PWM and music playback

[anuragmash]

I have the .wav samples (unsigned 8-bit PCM) of a music file that I want to play using the AVR microcontroller. I have enough flash to store these samples and would like to know if it would be easier to play it directly from the flash or should I store them on the RAM and then play the samples using PWM.

Also I would like to know how PWM works in such cases i.e. in playback of an audio signal. These samples are all 8-bit unsigned samples and the sampling frequency is 16kHz. How do these samples decide the duty cycle of the waveform i.e the pulse train of the PWM.

 

[kripacharya]

Your duty cycle register can be directly fed with the PCM bytes - assuming you are using an 8-bit pwm channel. Should be easy enough from flash itself - no need for RAM.

However the PWM frequency needs to be much higher than that of the audio content, which i suppose is < 8KHz. A 10x factor is OK. Higher is better. However with the standard 8-bit AVR's the fast PWM can only do around 78KHz @20Mhz. If you are using an AVR with internal PLL then you can go higher.

The trick is to change to the next PCM byte/ PWM duty cycle after exactly 62.5uS or at the same rate as the original sampling of 16KHz. This can be easily done by using a 2nd timer.

Your output needs to be LPF'd, and usually a simple RC is enough. Since the audio is < 8Khz, and PWM is ~78Khz this should be enough.
This output will be centered at approx Vcc/2 ~ 2.5v typically.

Which AVR are you planning on using ? Note that each 1 second of your wav file will occupy 16Kbytes, and your code needs to be there too !!

 

[anuragmash]

thanks for your valuable response, good sir!!!