Another video interruption - serial audio with a PIC 16F1825

While writing up how SD cards work, and reading files using FAT16, we've also been developing our little audio board a bit further. We got as far as playing audio files from our SD card and fitting them to the alarm clocks for a tester project the other day.

We've now taken the project one stage further, and introduced serial/uart support.
So rather than play a file immediately, you can send serial commands to the audio board.

• # - send back a list of all files found in the root directory
• | - pause playback
• > - resume playback 
• \ - half current volume (max four times)
• / - double current volume (to max)

At any time you can send in a filename over serial.
If the filename is in the format xxxxx.yyy (where xxx is any string up to 8 characters and .yyy is a full stop followed by a three-character extension) the chip will search the root directory for a file with the same filename and extension.

If a file is found, it begins playing immediately.
If the chip is in the middle of playing a wav, the current wav file is stopped, the current sector is closed, and the chip plays the new sound from the start.

Note - our video is blurry (again) thanks to wonderful camera-phone technology! And trying to hold the speaker in one hand, the phone in the other, and typing at the same time ;-)

The PWM carrier signal generates a hum which is much more noticeable in the video than in real life, but in a future revision, we'll look to remove this using a low-pass filter or capacitor across the speaker terminals.

At present we're playing raw (headerless) PCM wav files.

The frequency is hard-coded to 22khz and all sounds are played in mono at 8-bit. This requires the sounds to be prepared before copying them to the SD card. A future revision will be able to play different types of wav files, but for now, we've a single-chip, low-cost, low component-count audio player board which works with all SD cards we've tried it with!