Feasibility ultrasonic recorder

Other Hardware Audio
1

Hi,
With mems ultrasonic microphones
https://www.invensense.com/products/ics-41350/
Available at $2, it seems feasible in principle to have a very cheap ultrasonic recorder up to 40kHz. TDK have good docs.

Originally I was thinking of creating a small device using the audio jack of an android phone and send the clock signal down the audio channel and take the output from the mic channel. Then say use spectrum analyser app

However I sense Arduino may be a better platform or at least developers more hardware oriented....

  1. Is this feasible ? Is power sufficient. Will clock signal be clean enough ?
  2. Might anyone be interested in getting a prototype made ?

There are multiple applications and bat detectors sell at $1000+.
Also cheap high powered ultrasonic animal repellants can be weaponised and cause hearing damage in humans.
So there is certainly a potential market for ultrasonic detector.

2

Bats can go way above 40kHz.

3

You need a clean (low jitter) clock between 4.1MHz and 4.8MHz to get the device working
in high performance mode, the quality of the signal depends ultimately on this clock.

Some fairly agressive low pass filtering to reject 4MHz and let 40kHz through is needed,
probably RC passive followed by 2-pole opamp active filter using a fast opamp (standard
audio parts may not be good enough with 4MHz energy flying about). Or an LC notch filter on
the clock frequency might be useful as a first stage.

After that standard ADC sampling at 100kHz or more is needed, possible with an external SPI
ADC.

Then you have to handle the raw data rate...

No phone audio will handle anything above 20kHz, they all filter ultrasonic out, and an audio
jack doesn't handle a digital clock at all.

4

Thanks Mark.
Not sure I fully understand.

  1. The mems mic has pdm output, from 4th order sigma Delta. Is all the filtering you've quoted in addition to that ?

2.why will phone reject 20+kHz ?
Eg the fft spectrum analyser app spectroid gives an option to.sample at 96kHz......

3 what platform would you recommend for such a project with say 30kHz limit ?

Many. thanks.
PS in OP I said read from the audio channel, I meant read from the mic channel obviously.

5

IIRC the Teensy audio library can now handle pdm. Get a Teensy 3.6 and have fun.

Pete

6

Cumfy:
Thanks Mark.
Not sure I fully understand.

  1. The mems mic has pdm output, from 4th order sigma Delta. Is all the filtering you've quoted in addition to that ?

The mic output is at ~4MHz, that's needs filtering out as explained before sampling.

2.why will phone reject 20+kHz ?

Because the digital signal processing in the device is designed by someone who knows what they are
doing, nothing above 20kHz is useful to humans - typical maximum sampling rates in a phone
would be 44.1kSPS or 48kSPS.

Eg the fft spectrum analyser app spectroid gives an option to.sample at 96kHz......

Some phones may have that option, but most don't, and no-one's going to care if the bandwidth
is well belong 40kHz - its not a high-end digital mixing desk or a bat-detector, its a phone. If you
can find out which I2S chip is used in a phone you'll be able to determine if the device is physically
capable of 96 or 192kSPS.

If you have a phone with higher sampling rates, it may be all you need anyway...

3 what platform would you recommend for such a project with say 30kHz limit ?

Something with a DSP or FPU to do the heavy lifting, and a suitable ADC of course. A good I2S
ADC with 96 and 192kSPS settings might be the thing, but check what digital filtering is performed
on the ADC chip itself - if it has a 20kHz brick-wall filter built-in, then its not going to be useful to
you.

Many. thanks.
PS in OP I said read from the audio channel, I meant read from the mic channel obviously.

I'd go and checkout whether the Teensy audio library handles ultrasonic signal frequencies,
what I've read says all audio is CD standard (44.1kSPS) and thus not able to handle ultrasonic.

7

The Teensy audio board can handle ultrasonic frequencies. There's a program on the Teensy forum which can be used to receive the DCF77 time signal radio broadcast on 77.1kHz using the audio board's microphone input connected to an antenna (I've modified it to receive WWVB on 60kHz). This is done by changing the boards sampling rate from 44.1kHz to 192kHz.

For bat detector:

for PDM microphone see #5 and #6:

Pete