How does I2S work?

[joshmarinacci]

I'm trying to add audio support for the T-Deck. I've been able to get this QA example to work but I don't really understand it so I can't extend it.

• Do you have to use DMA with I2S or can you just write some bytes directly?
• How do you control the volume?
  • Does it depend on the hardware to have direct volume control?
  • Do you have to manually change the amplitude of your samples?
• How do you continuously stream bytes in from a file?
  • I can read from a file on an SD card, but what is the preferred way to feed longer byte streams to I2S?
• How would I change the frequency of the example sine wave?
  • It seems like someone must have already made a sound synth crate but I couldn't find much for no_std.

Thank you. I'm documenting all of my lessons to publish new docs.

[Dominaezzz]

At the moment the I2S driver can only be used with DMA. See #2574

Volume can be controlled by adjusting the data. You should be able to just multiply/divide the bytes by some value to adjust the volume. There could also be a hardware volume rocker which can also control/influence the volume but I don't see these often.
So in summary, yes and yes 😄

I'm not too sure how to answer the continuous streaming question. You've seen the example so you know how the streaming I2S APIs work. All I can say is "Do what you want" I guess. If you clarify what kinda of answer you're looking for, I might be able to give a better answer.

The frequency can be adjusted here

esp-hal/qa-test/src/bin/embassy_i2s_sound.rs

Line 75 in edcc63a

Rate::from_hz(44100),

. When I adjusted that I noticed the difference in pitch.

[bjoernQ]

Some devices use audio codecs like ES8310 (popular on Espressif dev-kits) which also allow controlling the volume but looking at the schematics of the T-Deck, it's only using a MAX98357A so the approach to adjust the volume described by Dominic should be the right advise

[joshmarinacci]

I’ve made some progress, but I’m still having some trouble. I have successfully been able to stream bytes out of WAV file from an SD card, and then parse the WAV header, but I’m doing something wrong with the circular DMA. It works fine for the first couple of seconds, and then the audio stops and see errors that say the transfer is late. Does that mean I need smaller chunks? Or that I need to do the file loading work in a different order?

Here’s the code. The DMA transfer is on lines 278 - 305.

https://github.com/joshmarinacci/rust-tdeck-experiments/blob/main/src/bin/streaming_wave.rs

[Dominaezzz]

and then the audio stops and see errors that say the transfer is late

It means you're not supplying data fast enough to the transfer

[joshmarinacci]

But why. Does that mean the SD card read is too slow? Would reading in bigger chunks help, or is the SD card not fast enough to supply uncompressed wav files in real time? There’s not enough ram to buffer the whole thing but I could probably load more into ram at a time.

[Dominaezzz]

Hard to say. See what you can do to increase the bandwidth/throughput.

[joshmarinacci]

I was able to get it to work. Measuring the SD card read speed I was able to get 176kb/sec which is more than enough for a 44.1kb/sec wav stream. The trick was switching to an async circular buffer and only reading in as many bytes as necessary for each DMA. I put some example code here.

The relevant part:

    let mut transaction = i2s_tx.write_dma_circular_async(tx_buffer).unwrap();
    loop {
        transaction.push_with(|dma_buf| {
            // len = length of dma_buffer in bytes
            let i32_len = file_buffer.len();
            // use the shorter of dma and file buffer lengths
            let file_len = (dma_buf.len()/4).min(i32_len);
            // file_buffer is u32 so we use 1/4 of the len
            let slice = &mut file_buffer[0..file_len];
            fill_frames_from_sd(&mut file, slice, &wav);
            for (dest_byte, source_i32) in dma_buf.chunks_exact_mut(4).zip(slice.iter()) {
                dest_byte.copy_from_slice(&source_i32.to_le_bytes())
            }
            file_len*4
        }).await.unwrap();
    }