Tympan at High Speed (Ultrasonic!) Sample Rates

While we designed the Tympan as a platform for trying hearing aid algorithms, it's flexible enough to be used for many different audio tasks.  For example, by increasing the Tympan's sample rate, you can see signals above the range of human hearing...to explore ultrasound!  The question is, how high into the ultrasonic range can the Tympan go?

Sample Rate and Nyquist.  The Tympan is a digital audio device; it samples the voltage of a signal at discrete moments in time.  It acquires audio samples at a constant rate, the "sample rate".  If you wish to sense a certain frequency of audio (say 10 kHz), you need a sample rate that is fast enough to capture this frequency.  Thanks to Nyquist, we generally say that the sample rate needs to be at least twice the frequency of the signal that you want to sense.  So, to sense 10 kHz, our sample rate must be *at least* 20 kHz.  Typically, digital audio systems run at 44.1 kHz or 48 kHz so that they can comfortably span the 20 kHz maximum range of human hearing.

//set the sample rate and block size
const float sample_rate_Hz = 48000.0; //for audible sound
const int audio_block_samples = 128; 

Ultrasound.  For sensing ultrasound, we need to sense frequencies higher than 20 kHz.  Many inexpensive ultrasonic range-finders, for example, operate near 40 kHz.  If we want to explore these signals, we need to increase our sample rate to 80+ kHz.  Some rats and bats make vocalizations that extend up to 80 kHz, so that would require a sample rate of 160+ kHz.  Can the Tympan sample this fast?

Changing the Sample Rate.  Changing the sample rate of the Tympan is easy.  Near the top of every Tympan example, you can see where to change the sample rate.  This example is even called "ChangeSampleRate".  So, that part is easy; simply write in a sample rate that is higher!  The question is whether the Tympan produces useful data when running at these higher speeds.  Let's test it!

//set the sample rate and block size
const float sample_rate_Hz = 96000.0; //for ultrasound
const int audio_block_samples = 128; 

Test Setup.  As shown in the photo at the top of this post, I used a function generator to make a sine wave.  I then ran its signal through an attenuator to make sure that I wasn't overdriving the input of the Tympan.  I used a Tympan RevE and inserted the signal via its pink input jack.  

Tympan Software.  On the Tympan, I used one of the example sketches that records audio to the SD card.  In the code, I made two changes: (1) I told it to record from the pink jack as line-in and (2) I changed the sample rate to whatever I was testing.

Test Method.  For each test, I started the Tympan's SD recording and then I manually turned the knob on the function generator to sweep up through the frequency range.  I then stopped recording, pulled out the SD card, and made a spectrogram of the recording on my PC.  I used Matlab, but you could also use Python or Audacity for your spectrograms.

Results, Clean Audible Signal (fs = 48 kHz).  I started with a known-good traditional audio sample rate.  The figure below shows my frequency sweep when using a sample rate of 48 kHz.  The spectrogram shows that we could see frequencies up to 24 kHz, as expected based on Nyquist.  The spectrogram looks great; the signal is clean and the background noise looks like background noise.  This is what "clean" and "good" look like.

Results, Clean Ultrasound Signal (fs = 96 kHz).  I then turned up the sample rate to 96 kHz and repeated the measurement.  The spectrogram below is the result.  It looks great.  We see our signal up to 48 kHz, as expected.  There's a little bit of aliasing as the input signal continued past 48 kHz (we see that the signal's line in the spectrogram bounces downward a little bit when it hits 48 kHz).  The aliasing stops quickly, so this seems fine.  I think that this spectrogram looks great.

Results, Marginal Quality (fs > 96 kHz).  When I increase the sample rate beyond 96 kHz, the results start to look less good.  Below are the results for 100 kHz, 105 kHz, and 110 kHz.  As you can see, the signal itself looks OK, but strange artifacts start to appear in the background noise.

Results, Bad Quality (fs > 110 kHz).  Finally, by the time we get to a sample rate of 115 kHz, the recorded audio is bad.  Bascially, any sample rate above at 115 kHz and above is unusable.

Conclusion.  The Tympan is good for recording at sample rates up to 96 kHz.  You can maybe even run up to 110 kHz.  But, at 115 kHz and above, your signal will be corrupted.

Improving High-Frequency Performance.  The audio codec used to do the sampling is a very complicated device.  There are many settings and many ways of clocking the device.  It is possible that there is a different combination of settings that would provide good-looking data at sample rates higher than 96 kHz.

96kHz is Still Good!  Luckily, 96 kHz is still a very useful sample rate for ultrasound.  Running at 96 kHz is fast enough to give good access to signals around 40 kHz.  This is a very important region for ultrasound in air.  There are many ultrasonic range finders and motion sensors that operate in the 40 kHz range.  So, you can explore and do many fun things running your Tympan at 96 kHz.  Furthermore, we also know that the Tympan's on-board microphones are sensitive up into this region, so you don't even need any additional hardware to sense the ultrasound!  You can just change the system's sample rate and then go have fun!