Can I use CPU2

[phpbg]

Hi,
this looks really promising, thanks!

Can you give more details on how it works?
More especially, you say it is interrupt driven, but is there any way so select on which CPU decoding happen?

I have an UI that is a bit heavy, and I want to use both CPUs of my RP2040: decoding on CPU1 and UI on CPU0 for example...

Regards

[earlephilhower]

It's driven off the DMA done interrupts of the output device. So if you start things on the 2nd core, it will do all processing there.

But, as always, you need to make sure you feed it data from the same core it's running on or you could find some "interesting" behavior because there's no cross-core synchronization in Arduino.

[phpbg]

Thanks for the feedback.
I've never used both cpu on rp2040 before, I'll give it a try.
I'll try to keep synchronization "light": UI will run on core #0 and pass a filename to core #1 that will read it from SD and feed background audio...

To help me understand better, does the decoding process occur within an interrupt routine? or the interrupt is just used to signal another block should be decoded (outside of the interrupt routine)?

One more question: do you plan to add audio codecs? (thinking of flac)

[earlephilhower]

To help me understand better, does the decoding process occur within an interrupt routine? or the interrupt is just used to signal another block should be decoded (outside of the interrupt routine)?

On the Pico we don't have an OS, so the only way to have a "decode task" is to do it w/in an IRQ vector. The current code gets a normal priority IRQ callback when a DMA block has been sent to the PWM or I2S and then fires a low priority IRQ and returns (to avoid blocking things like GPIO IRQs) which does the actual decode. (For WAV, because there is no actual decoding, just memcpy, we do write the next buffer in the normal prio IRQ).

On the ESP32, because they always use FreeRTOS (their WiFi radio stack in run on the host CPU so it needs an OS) we use a Task which is triggered from inside the DMA done IRQ handler.

One more question: do you plan to add audio codecs? (thinking of flac)

FLAC is not really on my radar here (see ESP8266Audio for a app-driven audio pump).

The issue is that the way things work now, we need to have non-blocking decode operations. It's fine to send 0s out to the playback device if there's not enough data, but we can't ever wait for data or the whole architecture falls apart.

For MP3 and AAC everything's in a nice self-contained packet format (which is why you can cut-n-paste pieces of MP3 files together without issue). Great for streaming (which is from where the formats originated).

For WAV I have to implement a multi-stage FSM that can parse byte-by-byte because WAV has a defined and single header block that needs processing before the audio data appears. I could do that because even though there's a lot of stages, it's relatively straightforward.

FLAC, however, is more complicated. It combines the separate header parsing of WAVs with the streaming of data packets of MP3. It's probably possible to implement, but I haven't really dived into any of the FLAC streaming APIs or file format.

[phpbg]

Thanks for all the details!