PDMIn on RP2040 sample rate factor-of-two error?

[mediumcrab]

CircuitPython version

Adafruit CircuitPython 9.2.1 on 2024-11-20; Waveshare RP2040-Plus (16MB) with rp2040
Board ID:waveshare_rp2040_plus_16mb

Code/REPL

import array
import time
import audiobusio
import board
import math

# Constants
SAMPLE_RATE = 22050  # Sample rate in Hz
DURATION = 0.4      # Recording duration in seconds
BUFFER_SIZE = int(SAMPLE_RATE * DURATION)  

# Prepare a buffer to record into
buffer = array.array("H", [0] * BUFFER_SIZE)

with audiobusio.PDMIn(
    board.GP2, 
    board.GP3, 
    sample_rate=SAMPLE_RATE, 
    mono=True,
    oversample=64,
    bit_depth=16,
    startup_delay=0.2
) as mic:
    # discard short period of data
    dump_length = int(SAMPLE_RATE*0.01)
    tmp_buffer = array.array("H", [0] * dump_length)
    while(dump_length > 0):
        samples_read = mic.record(tmp_buffer, len(tmp_buffer))
        dump_length -= samples_read
    print("Recording...")
    offset = 0  
    while offset < len(buffer):
        chunk_size = min(512, len(buffer) - offset)  
        samples_used = mic.record(memoryview(buffer)[offset:offset+chunk_size], chunk_size)  
        offset += samples_used  
    print("Recording complete.")
    with open(f"/audio_data{mic.sample_rate}.bin", "wb") as file:
        file.write(buffer)
        file.flush()
    print("Data written to file")

Behavior

When using the above code, I find that the audio data is half the expected sample rate when I inspect the data. I.e. recording a 500Hz sine tone using this code, I would expect a 22050Hz sample rate, but on inspection I see a 11025Hz sample rate because the plotted waveform from my saved data has a 1kHz frequency.

Attaching a logic analyzer to the Clock and Data pins, I see that the clock frequency is measured at 705kHz. With the args passed to the ctor to the PDMIn object, I would expect 64*22050 = 1411200 i.e. 1.41120MHz clock frequency.. I think?

Happy to dive deeper and see if I can find out what is going on and submit a PR, but just want to know first if I've misunderstood how it is meant to work! Thanks!

Description

The hardware is a Waveshare Pico Plus board, and the PDM mic is the Adafruit PDM MEMS Microphone

Additional information

No response

[dhalbert]

Do you have any other non-RP2040 CircuitPython boards to try, to see if they produce the same result or show the correct frequency?

[mediumcrab]

I think I have a Teensy 4.1 in the drawer - I'll see what it does and report back 👍