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I'm trying to understand how to implement audio playback from scratch on attiny85. The goal is to play a short sound (cat meows, so i want it to remain recognizable) from an array representing strength of audio signal sampled at fixed interval.

As far as i understand, signal strength is linearly mapped to voltage of analogue audio signal. As far as I know, audio cards are Digital to Analogue Converters, but attiny85 probably doesn't have that.

I'm curious if I can use pwm to play the sound back. Since pwm changes average voltage by changing duty cycle of alternating high and low phases of signal, it most likely would result in the drop of audio quality. Wav sampling rates can differ between 1 HZ and 4.3 GHz according to google. Attiny85 has internal clock with frequency up to 8MHz (which I hope is same for it's pwm generator).

Considering reconfiguring the timer and pwm settings as well as looping in the array, what is the maximum sampling rate of audio i can reliably play? And should i even try to do it with pwm, or there are better options?

Nikolai Savulkin
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2 Answers2

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A simpel sinus can be generated by using a 50% PWM signal and varying the frequency. Given some filtering effect through the speaker, it would mimik a single tone audio signal.

Making more advanced tones (needed for natural sound) quickly gets more complicated and the duty cycle of the signal can also be used to trick the human ear into hearing harmonics. Check out the arduino function tone() for some inspiration.

Be carefull when connecting a small speaker to the Arduino, preferably a transistor/buffer/small amplifyer should be place between the Arduino and the speaker.

Jesper J
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Given a system clock of 8 MHz, you can use PWM to generate mono (single-channel) audio.

Consider a PWM period of 1000 clocks, giving you about 10 bit resolution. The sample rate will be 8000 Hz then, which gives you some kind of lo-fi audio.

If you reduce your signal resolution to 8 bits, you'll get 8 MHz / 28 = 31.25 kHz sample rate. This gets near hi-fi.

Synchronize your sample output with the PWM generator, and use an appropriate analogue filter.

Many years ago I built a digital door bell with a sample rate of 8 kHz and 8 bit samples. It played nice sounds in the quality of telephones. The microcontroller was a 8051 derivative and it used an R-2R ladder as DAC.

the busybee
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