Arduino Audio Tools: Audio over Serial

So far I have always preferred to send audio using the networking functionality of the micro-controllers. This is usually fast and reliable. Many examples can be found in my communications examples folder using different protocols.

Using Wires

But we can also send audio signals over a physical wire. I prefer to use the Serial protocol because this is well supported across all environments.

Just to proof the case I had a simple demo which was sending some some audio via the Serial TX pin and receives them back on the RX pin in order to play it back

Serial Communication gets unreliable at high bit rates and it can happen that a single byte can get lost. This has the consequence that some nice audio is turning suddenly into noise! A usual sample rate is 44100 samples per second. If we use stereo with 16 bit we will need a baud rate of >= 1587600 which is well above the reliable range!

Using 8 bit Samples

When we use 8 bit samples instead of 16 bits we not only can half the needed baud rate, but we also solve the issue of the lost bytes: If some bytes get lost, this will not be audible. The consequence is only that the channels get switched. In our framework we can use a codec to translate between the 8 and 16 bits. The minimum baud rate for 44100 samples stereo is 793800.

Here is the new example that uses 8 bits.

This is by far my preferred solution: It is technically very simple and efficient!

Using A Codec – E.g. ADPCM

ADPCM is an efficient codec that we can use on micrcontrollers and that provides a compression ratio of 1:4. But here as well, any lost bytes will lead to decoding errors. I noticed in my tests that a baud rate up to 115200 is usually very reliable.

I added some container functionality that can provide next to the audio segments also meta data and other information. The audio information is checked with a checksum and invalid segments are discarded.

Here is the related example that can handle stereo with 44100 samples / second.

Dependencies

• arduino-audio-tools
• adpcm