MCSI breakthrough

Rafael Diniz rafael at
Sun Apr 7 22:26:11 UTC 2019

Thanks for the advice, DS. When I do it, I'll for sure attenuate the
sound card line output before connecting the fcdev3b mic input. But
first I want to do what you did - connect a MIC and a speaker.

Rafael Diniz

On 4/7/19 4:02 PM, Das Signal wrote:
> Hi Rafael, Mychaela,
> I just tested an analog phone call on the FCDEV3B and I'm glad to report
> it worked very well! The test setup is as follows:
> - a standard 8ohm speaker which I soldered to 2.54mm female cables and
> connected to the appropriate header on the board
> - an electret microphone also soldered to 2.54mm cables and connected with
> the correct polarity to the FCDEV3B
> I recompiled the fc-magnetite firmware, flashed it and followed Mychaela's
> instructions in
> Then I tested voice calls with ATD<phone number>; and it worked perfectely,
> audio was crystal clear on both ends. So this configuration is indeed shown
> to be working fine! Although Rafael be careful when connecting a sound card,
> you don't want to overload the microphone input which could cause in the worst
> case permanent damage to the amplifier if the input level is too high.
> --DS
> On Sun, Mar 31, 2019 at 08:53:52PM -0300, Rafael Diniz wrote:
>> Hi Mychaela,
>> Thanks for the support. I prefer first to have the speaker and Mic
>> you'll provide us working fine here. As a later experimentation, I'd
>> like to try to match the impedance of the sound card with the fcdev3b
>> audio I/O and may be add a switch to change between official mic/speaker
>> and AUX audio I/O. Just ideas for now.
>> Cheers,
>> Rafael Diniz
>> On 3/31/19 7:54 PM, Mychaela Falconia wrote:
>>>> Hi Mychaela and Das Signal,
>>>> Is there a way I could install two audio connectors for MIC and Speaker?
>>>> For the use I have in the lab with the FCDEV3B it would be more interesting.
>>> I am afraid that I don't really understand the question.  Are you
>>> asking for a way to convert the speaker and mic hook-ups on the FCDEV3B
>>> to audio connectors like the ones on a laptop, such that you could then
>>> plug off-the-shelf end-user audio stuff into them?
>>> If that is your question, then I don't have an answer to it, as I am
>>> quite illiterate in the realm of audio myself - just producing the
>>> circuits which are currently on the board was an extremely difficult
>>> struggle for me during the design phase.  Someone who understands
>>> analog audio much better than I do would need to review the circuits
>>> which are currently on the board and figure out what can be done with
>>> them, other than what I found to work.
>>> As I already said before, I would be more than happy to send one of my
>>> voice kits (speaker and mic outfitted with wires and connectors that
>>> connect to the FCDEV3B) to each of you (both Rafael and DS), but if
>>> you desire something different, you will have to figure it out
>>> yourselves.  You will need to start with the Leonardo schematics:
>>> TI's Leonardo speaker and mic circuits are depicted on page 3 of the
>>> above PDF; the corresponding circuits on the FCDEV3B are almost
>>> identical, with only a few differences:
>>> * Where the original Leonardo schematic drawing shows physical speaker
>>> and mic parts, on our FCDEV3B we have a two-post header in the place
>>> of each, for the physical speaker or mic to reside off-board;
>>> * The differential input to the TPA6203A1 loudspeaker amplifier comes
>>> only from EARN&EARP on the FCDEV3B (not from AUXON&AUXOP), and the
>>> gain control resistor values are different, tuned to produce 3.6 V of
>>> swing on each leg of TPA6203A1 output in our configuration when the
>>> output from the Iota ABB (input to the TPA6203A1) is at the maximum;
>>> * C333 and C334 footprints are omitted;
>>> * HSP3xx ESD protection features are also omitted.
>>> The above is all I know; from here onward you have to use your own
>>> audio knowledge, which I lack.
>>> Sorry for being of so little help,
>>> Mychaela aka The Mother
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