[aprssig] slightly OT talking about 9k6

John Langner WB2OSZ wb2osz at comcast.net
Sun May 8 12:26:58 CDT 2016

> What's a good sound card dongle to use with a Pi?

I've had good results with this one:  http://www.amazon.com/gp/product/B001MSS6CS 

Just make sure that you remove pulseaudio when running on a Raspberry Pi  (pulseaudio is fine on x86 Linux).  Detailed instructions here:  https://github.com/wb2osz/direwolf/raw/master/doc/Raspberry-Pi-APRS.pdf  

>  The 9600 is still a real problem with USB soundcard dongles.

My test results do not agree with that assertion.   I've run tests, with the same audio going into a USB audio adapter, and a motherboard "soundcard," at the same time, and found them to be essentially the same for this application.

In my side-by-side test, the soundcard software modem decoded more frames than the modem inside the TM-D710.  See https://github.com/wb2osz/direwolf/raw/master/doc/A-Better-APRS-Packet-Demodulator-Part-2-9600-baud.pdf  for test method and results.

> There are issues with transformers and DC blocking capacitors.

Don't use audio transformers in this case.  9600 baud requires wide flat audio bandwidth out to about 5 kHz.  Alternating data bits, at the maximum rate, would be a 4800 Hz square wave.

Don't waste your time trying to use the microphone and speaker connectors.  The pre-emphasis, de-emphasis, and other filtering distort the signal making it useless.  Most modern mobile VHF/UHF rigs have a "data" connector for use with external modems.  This bypasses the normal audio processing that is good for voice but bad for digital data.   Some surplus commercial radios have suitable connections available inside.

More discussion about radios suitable for 9600 baud operation:  https://groups.yahoo.com/neo/groups/direwolf_packet/conversations/topics/768   

> but I don't think a pi could handle that with 2 1200 tncs, 2 9k6 links running "soundmodems".

The 1200 and 9600 baud software modems each take about 5% of the CPU power available on an RPi model 2.  You could plug in four USB audio adapters, run two 1200 baud radio channels, two 9600 baud radio channels, a cross band repeater, and still have about 3/4 CPU idle time.

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