LINUX DRIVER FOR SOUNDCARDS AS AX.25 MODEMS

       Thomas M. Sailer, HB9JNX/AE4WA, <sailer@ife.ee.ethz.ch>

This driver allows either SoundBlaster (sbc) or WindowsSoundSystem (wss)
compatible soundcards to be used as either 1200 baud AFSK or 9600 baud FSK
AX.25 packet radio modems. Only half duplex operation is supported by
the SoundBlaster driver. Fullduplex is supported for WSS chipsets capable
of addressing two DMA channels. The driver needs some processing power!
A 486DX/2 66MHz is a minimum requirement, otherwise interactive performance
of the computer may become sluggish.


The Interface of the driver

The driver provides a kernel network drivers named sm[0-3]. sethdlc 
from the ax25 utilities may be used to set driver states etc. Users
of userland AX.25 stacks may use the net2kiss utility (also available
in the ax25 utilities package) to converts packets of a network interface
to a KISS stream on a pseudo tty. There's also a patch available from
me for WAMPES which allows attaching a kernel network interface directly.


Configuring the driver

Some sound cards need to be initialized before they operate in either
SoundBlaster or WSS compatibility mode. The driver does _NOT_ do this;
you may use the standard linux sound driver to initialize the soundcard;
compile it as a module, and do
  insmod sound
  rmmod sound
The soundcard should then be initialized correctly. isapnptools is another
package that can configure PnP soundcards. If your soundcard contains
a Crystal CS423x chip, you may also try the setcrystal utility in the
ax25-utilities package.

Every time the driver is inserted into the kernel, it has to know which
modems it should access at which ports. This can be done with the sethdlc
utility. If you are only using one modem, you can also configure the
driver from the insmod command line (or by means of an option line in
/etc/conf.modules).

Examples: (use either method, not both)
  insmod soundmodem mode="sbc:afsk1200" iobase=0x220 irq=5 dma=1
  sethdlc -i sm0 -p hw sbc type afsk1200 io 0x220 irq 5 dma 1

Both lines configure the first port to drive a soundblaster card
in 1200 baud AFSK mode.

The channel access parameters can be set with sethdlc -a or kissparms.
Note that both utilities interpret the values slightly different.


Input and output levels

It is important that the input and output levels are adjusted properly.
There are two utilities, available in the ax25 utilities distribution,
to facilitate this: smmixer and smdiag. smdiag allows you to display
the input signal in an oscilloscope like display or an eye diagram.
smmixer allows you to adjust input/output levels. See the respective
man pages. These two utilities have two counterparts: xfsmmixer and
xfsmdiag. They both require the FORMS to compile, and provide a nicer
user interface. Functionality is the same, though.


Transmitter keying

Since soundcards do not have a DC coupled output; PTT keying options include
the following:
* VOX circuitry
* Serial port pin
* Parallel port pin
* MPU401 MIDI output via a retriggerable monoflop.
Circuit schematics may be found at
http://www.ife.ee.ethz.ch/~sailer/pcf/ptt_circ/ptt.html.


Compatibility with the rest of the Linux kernel

The sound driver and the soundcard modem driver compete for the same
hardware resources. Of course only one driver can access a given
interface at a time. Worse yet, the sound driver grabs the soundcard
at startup time. Therefore the soundcard modem driver subsequently won't
be able to access the soundcard. You might therefore find it necessary to
unload the sound driver before using the soundcard modem driver.


Further reading

Please take a look at http://www.ife.ee.ethz.ch/~sailer/ham/linux/hdlc.html
for further informations on the driver.


vy 73s de
Tom Sailer, sailer@ife.ee.ethz.ch
Packet Radio: hb9jnx@hb9w.che.eu