[Linrad] Duplex

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi All,

For Linrad to work as a transceiver with 96 kHz I/Q as input
and output for the RF signal, a Delta44 soundcard works fine.
Two more audio channels are needed however, one for loudspeaker
output and another for microphone input. (The loudspeaker has 
to be stereo for full receive performance.) 

Good old ISA soundcards like the Soundblaster 16 would fit nicely,
but modern computers do not have any ISA slot. The AC97 or INTEL ICH5 
which is built into my motherboard would have been fine if it
had been possible to disconnect the input from the output.
I have tried both 4Front and ALSA but none of them has a mixer
software (that I can find) that allows me to prevent the loudspeaker
signal that Linrad sends to the card to appear on the microphone
input line. The good old soundblaster had llinesw and rlinesw for this
purpose in ossmix.

The AC97 mixer has a myriad different controls but none of them affects
the presence of the output on the input stream.

What have I missed? I find it hard to believe that the architects
behind a complicated system like the AC97 did not think about this.
Maybe the authors of ossmix/alsamixer just did not provide access
to the appropriate control register?

Is there anyone who knows? As things stand now, I have to use three
different soundcards to make the Linrad transceiver operational.
Utterly stupid! I need the third card in another computer.....

Few of my soundcards are useful for the transceiver mode since only
those for which I can set a small DMA buffer give a decent timing.
I need less than about 50 ms from key-down to loudspeaker output
(10 ms key to antenna and 40 ms from antenna to loudspeaker.)
Having buffers that delay by 0.1 second both at the loudspeaker
output and at the microphone input really hurts. Setting a ridiculously
high sampling speed solves the problem partly, but being able to 
run the AC97 in full (independent) duplex would be far more satisfying
(although it uses a ridiculously high sampling rate internally.)

73

Leif / SM5BSZ


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