Precise audio presentation

When delivering a sound through the soundcard of a computer and a trigger through the parallel port, it’s important to ensure that the sound and the trigger are synchronized.

Python sleep precision

The time.sleep() function from the time module is not precise enough to halt the program execution for a precise amount of time. The variability depends on the operating system and python version and should be measured before using it in an experiment.

import time
import timeit

import numpy as np

times = timeit.repeat("time.sleep(0.005)", repeat=3, number=100, globals=globals())
times = np.array(times) / 100
print(f"Mean: {np.mean(times):.8f} seconds")
print(f"Standard Deviation: {np.std(times):.8f} seconds")

Locally, I measured 5.06822 ms ± 61.4 μs per loop (mean ± std. dev. of 3 runs, 100 loop each).

If greater sleeping precision is needed, the sleeping period can be cut into segments waited with time.sleep() and a last segment where a time.perf_counter() is used to wait for the remaining time.

Note

Note that using time.sleep() is important as it gives back the control to the operating system, allowing other processes to run.

def high_precision_sleep(duration: float) -> None:
    """High precision sleep function."""
    start_time = time.perf_counter()
    assert 0 < duration, "The duration must be strictly positive."
    while True:
        elapsed_time = time.perf_counter() - start_time
        remaining_time = duration - elapsed_time
        if remaining_time <= 0:
            break
        if remaining_time >= 0.0002:
            time.sleep(remaining_time / 2)


times = timeit.repeat(
    "high_precision_sleep(0.005)", repeat=3, number=100, globals=globals()
)
times = np.array(times) / 100
print(f"Mean: {np.mean(times):.8f} seconds")
print(f"Standard Deviation: {np.std(times):.8f} seconds")

Locally, I measured 5.00070 ms ± 0.08 μs per loop (mean ± std. dev. of 3 runs, 100 loop each).

Psychopy and Psychtoolbox

Psychtoolbox is a compiled toolbox for MATLAB that allows to present stimuli with precision. Despite a lack of funding and resources to support it, it’s still one of the most used stimuli presentation toolboxes. Some of its functionalities are available in Python, especially through the Psychopy library.

# In this example, we will use the python interface of `Psychtoolbox`_ combined with
# triggers from :class:`byte_triggers.ParallelPortTrigger` to deliver synchronize audio
# stimuli and triggers.

import psychtoolbox as ptb
from byte_triggers import ParallelPortTrigger
from psychopy.sound.backend_ptb import SoundPTB

# create the sound and trigger object
sound = SoundPTB(value=440.0, secs=0.2, blockSize=16)
trigger = ParallelPortTrigger(0x2FB8)  # or "/dev/parport0" on linux

# loop 10 times and deliver 10 sounds
for k in range(10):
    now = ptb.GetSecs()
    sound.play(when=now + 0.2)  # schedule the sound in 200 ms
    high_precision_sleep(0.2)  # wait for 200 ms
    trigger.signal(1)  # send the trigger
    print(f"Sound {k + 1} delivered.")
    high_precision_sleep(0.5)  # wait between sounds

The key elements are to schedule the sound with the when argument of the Psychtoolbox backend, wait for the scheduling duration, and deliver the trigger. With this method, the trigger to sound delay should be less than 1 ms.

The measure above was done with a 1024 Hz sampling rate on an ANT Neuro EEG system. Measuring algorithmically the delay between the trigger and the sound is possible through a threshold on the absolute value of the hilbert transformed signal, yielding:

Note

Note that the measure through the absolute value of the hilbert transformed signal is a good automatic sound onset detection method, but it’s not perfect and the thresholding inherently adds jitter to the measure.

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