|An inexpensive DIY ultrasonic audio player|
This project is about a do-it-yourself portable ultrasonic audio player, built out of inexpensive modules available on sites like AliExpress.
A typical use case for this is a kind of "lure" for biology researchers to improve the results of trying to catch bats in a net. The player emulates (social) bat calls which attracts the bats to the net, increasing the chance for them to be caught.
The ultrasonic player acts as a USB sound card with high sampling rate plus a speaker to turn it into actual ultrasonic audio. You have to provide a device with a media player yourself, like a tablet or a laptop.
2017-04-16: created a video of current progress. 2016-10-11: received the amplifier boards. 2016-10-02: ordered the parts (USB audio, amplifier, a few step-up circuits).
measure the frequency range of the USB audio card, using artificial waveform(s), like sweeps and fixed-frequency tones. I can create these waveform in Audacity (which supports the 384 kHz sample rate).
measure the frequency range of the amplifier circuit
connect the 5V-12V step-up circuit and use it to power the amplifier
measure the current consumption of the amplifier-> about 200 mA
connect the speaker and verify sound production using a bat detector-> works, but audio is not really loud yet. Making it louder introduces a lot of distortion.
- perhaps try to reduce the input signal to the amplifier and let the amplifier just run at full power.
- check whether devices can actually play ultrasonic audio over this sound card: my laptop, perhaps some cheapish android tablets (using an OTG cable). Bus load is 1536 kB/s (stereo 16-bit @ 384 kHz). Can a Raspberry Pi do it too? -> works fine on laptop
- figure out a better way to tune the amplifier (it requires very delicate tuning of the pot meter now)
- reverse engineer the schematic
- work on automatically mounting USB stick and playing the file on it
The hardware consists of the following parts:
- Some kind of media player which takes care of the storage and playback of the ultrasonic files, this can be a tablet or laptop
- A USB audio card to create the analog ultrasonic signal
- An amplifier to amplify the ultrasonic signal
- A speaker to turn the signal into actual ultrasonic audio
This is currently the greatest unknown where requirements are most vague.
This could be some kind of Linux mini computer (e.g. a Raspberry Pi / Orange Pi Zero / etc) working as follows:
- the user puts in a USB stick (formatted as FAT32 for windows compatibility) with some wav files, and possibly a playlist file (.m3u)
- the player mounts the USB stick as a read-only device, e.g. using udev
- the player first looks for a playlist file and attempts to play files from the playlist, on repeat
- if no playlist file is found, it just plays all files in undefined order, on repeat
- when done playing, the user pulls out the USB stick, the playback process is stopped automatically
- Starting playback upon USB stick plugin, using udev?
- Stopping playback upon USB stick plugout, using udev?
- Linux experts saying: "You can't pull the stick without unmounting! That would be bad!"
USB sound card
I'm considering this one: USB sound card based on a SA9227+PCM5102 chip.
It allows a maximum sample rate of up to 384 kHz, or equivalently audio op to about 170 kHz. Price: about E30,-
To play back the audio file noise.wav:
aplay -D plughw:CARD=Audio,DEV=0 noise.wav -v
On the right, rom top to bottom: the signal at 50 kHz, 100 kHz, 150 kHz respectively
Measurements of amplitude vs frequency:
- on the scope you can clearly see the sampling steps: at 100 kHz one wave is sampled using only 3.84 samples.
- the amplitude drops a bit when going higher, but only like 30% at 150 kHz compared to 50 kHz
I'm considering this one: amplifier board based on a TDA2030 chip.
The TDA2030 chip has a claimed audio bandwidth of up to 140 kHz. Price: about E1,-
Measurements with a separate power supply for the amplifier:
- seems to be able to handle 50 kHz and 150 kHz audio input equally
- additionally, it seems the amplifier still works down to 5 or 6V power supply voltage.
I'm considering this one: Vifa/Tymphany XT25SC90-04.
This speaker is also used in other products that produce ultrasonic audio.
Price: about E22,-
For the power source, there are two options:
- draw power from the USB port from the tablet/laptop
- use our own power from a separate battery
In the first case, I'll have to consider the maximum current that an average USB port can supply, which is 500 mA typically. In the second case, I'm thinking of just using a 5V USB battery. There are plenty of models to choose from, in varying capacity ranges and prices.
If we're powering everything from 5V, we may need some kind of step-up converter to supply the amplifier with 12V. I'm considering this one: 5V-to-12V step-up cable or possibly this USB 5V to 12V converter. An important consideration for the step-up converter is that the switching frequency is considerably higher than any ultrasonics frequencies we are interested in.
Measurement results for AL519, running into a 1 kOhm load:
- switching period is about 134 us
- peak-peak ripple of about 200 mV
Measurement for board with AL697-chip (I've got three of these):
Current measurement (total current over USB)
- normally 0.36A (USB audio + step-up + amplifier)
- when playing: 0.39A