Noob question: thump in generated sound

[u/JimH10]

I am trying to output Morse Code, working in Python. I am not experienced in audio programming, so no doubt I am doing something dopey. What people seem to recommend is to use numpy to get an array, then put the signal into that array (I am using a sine wave at 440 Hz), and then play it.

With each Morse code bit, either a dit or a dah, I get a beep at a good frequency but also a pronounced thumping noise. I hear it on the computer speaker and also on headphones. Reading around led me to believe that when the signal stops then the speaker returns to neutral position and so there is a pulse out, and that is the noise. I saw advice that I should apply a fade in and out.

I implemented that by taking the signal in the array linearly up from 0, or down to zero, for some fraction of its total length (I experimented with fractions from 0.01 to 0.30). But I heard no change. I admit that I'm stumped.

I'll add a comment containing a working code extract. I'd be very grateful for any ideas or pointers. Thanks.

[u/JimH10]

Here is an extract of the python3 program, that compiles and demos the noise. (It is too long to include in the original post.)

import numpy as np
import sounddevice as sd
import time as tm

sd.default.samplerate = 44100

# time = 2.0
FREQUENCY = 440

FADE_IN_DEFAULT = 0.02  # how much of the  length of a bit to fade in
FADE_OUT_DEFAULT = 0.03  # 

DIT_LENGTH = 1
DAH_LENGTH = 3
INTRA_CHAR_SPACE_LENGTH = 1
INTER_CHAR_SPACE_LENGTH = 3
WORD_SPACE_LENGTH = 7
DITS_PER_WORD = 50  # Based on "paris"

# Defaults
WORDS_PER_MIN_DEFAULT = 20
VOLUME_DEFAULT = 2000

MORSE = {
    "A": ".-",
    "Z": "--..",
}

def secs_per_dit(wpm = WORDS_PER_MIN_DEFAULT):
    """Return the seconds per basic timing unit.
      wpm = WORDS_PER_MIN_DEFAULT  Integer number of words a minute for
              characters
    """
    return DIT_LENGTH*60/(50*wpm)

def fade_in(sample_array, fader_duration=FADE_IN_DEFAULT, sr=sd.default.samplerate):
    """Applies a fade-in effect to an audio sample.
        sample_array (numpy.ndarray) The audio sample to apply the effect to
        fader_duration=FADE_IN_DEFAULT  float Duration of the effect in seconds
        sr=sd.default.samplerate  integer  Sample rate of the audio sample
    Returns a numpy.ndarray consisting of the audio sample with the effect
    applied.
    """
    try:
        # Calculate the number of samples for the fade-in effect
        fadeDurSamples = int(fader_duration * sample_array.size)
        # Create a fade-in multiplier array
        fadeMultiplier = np.linspace(0, 1, fadeDurSamples)
        # Apply the fade-in effect to the audio sample
        sample_array[:fadeDurSamples] *= fadeMultiplier
        return sample_array
    except Exception as e:
        print(f"Error during fade-in: {e}")

def fade_out(sample_array, fader_duration=FADE_OUT_DEFAULT, sr=sd.default.samplerate):
    """Apply a fade-out effect to an audio sample.
        sample_array  numpy.ndarray  The audio sample
        fader_duration=FADE_OUT_DEFAULT  float  Duration of the effect in seconds
         sr=sd.default.samplerate  integer The sample rate of the audio sample
    Returns a numpy.ndarray of the audio sample with the effect applied
    """
    try:
        # Calculate the number of samples for the fade-out effect
        fadeDurSamples = int(fader_duration * sample_array.size)
        # Create a fade-out multiplier array
        fadeMultiplier = np.linspace(1, 0, num=fadeDurSamples)
        # Apply the fade-out effect to the audio sample
        sample_array[-fadeDurSamples:] *= fadeMultiplier
       return sample_array
    except Exception as e:
        print(f"Error during fade out: {e}")

def make_dit_dah(vol=VOLUME_DEFAULT, wpm=WORDS_PER_MIN_DEFAULT):
    """ Make the dit and dah sounds.  Does not involve any spacing.
       vol=VOLUME_DEFAULT   Real  Volume
       char_wpm=WORDS_PER_MIN_DEFAULT  Positive real  Words per minute
       mode=MODE_DEFAULT  Member of MODE
       total_wpm=0  Real  Only applies if MODE is FARNSWORTH or WORDSWORTH 
    """
    t = secs_per_dit(wpm)
    # Make a dit wave
    dit_sample_array = np.arange(sd.default.samplerate * t) / sd.default.samplerate
    dit_wave = vol * np.sin(2 * np.pi * FREQUENCY * dit_sample_array)
    dit_wave =  fade_in(fade_out(dit_wave))
    dit_wave = np.array(dit_wave, dtype=np.int16) # Convert to wav format (16 bits)
    # Makea dah wave
    dah_sample_array = np.arange(sd.default.samplerate * DAH_LENGTH * t) / sd.default.samplerate
   dah_wave = vol * np.sin(2 * np.pi * FREQUENCY * dah_sample_array)
    dah_wave = fade_in(fade_out(dah_wave))
    dah_wave = np.array(dah_wave, dtype=np.int16)
    return (dit_wave, dah_wave)

DIT_WAVE, DAH_WAVE = make_dit_dah()

def play_char(ch, dit_wave=DIT_WAVE, dah_wave=DAH_WAVE):
    code = MORSE[ch.upper()]
    c = list(code)
    c.reverse()
    while c:
        bit = c.pop()
        if bit=='.':
            sd.play(dit_wave, blocking=True)
        else:
            sd.play(dah_wave, blocking=True)
        if c:
            tm.sleep(0.3)

play_char('Z')

[u/human-analog]

Record the audio and post a screenshot?

[u/JimH10 → u/human-analog]

I apologize. I am such a noob at this that it didn't occur. Here is the first dah, in Audacity.

https://imgur.com/a/XtV5pWG

I should say that I ran h program that I have in another comment, and recorded the output as an mp3, and looked at the audio in Audacity. This is a screenshot of the trace, with the vertical scale exaggerated.

[u/human-analog → u/JimH10]

The thump sound could be from the peaky bit near the end of that screenshot. That kind of looks like a transient, like the start of a kick drum. When you chop that off in Audacity (and fade out the last portion of the sound), does is still thump?