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Hi Jason,<br>
<br>
No ETA at this stage (this is just a hobby for me...)<br>
<br>
<br>
No, you can use multiple I2S audio devices on a Raspberry Pi. I
believe that the Raspberry pi only has a single I2S audio port, the
short answer is that this limits you to full-duplex stereo.<br>
The long answer is that you might be able to do a dirty hack and
multiplex more than two channels over the I2S interface (eg run the
Raspberry pi's I2S interface in 32bit sample mode and use the high
word of a sample for one 16 bit channel and the low word for
another) - this of course would require special "non-standard"
drivers to make ALSA etc mux two logical sound cards to the one
physical I2S interface, and some custom logic between the I2S
interface and the codec chips, so that the 16 bit samples from each
codec are muxed/demuxed to/from the correct word in each 32bit I2S
sample.<br>
<br>
<br>
Eliminating the VOX function (PTT control only by GPIO) would reduce
the requirements for the microcontroller on my audio adaptor to that
of a simple PTT watchdog timeout (I specified the STM32F301 as it
has a fast I2S port that can be used to snoop on the I2S stream to
determine if a channel is transmitting). At the cost of increasing
complexity (add an I2S clock generator and run the codec in slave
mode), a true DC RX response could be obtained (instead of rolling
off below about 4 Hz).<br>
<br>
If the isolation barrier between the Raspberry Pi and the codec was
removed, and instead an isolated 5V supply was provided to the
Raspberry Pi (eg isolated 13.8V to 5V PSU), then the design of my
board would be greatly simplified, with the downside that connection
of any non-isolated peripherals to the Raspberry Pi (eg a
non-isolated USB-RS232 converter, connected to say a hardware TNC)
would risk creating a ground loop. The Raspberry pi's ethernet
interface would probably be safe to use though, as it is transformer
isolated. This is of course quite similar to the UDRC: <a
href="http://nwdigitalradio.com/wp-content/uploads/2012/04/UDRCDS.pdf"><a class="moz-txt-link-freetext" href="http://nwdigitalradio.com/wp-content/uploads/2012/04/UDRCDS.pdf">http://nwdigitalradio.com/wp-content/uploads/2012/04/UDRCDS.pdf</a></a>
except that as far as I can tell (maybe I am wrong?), the UDRC does
not have balanced audio I/O, an isolated power supply, or good
common mode rejection.<br>
<br>
<br>
Another approach, which I have thought about, but have not expended
any design effort on, is to replace the Raspberry pi with a 32bit
micro with ethernet and I2S interface (eg STM32F4xx or LPC43xx). Run
JACKAUDIO and Ser2Net (RFC2217) servers on the embedded micro to
allow software on Linux/Windows/MacOSX machines (Raspberry Pi,
Odroid, Beaglebone, or full blown PC) to use the audio interface and
control the PTT via ethernet. This is similar to the SignaLink USB,
except with better audio IO and ethernet comms instead of USB. Of
course, if someone was sufficiently inclined, I am sure that it
would be feasible to program the embedded micro to function as a
full-blown 1k2/9k6/56k/etc TNC with an ethernet interface.<br>
<br>
<br>
73 ZL2WRW<br>
Ross Whenmouth<br>
<br>
<blockquote cite="mid:mailman.2.1462726802.16549.aprssig@tapr.org"
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<div class="headerdisplayname" style="display:inline;">From:
</div>
Jason KG4WSV <a class="moz-txt-link-rfc2396E" href="mailto:kg4wsv@gmail.com"><kg4wsv@gmail.com></a></td>
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<div class="headerdisplayname" style="display:inline;">Date:
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09/05/16 02:26</td>
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<div class="headerdisplayname" style="display:inline;">To:
</div>
Brian D Heaton <a class="moz-txt-link-rfc2396E" href="mailto:ky9k-lists@ky9k.org"><ky9k-lists@ky9k.org></a>, TAPR APRS
Mailing List <a class="moz-txt-link-rfc2396E" href="mailto:aprssig@tapr.org"><aprssig@tapr.org></a></td>
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<pre wrap="">ETA?
Is the design such that the units can be stacked for multiple units on a single Pi?
-j
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