Hopefully this is the right sub to ask this question. Anyways, I'm working on a USB Audio device on a microcontroller and I'm having issues with getting a consistent audio input.
As of right now, I have connected the mcu and codec via I2S (codec is slave) with the following configurations:
MCLK = 11.2896 MHz BCLK = 2.8224 MHz LRCLK = 44.1 kHz USB CLK = 12 MHz
When I do a simple DSP bypass (codec input -> mcu -> codec output), the audio is glitch free and awesome. However, when I enable USB, that's when things get weird. I'll get occasional glitches and the signal sometimes repeats itself. I'm not totally sure if this is because I'm not using enough buffers when writing to USB.
Right now my codec reads and USB writes are happening separately:
Codec Flow: Codec Input -> MCU I2S RX -> DMA Interrupt when I2S RX buffer is full -> Write this buffer into USB buffers -> Calculate next input frame len USB: USB initiates input -> Write buffer to USB endpoint -> USB Write Callback called when USB transfer is complete -> Start reading next buffer
How I'm calculating my frame lengths:
#define AUDIO_POLL_INT 4 #define FRAME_BYTES 3 #define NUM_CHANNELS STEREO // Calculate the frame length uint16_t frame_len = 44 (44.1kHz/1000 samples) * NUM_CHANNELS * FRAME_BYTES; // Every 10 ms, calculate frame length with additional frame (only applies to 44.1kHz sample rate) if (!(frame_pos % 9)) frame_len += (1 * NUM_CHANNELS * FRAME_BYTES) // Increment frame position per usb call frame_pos = (((frame_pos + 1) / 8) * (2 << (AUDIO_POLL_INT-1))) % 10; frame_pos = (frame_pos + 1) % 10;
Actual Question/What I think is going wrong (?)
I'm guessing there is some jitter issues due to the codec MCLK and USB CLK rates being different (11.2896 vs 12). I'm only guessing this because the interruptions/glitches happen pretty periodically. I also have noticed that the codec buffers will "catch" up to the USB buffers every once in a while (codec buffer will overwrite a USB buffer even though the USB buffer is marked "full"). However, I'm not sure how to stop this from happening because the codec would go out of sync and I would lose audio data. I also cannot speed up the USB transfers obviously... I'm transferring a max of 270 bytes per USB write for a sample rate of 44.1 kHz.
If anyone has had experience with USB audio and synchronization, your help would be appreciated! I'm not sure how to approach the problem from here honestly. If you're wondering, USB Audio output is fine though (with the exception of some channels not being synchronized).