[aprssig] slightly OT talking about 9k6

Ross Whenmouth ross at topwire.co.nz
Sat May 7 17:45:30 CDT 2016


I am working on the design for a "Pi-hat" audio card for the Raspberry 
pi, specifically for amateur radio use (including APRS and 9k6 packet). 
I am targeting ~ 4Hz to ~ 90 kHz audio input bandwidth (DC - 90 kHz 
output), compatibility with 600 ohm balanced interfaces (and unbalanced 
interfaces too), low QRM, complete galvanic isolation and good common 
mode rejection.

The plan at this stage is to use 2x INA137/SSM2143 line receivers, 2x 
DRV134/SSM2142 line drivers and an AK4556VT 24bit 192kHz stereo codec 
with galvanic isolation between the Raspberry Pi and the codec/line 
transceivers. At this stage I am looking at using something like the 
Si8641/ISO7841 to galvanically isolate the I2S digital audio stream, 
with opto-couplers for the 2x PTT outputs.

The expected transmit audio out response goes all the way down to DC, 
but the receive audio input rolls off below about 4 Hz - according to 
the plot on page 8 of http://sss-mag.com/pdf/gmsk_tut.pdf it looks like 
a 4 Hz rolloff is unlikely to be a problem for K9NG/G3RUH 9k6 
packet/APRS. The RX chain, all the way to the codec ADC is DC coupled, 
however, if the digital DC-block filter in the codec is disabled, the 
codec can only be operated as an I2S slave (I plan to operate the codec 
as a master with its own 24.5760 MHz clock, because when the Raspberry 
pi is operated as an I2S master, its I2S clock output has poor phase 
noise). Yes, I could choose a different codec, but the AK4556VT also 
seems to be very easy to interface with, and hand-solderability (only 
just!) at 192 kHz sampling rate are not such a common combination.

I am considering attaching an STM32F103C6Tx/STM32F103C8Tx to the I2S bus 
and Raspberry pi GPIO on the Raspberry pi side of the galvanic isolation 
barrier, and using this MCU to control the PTT optocouplers. The idea is 
to be able to software configure PTT watchdog timeout (short for APRS, 
long for WISPR, off for a DMR repeater, etc), and for PTT control by 
either I2S triggered VOX, or Raspberry Pi GPIO (eg use RTS for the 
Raspberry Pi's uart AMA0).

Ross Whenmouth

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