Over-the Air (RF) PCM Playback with NRF24L01+ and Arduino

Previous posts regarding the playback of wav files from an SD card were related to another project I am working on involving NRF24L01 radio modules. Since these are capable of around 20KB/s in a real-life scenario, I thought it would be worth a shot to try sending the audio over the air. Basically, I wanted to get feedback from an Arduino operating remotely in the form of sounds, but I don't have room for an SD card locally on the controller, and I also wanted the remote Arduino itself to be able to give feedback in the form of sounds when events are triggered or commands are received, etc.

This is the sketch I have been using to work out some of the details and the bugs. Right now it is geared up to work with two 'controllers' in that if you send the letter 'g' over the serial port to the 'transmitter', it will randomly play a beep on one of the three devices and continue randomly on as fast as it will go. (audio files not included) There is also a data transfer rate test that can be enabled, so one can see exactly how fast data is flowing. It will also play a sound file over the RF link, but thats kind of boring. The sketch is being posted mainly for my own reference, because in its current form I can't do much with it, and its not really useful for anything, but it does demonstrate the capabilities and how to use them.

This RF side of this also uses improved code for PCM playback, which I will likely include in the library previously posted. The timer is run at a minimum of double the actual sample rate, so as to maximize audio quality. Low quality 8khz sample rate sounds sound much, much better this way. This is very similar to the PCM libary, so sounds can be anywhere from 8-20khz sample rate, 8-bit, Mono, but it won't handle 20khz very well, since that is basically the max transfer rate. Speaker is pin 9 on Arduino Uno,Nano, etc, and 11 on Mega.

The system works using ack-payloads and interrupts to coordinate the data transfer and simultaneous playback. What is referred to as the 'transmitter' is actually set to PRX(primary reciever) and sends sound data in the form of ACK(acknowledgement) packets, with 32bytes payload. The 'receivers' just send out commands till one is answered, and then request the data as they need it. The 'transmitter' only sends out data in response to an incoming command.

 This could theoretically work on a larger number of devices, since it only uses a single data pipe at a time with different addresses for each device, but I don't really have any use for that.Change the data pipe to [1] before uploading to a second receiver.

The SD 'Transmitter' is a big sketch, and at the moment is just a bit big for the ATMega328, and I have been testing using a Mega 2560. The receivers are tested with a Nano328. Again, these are just rough sketches, but they work fairly well at this point: 

Controller/Receiver: here

SD Transmitter: here

This is the library for the NRF24L01+ I have been using: nrf24l01  Disclaimer: I did not author this library, it was found here . I just hacked in some extra code for testing which is required for this version of the sketch, and wanted to archive it for future reference.


Asynchronous WAV/PCM: Arduino Audio Library Update:

 This library allows asynchronous playback of WAV files using only an Arduino, SD module, and a speaker.

 A little while ago, I decided to create a library for simple wav file playback using an Arduino, since I couldn't find any that fit my needs

Logical Functionality: 

I posted two previous versions of this libary, one that used a buffer, and one that used interrupts to load the data. Each had its tradeoffs, and neither were perfect. The interrupt based version had noticeable sound quality issues, and the buffering version could not be easily stopped during playback, or the volume adjusted, etc.
 The problem as I understand it, is that a read from the SD card will actually read 512 bytes at a time, so the buffering interrupt would not always complete before the music interrupt was set to trigger next. Since only 1 interrupt will trigger at a time, timing was an issue and so created other issues.

Searching through the datasheet for some functionality that would allow me to do what I wanted, I stumbled across mention of 'nested' interrupts. It took a little bit of time to figure out exactly how to use them in this application, but here is a brief overview of how the timer and interrupts work together:

OVF: This is an interrupt overflow vector that is triggered everytime the timer 'overflows'. (every cycle) Here, it reads a byte from the buffer into OCR1A, and therefore changes the pwm duty every cycle (@16khz)

COMPB: This is an interrupt compare match vector that is triggered when compare match is made during the timing cycle (TCNT1 == ICR1). This interrupt vector is used to read data into the buffers. Can be interrupted by other interrupts via 'nested interrupts'.

a: Interrupt vectors enabled: OVF, COMPB
b: When COMPB is triggered, it disables itself, but leaves OVF enabled. Global interrupts are automatically disabled while an interrupt completes. To enable nested interrupts, global interrupts are enabled manually before reading from the SD card.
c: If ready to buffer data, it begins (OVF can now interrupt COMPB while it bufferrs data)
d: COMPB completes, and re-enables itself to trigger again while it waits to buffer more data

Thanks to nested interrupts, I finally have what I wanted, with the basic functionality one would expect. I think the code can still use a bit of tweaking though, since I haven't fully tested its limits.

Updated Features:
- Sound Quality/Distortion issues have been resolved
- Uses a single timer (timer1)
- Asynchronous (interrupt driven) playback and buffering allows other code to run while music plays 

iTunes Conversion: 
a: Click Edit > Preferences > Import Settings
b: Change the dropdown to WAV Encoder and Setting: Custom > 16.000kHz, 8-bit, Mono

c: Right click any file in iTunes, and select "Create WAV Version"

d: Copy file to SD card using computer

Function Usage:"filename"); //plays a file
tmrpcm.speakerPin = 11; // set to 11 for Mega, 9 for Uno, Nano, etc
tmrpcm.volume(1); //raises or lowers the volume: 1 or -1
tmrpcm.disable(); //disables the timer on output pin and stops the music
tmrpcm.stopPlayback(); //stops the music, but leaves the timer running

Individual Files:

Library Package: (OLD)
(now hosted on GitHub here)

Added Functionality:
Automatic detection of sample rate (8000 - 22000Hz)
WAV format verification
Memory buffer 300 bytes
Phase/Frequency-Correct and Fast PWM modes
Added functions: 
tmrpcm.isPlaying();  //returns 1 if music playing, 0 if not
tmrpcm.pause();  //pauses/unpauses playback
tmrpcm.pwmMode = 1; //set to 1 for phase/frequency correct mode, 0 for fast pwm 
tmrpcm.volume(0); //CHANGED from prev version, now uses either a 1(up) or 0(down)

Tested with: Arduino Nano/328 and Mega2560  (OLD)

(Current version on GitHub here)

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