CODEWITHSUNDEEP
javaandroidaudio
Flynn
Flynn

Reputation: 6181

Generate a single period of a frequency?

I would like to be able to take a frequency (eg. 1000hz, 250hz, 100hz) and play it out through the phone hardware.

I know that Android's AudioTrack will allow me to play a 16-bit PCM if I can calculate an array of bits or shorts. I would like to calculate only a single period so that later I can loop it without any issues, and so I can keep the calculation time down.

How could this be achieved?

Upvotes: 1

Views: 407

Answers (1)

Alnitak
Alnitak

Reputation: 339776

Looping a single period isn't necessarily a good idea - the cycle may not fit nicely into an exact number of samples so you might get an undesirable discontinuity at the end of each cycle, or worse, the audible frequency may end up slightly off.

That said, the math isn't hard:

float sample_rate = 44100;
float samples_per_cycle = sample_rate / frequency;
int samples_to_produce = ....

for (int i = 0; i < samples_to_produce; ++i) {
   sample[i] = Math.floor(32767.0 * Math.sin(2 * Math.PI * i / samples_per_cycle));
}

To see what I meant above about the frequency, take the standard tuning pitch of 440 Hz.

Strictly this needs 100.227 samples, but the code above would produce 100. So if you repeat your 100 samples over and over you'll actually play the sample 441 times per second, so your pitch will be off by 1 Hz.

To avoid the problem you'd really need to calculate several periods of the waveform, although I don't know many is needed to fool the ear into hearing the right pitch.

Ideally it would be as many as are needed such that:

i / samples_per_cycle

is a whole number, so that the last sample (technically the one after the last sample) ends exactly on a cycle boundary. I think if your input frequencies are all whole numbers then producing one second's worth exactly would work.

Upvotes: 2

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