|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.
Another use case is to use it as an educational tool, to train people in the use of a bat detector. The player makes it easy to play the exact same sound again and again, allowing people to experiment with finding the best setting for their equipment and to improve their determination skills.
The ultrasonic player consists of a USB sound card with high sampling rate plus a speaker to turn it into actual ultrasonic audio. A built-in Linux single-board-computer (something like a Raspberry Pi) controls playback of audio files. The idea is that you prepare one or more USB sticks with bat calls and just plug one in the player, then the player automatically plays them in a loop
2017-06-24: successfully played audio back from a raspberry pi 3, running the amplifier from one of the USB ports. The rpi3 shows a little lightning bolt indicating undervoltage while playing. 2017-06-18: realized that an orange pi zero is not going to work: it doesn't have enough USB ports 2017-06-16: playing a bit with udev scripts to automate actions upon USB stick plugin: I can do short actions (like mount/unmount) but no longer running actions from udev! 2017-05-01: replaced a feedback resistor on the amplifier, to reduce the gain from about 30 to 3, this also makes it easier to adjust the gain using the potmeter 2017-04-30: found the schematic for the amplifier, plan to modify it for lower gain (better level control and higher bandwidth) 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).
- work on automatically mounting USB stick and playing the file on it
- simple alternative: just put in a USB stick and reboot
- simplest alternative: just put the audio files on the sd card and forget about the USB stick for now
- design a simple lasercut case, possibly just a designed basic on a simple makercase box
- customised with extra holes for USB battery, USB stick speaker
- drill holes for tie-wraps to hold everything together on the inside of the box
The hardware consists of the following parts:
- Some kind of media player which takes care of the storage and playback of the ultrasonic files, for example a single-board computer like a Raspberry Pi
- 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
As a media player, I'm using a raspberry pi, in particular a raspberry pi 3.
The player hardware needs at least two high-speed USB 2.0 ports, one for the USB stick containing the wav files and another one for the USB audio card. Bandwidth needed over USB is equivalent to 2-channel 16-bit audio at 384 kHz, which is 12.28 mbps, exceeding USB 1.0 full-speed throughput of 12 mbps.
The main use case of the entire player is as follows:
- user switches on the ultrasonic player
- the user plugs in a USB stick (formatted as FAT32 for windows compatibility) with some wav files
- the player automatically mounts the USB stick as a read-only device, e.g. using udev. Mounting it read-only should prevent corruption of file data on the USB stick.
- the player automatically plays all files from the USB stick, on repeat.
- when done playing, the user just pulls out the USB stick, the playback process is stopped automatically.
- user switches the ultrasonic player off
- 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,- 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
- removed the yellow audio connector, soldered on a simple 3-pin 2.54 mm pin header.
I'm considering this TDA2030-based module: 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.
Modification: replace R5 (150k) with a 15k resistor. This reduces the gain of the amplifier from about 30 to 3 times, making adjustment of input level easier and also improves bandwidth. We don't need a gain of 30, the input signal is already at about 1V level.
I'm considering this one: Vifa/Tymphany XT25SC90-04.
This speaker is also used in other products that produce ultrasonic audio.
Price: about E22,-
I'm thinking of using a 5V USB battery. There are plenty of models to choose from, in varying capacity ranges and prices.
To supply the amplifier with 12V, I'm considering this voltage converter: 5V-to-12V step-up cable or possibly this USB 5V to 12V converter. A 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 the "AL519" converter, running into a 1 kOhm load:
- switching period is about 134 us (7.5 kHz)
- peak-peak ripple of about 200 mV
Measurement for board with "AL697"-chip into a 1 kOhm load
- switching frequency is about 15 kHz
- peak-peak ripple of about 200 mV
Current measurement (total current over USB)
- normally 0.36A (USB audio + step-up + amplifier)
- when playing: 0.39A
- current consumed by USB audio: 0.18A idle, 0.19 when playing
Mounting it in a case
The various parts have the following dimensions (length x width x height) approximately:
- USB battery: 111x68x?? mm (without plugs)
- USB audio: 66x51 mm (PCB only), 77x51 mm (including connectors, without plugs)
- speaker: 66 mm (outside diameter), 53 mm (mounting hole diameter). Hole coordinates (mm): (0, 53) / (45.9, -26.5) / (-45.9, -26.5), big mounting hole: 47 mm diameter
- raspberry pi 3
- amplifier: 32x25x24mm
- step-up converter: 42x15x12 mm (without plugs)
- Put it all in a practical case. For the first prototype, I'm thinking about just laser-cutting a basic enclosure, then use zip-ties to tie stuff to the inside of the box. Make part of the USB battery stick out, so we can use it as an on/off switch and allow access to the charge port.
Perhaps I can use a raspberry pi to solve the USB port problem, see this table on wikipedia for comparison. For example the model B, generation 1+ has four USB port. Also it has modest power requirements.
As an operating system, I prefer Debian Linux, because I'm familiar with it on the desktop. This distribution allows a small basic minimal image without a graphical environment and systemd support.
Software will be added to my batplayer github archive, things like:
- udev/systemd plugin script: mounts the USB stick (read-only), invokes the playback script
- udev/systemd plugout script: stops the playback script, forces unmount of USB stick
- playback script, probably written in python, gets the mounted path as argument, scans the path for wav files and tries to play each file using aplay.
- udev scripts, to automatically mount and unmount USB sticks
To play back an audio file, I just use aplay (from package alsa-utils), for example:
aplay -D plughw:CARD=Audio,DEV=0 noise.wav -v
Suitable audio files for playback:
- the 'synthesized' versions from batcalls.com by AviSoft, these are mostly mono 16-bit PCM, sampled at 250000 Hz