Audio delay and VOX using ESP32

Source files for both the simple delay and VOX repeater have been updated.

Along with better commented tabbed code, there are some simple fixes and one major change, being the way the leading beep is implemented in the repeater. The original calculated waveform actually caused lost interrupts not being serviced due to floating point delays.  A lookup table for a sine wave had reduced overhead by a factor of 4.

The delay-only code has been updated so that the only thing that needs to be configured is the delay time in seconds.  The program automatically allocates the biggest buffer it can from heap space and increases the sample rate to the maximum for best quality. However, the sample rate is capped at 40k/sec as in practice the quality is really going to be limited by the 256 level output of the Digital/Analog converter.   A 12 second delay still results in quite reasonable sound.

As I wanted to be able to use transmitters that didn't externalise the PTT, and also because I wanted to alert the final receivers that the transmission was a crossband one, I have added a beep that sounds before the delayed voice.  This can be heard by the receivers, and also serves to give the transmitter VOX something to trigger it quickly.

In addition, I added a flag to stop DAC output unless the delay VOX was triggered so that the quiescent input signal to the transmitter VOX would be as quiet as possible.

Code update soon.

10 Dec 18: Added circuit diagram for an antialiasing filter which can also be used as a smoothing filter for the DAC output for sampling rates above 20000/sec.

With a new ESP32S I was able to test the pre and post filters.  The result looks pretty clean (see the project gallery for some scope shots of input and output).

10 Dec 2018:  While trying out an anti-aliasing filter, I accidentally let the magic smoke out of my test rig.  Waiting for a replacement ESP32!

05-Dec-2018:  Posted first iteration.  This version works, but includes no external filtering or antialiasing circuitry to get optimum results.