Use Any Pocket Recorder As An EMG

Read on for instructions on making a simple buffer so you can use any WAV-capable recorder, instead of shelling out for a Zoom.

The whole reason for using the Zoom H4N was because it had a high input impedance (2MOhm) on the 1/4″ jacks. This means that it could pick up even an incredibly low-power signal… the Zoom’s 1/4″ inputs are light and nimble like a sheet of paper, whereas most recorders’ inputs are more like a plank of wood.

The problem is the Zoom recorder is expensive: several hundred Euros (or dollars) at least, and it’s not so easy to find in shops.

On the other hand, 100 Euros will get you a quite functional 44kHz/16bit PCM WAV recorder, just about anywhere.

So how can you make that 44/16 PCM WAV recorder light and nimble like the Zoom?

Make a buffer. A buffer is just an amplifer that doesn’t amplify: you use an amplifier chip (which has very high-impedance inputs), and leave out some parts that tell it to amplify the signal. Since the signal goes through the amplifier chip nevertheless, the low-impedance ‘plank of wood’ recorder isn’t getting pushed around by the fragile EMG signal anymore. It’s getting pushed around by the strong output stage of the amplifier chip.

(The chip doesn’t need to amplify because the recorder is already designed to pick up low-voltage signals. In fact, I had to add some resistors to the output to REDUCE the signal level coming into my recorder! I left those out here for simplicity, but it’s not hard. Look up a “voltage divider” if you want to try it. Just be sure to use fairly low-value resistors — between 100 and 1000 ohms probably — so the amp can still drive the recorder’s input stage whatever its impedance is.)

The amplifier chip I used for this is the LM358, a really cheap chip you can get anywhere. It’s not perfect, so if you zoom in really close on a high-frequency signal you’ll see “ringing” and “overshoot”, but that doesn’t matter too much for our purposes. And ringing and overshoot can even help you analyze a signal that’s too fast for the recorder to capture otherwise — the worse the ringing is, the faster the signal was!

Apart from the chip, you’ll need two 4.7K resistors, two 2M ohm resistors (you can make a 2M ohm resistor by putting two 1M ohm resistors back-to-back), some capacitors, a 9V battery clip, and some audio jacks. You’ll also need some prototyping circuit board — get the kind with the little copper doughnuts around each hole, not the kind with strips.

(Instead of audio jacks, you can get a headphone extension cable — the kind with a jack and a plug — and cut it in half. Solder the wires from the jack half to the board where the input jack would go, and solder the wires from the plug half where the output jack would go. This means you don’t have to mess about plugging and unplugging another cable from the buffer to the recorder.

Even better still is if you put SMA or MCX plugs on the end of the RG-174 cable from the electrodes, and then solder short lengths of RG-174 with SMA or MCX jacks on them to the board.)

You’ll also probably want some kind of metal box or enclosure for the buffer, and you’ll want to connect a wire from the enclosure to ‘ground’ on the circuit. (I’d suggest the ground at the input jack.) This can be anything — I wrapped the board in wax paper, and then covered that in adhesive foil, with a wire running from the foil to ground. The idea is to keep radio interference and things like that out of the circuit: the whole point is that it’s sensitive.

Here’s the schematic as well as one way of arranging the parts on the board and wiring them up.


Strange signal

I’ve been making good progress on tracking down and eliminating unwanted interference like this… we’ll see if my current solutions (mostly shielding) hold up longer term.

Here’s a signal the EMG picked up that I haven’t quite been able to figure out… note how much faster it is than a normal muscle impulse.


Simple High-Performance DIY EMG based on the Zoom H4N

What is an EMG?
Like an EEG and an EKG, it measures the electrical impulses that make the human body work. Whereas an EEG measures the brain and an EKG the heart, an EMG looks at electrical activity over the rest of the body — which pretty much means the muscles.
To get your muscles to contract, the nervous system sends electrical pulses into the muscle fibers, which amplify those pulses and send them on to other fibers until the whole muscle is moving.
Here’s what a normal EMG might look like:
What’s it good for?
Recording muscle activity! In my case, I wasn’t sleeping so well, and I wanted to figure out whether I was tossing and turning in bed as much as I thought I was. An EMG provides the means to figure out just when and how that was happening.
But people use EMGs for all sorts of physical therapy and sports applications.
The heart of the DIY EMG is a Zoom H4N audio recorder hooked up to three electrodes — two measurement electrodes and a common ground. The resulting WAV files (I can get over seven hours of recording on a 16GB SD card, just about enough to cover a whole night) are the EMG output, easy to analyze in any audio editor (I use Audacity).
The Zoom recorder is recording at the highest quality setting, 24bit/96kHz. This gives it a maximum bandwidth of almost 50kHz and, at least in theory, a dynamic range of 145dB. In other words, perfect for recording voltage spikes that might be very quick while ranging from tiny to huge.
The Electrodes
I went through four designs for the measurement electrodes. Most of them weren’t capable of picking up the EMG waveforms completely: the inductance and capacitance were too high. I settled on 5cm x 1cm strips of self-adhesive aluminum foil for the contact material. They’re stuck to the fanned-out copper strands from the center conductor of of RG174 coaxial cable… backed with black duct tape, of course.
Here’s what the build looked like:
(adding foil)
The other end of the RG174 is soldered to the center conductors of some standard 1/4″ mono plugs. The RG174’s shield braid is connected the plugs’ grounds, this helps reduce the amount of interference picked up by this very sensitive setup. (By using the Zoom H4N’s 1/4″ inputs, we get an input impedance of 500,000 ohms. We need a really high input impedance because even on a good day the electrodes don’t form a particularly good connection: on the order of 500k ohms as well.)
Each electrode then goes on the area I want to take readings from. You have to be quite close to the muscle to get readings from it, the signals don’t travel very far. If you’re far away, the spikes get ‘mushy’. Instead of sharp peaks, you see rounded humps. (The closer you are to the muscle, the sharper the peaks.)
For the ground electrode, I’m using a similar but much larger square (5x5cm) connected with a length of standard electronic hookup wire. I place it far away from the measurement electrodes to provide a good reference. The ground electrode gets connected to either of the two 1/4″ connectors’ grounds.
All the electrodes are held in place with standard medical tape. A dab of sugar or glucose syrup (available at your local pharmacy, corn syrup would also work) on the aluminum foil helps make a good electrical connection. And, after a few minutes, it’s sticky enough to help hold the electrodes in place.
The Results
Impressive! The Zoom captures a lot more detail than a normal clinical EMG:
Here’s zooming in a little on the waveforms… You can see the spike in the middle there occupies less than 1/290th of a second. Compared to a normal EMG that samples 50 times per second, that means the Zoom is capturing things you normally wouldn’t see.
Not bad for a few hundred bucks from an audio recorder and less than ten for the electrodes.