MP3Decoder: Switching tracks can play part of the previous track

[jepler]

CircuitPython version

Adafruit CircuitPython 9.2.0-beta.0-3-g3a189e75d9-dirty on 2024-09-23; Raspberry Pi Pico 2 with rp2350a

Code/REPL

# https://github.com/adafruit/Adafruit_Learning_System_Guides/blob/main/circuitpython-audio-fx/polyphonic/code.py

Behavior

Install the project and hook up a button to GP4. When pressed and held, this says "track 4 hold loop".

Repeatedly press and release the button. Press for about 1 second at a time, trying to release when the voice is speaking.

Description

At the beginning of a subsequent play, you'll hear a snippet of the file playing at an earlier position. Then, it correctly plays from the beginning.

Additional information

It's not clear why this is happening, but it could relate to the fact that most of the MP3 player state is not cleared when a file is assigned; instead just a minimum amount of work is done.

[jepler]

You can also hear this if you use GP4 to trigger "track 4 hold loop", release mid-sample, and then press GP3 to trigger "track 3". However, you don't hear anything when going from GP3 to GP4, i.e., it's either only affecting looping samples OR samples that are stopped mid-playback. This also shows it's likely not a special case error of playing the same sample twice in a row.

[jepler]

Here's a standalone test to run on the unix port. Each time the first buffer's RMS should be zero. but instead in the reopen case it's nonzero:

import audiomp3, audiocore
import ulab.numpy as np

TEST_FILE ="tests/circuitpython-manual/audiocore/jeplayer-splash-44100-stereo.mp3"
def normalized_rms_ulab(values):
    values = np.frombuffer(values, dtype=np.int16)
    # this function works with ndarrays only
    minbuf = np.mean(values)
    values = values - minbuf
    samples_sum = np.sum(values * values)
    return (samples_sum / len(values))**.5

def print_frame_loudness(decoder, n):
    for i in range(n):
        result, buf = audiocore.get_buffer(decoder)
        print(f"{i} {result} {normalized_rms_ulab(buf):5.0f}")
    print()

# First frames
decoder = audiomp3.MP3Decoder(TEST_FILE)
print_frame_loudness(decoder, 8)

# First frames (fresh decoder)
decoder = audiomp3.MP3Decoder(TEST_FILE)
print_frame_loudness(decoder, 2)

# First frames (reopen)
decoder.open(TEST_FILE)
print_frame_loudness(decoder, 3)