SM 5 BSZ - Using ossmix to set up the A/D board
(Mar 27 2001)

General structure of A/D boards

Besides the A/D and/or D/A converter the ad board may have several other inputs and outputs. Inputs and outputs can be connected to each other by means of software controlled switches.

There may also be several volume controls, base and treble controls and an AGC control for the Microphone input.

Different boards have different architectures. The switches, volume controls etc. are known as the mixer. There is no reason to set control the mixer the signal processing program as I did in early versions (up to dsp00-13). These settings should never be changed once the correct settings for a specific hardware combination is found.

If OSS is used, the program ossmix should be used to set up the mixer. I have a script running ossmix several times to set several parameters to what my hardware requires.

Different boards may have different names for similar switches

When you have installed for the first time and try to run the dsp program you will most probably have direct sound in the loudspeaker. The default setting is usually that line input is connected to the loudspeaker output. For SB16 (Vibra16X or Vibra16C) the two following commands will disconnect:
/lib/oss/ossmix omix.llinesw OFF
/lib/oss/ossmix omix.rlinesw OFF

Presumably one can read about mixer controls somewhere but it is not difficult to find what you need by some experimenting. By typing /lib/oss/ossmix without any arguments you get a list over all controls and their current values. Add -dn as argument if you have more than one sound board. -d0 is default.

Volume controls

To get highest possible dynamic range one has to set volume controls low. Disconnect the cable from the analog hardware, set the volume control to zero and run dsp to see where the noise floor is. Then increase the volume until you see the noise floor change.

Do not use the microphone input. Make sure it is disconnected because it produces a lot of noise. It may be a good idea to set all swithces to off and volume controls to zero for unused devices like CD and MIDI.

If a single board is used you will find that there is some cross talk between loudspeaker output and analog input. Feed a vey weak audio signal to line input and run the program. The spectrum will show the noise floor and the very weak signal. When you press the mouse button and get a tone in the loudspeaker you will perhaps see a new signal in the spectrum. Different boards behave differently and you may use the volume controls to minimise the problem. With a Soundblaster PCI board setting the pcm control low will completely eliminate this problem while setting pcm to 100:100 causes this interference to give peaks 40dB above the noise floor!!!

Using separate sound boards for input and output

It is possible to use two sound boards with the dsp program. This way it is possible to set different speeds for input and output in case the input board will not allow a different speed for output. (Delta44)

The Vibra16 boards do not allow different speed settings for input and output but one can not use two identical boards, not even a combination of Vibra16X and Vibra16C because of the way PNP boards work.

Combining Delta 44 and Vibra16 works fine as well as combining a Soundblaster PCI with a Vibra16. The PCI board allows different speeds in and out and there is not really any reason to use a second board with it but there is no disadvantage either.

For the combination of a Soundblaster PCI and a Vibra16X I use the following ossmix settings:

Selected mixer 0/AudioPCI 97 (CS4297A)
Known controls are:
vol [:] (currently 0:0)
pcm [:] (currently 0:0)
speaker  (currently 21)
line [:] (currently 0:0)
line.rec ON|OFF (currently ON)
mic  (currently 0)
mic.rec ON|OFF (currently OFF)
cd [:] (currently 0:0)
cd.rec ON|OFF (currently OFF)
rec [:] (currently 40:40)
igain [:] (currently 0:0)
line1 [:] (currently 0:0)
line1.rec ON|OFF (currently OFF)
phin [:] (currently 0:0)
phin.rec ON|OFF (currently OFF)
video [:] (currently 0:0)
depth  (currently 0)

Selected mixer 1/Sound Blaster
Known controls are:
vol [:] (currently 100:100)
bass [:] (currently 75:75)
treble [:] (currently 75:75)
synth [:] (currently 75:75)
synth.rec ON|OFF (currently OFF)
pcm [:] (currently 75:75)
speaker  (currently 75)
line [:] (currently 75:75)
line.rec ON|OFF (currently OFF)
mic  (currently 32)
mic.rec ON|OFF (currently ON)
cd [:] (currently 75:75)
cd.rec ON|OFF (currently OFF)
mix  (currently 0)
igain [:] (currently 75:75)
ogain [:] (currently 75:75)
omix.llinesw ON|OFF (currently OFF)
omix.rlinesw ON|OFF (currently OFF)
omix.cdsw ON|OFF (currently OFF)
omix.micsw ON|OFF (currently OFF)
misc.agc ON|OFF (currently OFF)