows MPU-401 stardard that allows play and 
record at the same time.
As you can see, the sound card routes the signals DIRECTLY between inputs and 
outputs - so any input cound can appear at any output at ANY time, as long as 
the mixer has the channel enabled and sufficiently loud. The PC can on 
'listen in' on the signals passing thru the card, and enable which signals 
pass thru. The PC CANNOT perform simultaneous playback and record however
So, the 9 possible activities are:
FM/Midi-out   Digital-out    MIDI-in    digital-in
    N               N           N           N           Silence
    N               N           N           Y           Normal record
    N               N           Y           N           MIDI-Record
    N               N           Y           Y           odd one?
    N               Y           N           N           Normal playback
    N               Y           Y           N           odd one?
    Y               N           N           N           MIDI/FM Playback
    Y               N           N           Y           odd one?
    Y               Y           N           N           Used a lot by games
Here's a diagram of the 'paths' the sounds can travel around the card:
INPUTS                                                          OUTPUTS
+-------+                                                         +--------+
|speaker|------+                              +-------------------|MIDI-out|
+-------+      |                              |                   +--------+
               |    +------------+            |
+-------+      +----|in       out|------------+                   +--------+
|line-in|---|       |            |            |   +---------------|Line-Out|
+-------+   +-------|in          |            |   |               +--------+
                    |    MIXER   |            |   |
+-------+   +-------|in          |            |   | +---------+   +--------+
|mic-in |---+       |         out|---+--------*---+-|Power Amp|---|Spkr-Out|
+-------+      +----|in  in in   |   |        |     +---------+   +--------+
               |    +------------+   |        |
+-------+      |      |   |  |       |        |  *=see note1
|CD-in  |------+      |   |  +-<-$-<-+        |
+-------+        +----+   |      |            |
                 |        |      | $=see note3|
+-------+        |   +----+      |            |
|MIDI-in|--------+   |           |            |
+-------+            |           |         +--+
                     |           |         |
                     |           |         |
                     |           |         |
                     |           |         |
                 +------+   +-------+   +-------+
Note2------>     |FM-Out|   |Digital|   |MIDI-IN|
                 +------+   +-------+   +-------+
Note1: the mixer-out and midi-out lines do not connect! they cross over due 
to drawing limitations, but are seperate signal paths, they do not join each 
other.
Note2: These boxes represent the 3 'channels' the programmer can 
software-read and software-write on the card.
Note3: This is effectively a switch, controlled by wether the DMA mode is 
playback or record. In 'playback' mode the signal is switched ONLY to the 
mixer input. In 'record' mode the signal is switched ONLY to the mixer 
output. The '<' symbols show the signal path's direction. Remember that the 
mixer-out is NEVER connected to the mixer input.
How a GUS (for example) varies:
The GUS simply has more than one digital channel, and a mixer with an input 
for each digital channel. The card can also play back from RAM onboard the 
card
itself, so the DMA is used for loading the RAM, NOT for real-time playback. 
Thus, the GUS only realy needs one DMA Channel. Thus, the only real-time
information fed to the GUS is mixer adjustments and playback commnds. Much 
less
CPU Intensive.
Hope this helps everyone out there?
        Craig
--- FMail/386 1.20+
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