|                |
                                    |                |
    <Synth>----Synth---------------------SynthCap--->|
                    \               |                |
                     -------------->|                |
                                    |                |
                                    |                |
                                    |                |
    <PCM>------PCM----------------->|                |
                                    |                |
                                    |                |
                                    |                |
    <LineIn>----Line----------------------LineCap--->|
                    \               |                |
                     LineMute------>|                |
                                    |                |
                                    |                |
                                    |                |
    <Mike>------Mike----------------------MikeCap--->|
                    \               |                |
                     MikeMute------>|                |
                                    |                |
                                    |                |
                                    |                |
    <CD>--------CD------------------------CDCap----->|
                    \               |                |
                     CDMute-------->|                |
                                    |                |
                                    |                |
                                    |                |
                                 Master              |
                                    |                |
                               Treble/Bass           |
                                    |                |
                                  Ogain            Igain
                                    |                |
      ???               |<----------|                |
       |                |           |                |
    PC-Speak       [Amplifier]      |                |
       |                |           |                |
    <PC-Spkr>      <SpkrOut>   <LineOut>       <Digitizer>

• The blocks labelled *Mute are on/off gates that control whether the associated signal is added to the output bus.
• The blocks labelled *Cap are on/off gates that control whether the associated signal will be added to the input ("capture") bus.
• The alsamixer program gives you convenient independent control of the *Mute and *Cap gates -- thereby allowing full-duplex operation. In general, ALSA (Advanced Linux Sound Architecture) seems well behaved. I recommend downloading it and installing it. In contrast, the OSS xmixer program controls the two together, so that any given channel is connected to both the input and output bus, or neither -- thereby preventing full-duplex operation. I don't know whether it is possible to get OSS (Open Sound System) low-level IOCTLs to provide independent control of these gates.
• There is no way to mute the output of the MIDI Synthesizer, but you can record it. You can neither mute nor record the output of the PCM D/A converter.
• The following are 5-bit sliders, where slider=0 represents zero volume, and slider=31 represents 100%. Question: Are intermediate settings linear, or logarithmic?
  • Master
  • Synth
  • PCM
  • Line (input)
  • Mike
  • CD
• The Treble and Bass sliders appear to have 14 levels (slightly less than 4 bits) where 0 represents extreme de-emphasis, 7 is mid-scale, and 13 represents maximum emphasis. I have no idea what codes 14 and 15 do.
• The Ogain and Igain sliders appear to have four levels, where slider=0 represents unity gain, and slider=3 represents four times as much gain. There is no zero-gain setting. The PC-Speaker also has four levels, but I'm not sure of the semantics.
• The PC-Speaker is not hooked to the sound card in any machine I've checked, so I haven't ascertained where the sound-card's PC-Spkr circuits fit on the block diagram.