Arduino Audio: WAV Playback from SD Card  
*TMRpcm Library update*

Due to popular demand, the PCM audio library now supports 32khz sample rate audio for improved sound quality, and better performance at lower rates. Some of the notable improvements in this round of updates are documented below. See the Wiki on GitHub for usage information.

Multi Track Playback
Dual tracks can be played at once on separate pins. Limited to about 16-20kHz sample rate, see wiki page for updated usage and info.

SdFat Library Support
The library now optionally supports direct use of the SdFat library for lower program and memory usage and increased performance.

Metadata (ID3v2.3 and Info/List) Tag Support
The library now has a configurable option ( pcmConfig.h ) to handle and bypass metadata when reading audio files. It also provides limited support for retrieving song, artist and album information from audio files. Standard info (LIST) tags are supported as well as ID3 tags.

Easy WAV file generation and playback
One command to generate a standard WAV file and header, another to finalize it once data is written. Raw data from analog inputs or other sensors or information sources can be written to the file to generate digital audio that can be played on any device that supports WAV files, or easily converted to other standard formats.

Recording
Commands have been added to enable recording of WAV files to SD card. Performance is very much dependant on the write speed of SD card as well as the amount of SRAM used. Results seem to be ok with a class4 SD card.

Audio Formats and Options
The library is now capable of Stereo or pseudo 16-bit playback with a resistor ladder to a limited degree.
An option to operate using TIMER2 for compatility was added.
A function to start playback at a given number of seconds was added

Buffered SD Reads
After a bit of messing around with the standard SD library, I realized there was a buffered read that would improve the response quite a bit. It doesn't seem to be documented in the library reference, but the usage is as follows:

Normal Read:
byte buffer[32];
for(int i=0; i<32; i++){
   buffer[i] = File.read();
}

Buffered Read:
byte buffer[32];
int bytesRead = File.read((byte*)buffer,32);

*Update*PWM 'Pop' sounds on timer start/stop
*Edit* - There seems to be differences in how the timers are engaged on different boards. The library now autoselects between the ramping methods. See pcmConfig.h to manually select.
I suspected that the typical pwm 'popping' sound could be removed by creating an appropriate ramp, and was able to remove it via the following process on the Arduino Mega.

On Startup:
1. OCR1A and OCR1B are set for opposing PWM. If both are set at 0, one will be high and one will be low.
2. Before enabling the timer, OCR1A is set to 0, OCR1B set to maximum (Timer TOP value) so that there is no power flow on startup.
3. OCR1B is then ramped down to 0, and power is applied gradually
4. OCR1A and OCR1B can now be fed with the same value, without a popping noise

Between Tracks:
1. The OCR1A register always contains the value of the last sample from the previous track, unless disabled.
2. Read OCR1A, and read the first value of the next track.
3. Ramp the value of OCR1A from its current value, to the value of the next track 

On Disable:
1. Get the current value of OCR1A/OCR1B
2. Ramp OCR1B up to maximum (Timer TOP value) from the current value
3. Ramp OCR1A down to minimum (0) from the current value

On a Nano or Duemilanove board, the process is a bit different:

On Startup:
No ramp needed, handled by track transition ramp. There will be a pop the first time PWM is enabled, and no popping thereafter if disabled or enabled.

Between Tracks: 
Same as with Mega 

On Disable:
1. Get the current value of OCR1A/OCR1B
2. Ramp both OCR1A/OCR1B down to 0 from the current value

I applied a few more tweaks to bring the performance up a bit more, mainly converting 16bit variables to 8bit where possible, and improving on details of the playback process. The quality/oversampling option now defaults to 0. The update is available on GitHub:
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