tommy can you hear me?

Okay, so this week’s invention involves me being uncharacteristically-cheap. If you usually read about the gadgets I buy, you should know that I’m seriously into making things and using a variety of tools that I need to get these projects done. I’m happy to buy something if it’s worth the cost.

For these monthly talks I’ve been giving, I wanted to have a hands-free option for the Sennheiser FM transmitter. Because when you’re giving a talk and you’re also typing on the computer it just makes sense that both hands need to be free to do that. And yet, the Sennheiser has a propriety 1/8″ jack that makes it difficult to shop anywhere but their website for accessories.  And their cheapest headset is still hella expensive (~$180) and it’s just a standard headset with a standard microphone with a propriatary plug.  <_<  So I decided to try to build an entire rig instead.

The Gear

Of course, I’m starting things with a Raspberry Pi 3 at the moment but will likely port this over to a Raspberry Pi Zero W when I get things working.

raspberry-pi-3

I just picked up a digital USB microphone from Radio Shack (since they’re closing almost all the stores here in San Diego) so that was a mere $10 and has great quality in a tiny package.

mic

At REI, I snagged an FM radio so that I could do the development and listen in on the transmitted signal.

midland

The Sennheiser at the venue looks like this.  At the last monthly talk I took a photo using my phone so that I could record their tuning for their setup.

sennheiser

You can’t see the hand-held microphone on a cable from this stock photo but it’s kind of a pain, as it is right now.

Progress

So far, things are looking pretty good. I’m able to record from the microphone using the alsa-utils arecord program. I’m able to convert the output WAV file into something suitable for re-transmission. And I’m able to broadcast the signal from a GPIO pin on the Raspberry on a selected FM frequency. I believe I can make a longer antenna that should work out.

What’s missing at the moment is a way to (correctly) daisy-chain each of the commands together so that things will continuously transmit, say, upon startup.

arecord -D plughw:1 -f S16_LE -r 48000 - | ./pifm - - | sudo ./rpitx -m RF -i - -f 87900

Something like that, anyway. Any yet, it doesn’t seem to work like this.  The various, raw “-” hyphens as seen throughout are supposed to represent STDIN/STDOUT for streaming commands from one to the next. Many times this works as expected, albeit with the odd hyphen showing up here and there.

Anyway, things like this take a lot of hacking at the problem to get it solved. Perserverance usually wins a game like this.

Update

And of course the solution was a slight tweak to the earlier attempt.

arecord -D plughw:1 -f S16_LE -r 48000 /dev/stdout 2> /dev/null | ./pifm /dev/stdin /dev/stdout | sudo ./rpitx -m RF -i /dev/stdin -f 87900

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