pcm.py

# pcm.py
# Converts between .wav files and 1-bit pcm data. (pcm = pulse-code modulation)

import argparse
import os
import struct
import wave


BASE_SAMPLE_RATE = 16384

def convert_to_wav(filenames=[]):
    """
    Converts a file containing 1-bit pcm data into a .wav file.
    """
    for filename in filenames:
        with open(filename, 'rb') as pcm_file:
            # Generate array of on/off pcm values.
            samples = []
            byte = pcm_file.read(1)
            while byte != "":
                byte, = struct.unpack('B', byte)
                samples.append((byte & 0xf0) >> 4)
                samples.append(byte & 0xf)
                byte = pcm_file.read(1)

        # Write a .wav file using the pcm data.
        name, extension = os.path.splitext(filename)
        wav_filename = name + '.wav'
        wave_file = wave.open(wav_filename, 'w')
        wave_file.setframerate(BASE_SAMPLE_RATE)
        wave_file.setnchannels(1)
        wave_file.setsampwidth(1)

        for value in samples:
            value = max(16 * value - 1, 0)
            packed_value = struct.pack('B', value)
            wave_file.writeframesraw(packed_value)

        wave_file.close()


def convert_to_pcm(filenames=[]):
    """
    Converts a .wav file into 1-bit pcm data.
    Samples in the .wav file are simply clamped to on/off.

    This currently works correctly on .wav files with the following attributes:
            1. Sample Width = 1 or 2 bytes (Some wave files use 3 bytes per sample...)
            2. Arbitrary sample sample_rate
            3. Mono or Stereo (1 or 2 channels)
    """
    for filename in filenames:
        samples, sample_bins = get_wav_samples(filename)

        # Generate a list of clamped samples
        clamped_samples = [sample_bins.index(sample) for sample in samples]

        # The pcm data must be a multiple of 2, so pad the clamped samples with 0.
        while len(clamped_samples) % 2 != 0:
            clamped_samples.append(0)

        # Pack the 4-bit samples together.
        packed_samples = bytearray()
        for i in xrange(0, len(clamped_samples), 2):
            # Read 2 pcm values to pack one byte.
            packed_value = (clamped_samples[i] << 4) | clamped_samples[i + 1]
            packed_samples.append(packed_value)

        # Open the output .pcm file, and write all 1-bit samples.
        name, extension = os.path.splitext(filename)
        pcm_filename = name + '.pcm'
        with open(pcm_filename, 'wb') as out_file:
            out_file.write(packed_samples)


def get_wav_samples(filename):
    """
    Reads the given .wav file and returns a list of its samples after re-sampling
    to BASE_SAMPLE_RATE.
    Also returns the average sample amplitude.
    """
    wav_file = wave.open(filename, 'r')
    sample_width = wav_file.getsampwidth()
    sample_count = wav_file.getnframes()
    sample_rate = wav_file.getframerate()
    num_channels = wav_file.getnchannels()

    samples = bytearray(wav_file.readframes(sample_count))

    # Unpack the values based on the sample byte width.
    unpacked_samples = []
    for i in xrange(0, len(samples), sample_width):
        if sample_width == 1:
            fmt = 'B'
        elif sample_width == 2:
            fmt = 'h'
        else:
            # todo: support 3-byte sample width
            raise Exception, "Unsupported sample width: " + str(sample_width)

        value, = struct.unpack(fmt, samples[i:i + sample_width])
        unpacked_samples.append(value)

    # Only keep the samples from the first audio channel.
    unpacked_samples = unpacked_samples[::num_channels]

    # Approximate the BASE_SAMPLE_RATE.
    # Also find the average amplitude of the samples.
    resampled_samples = []
    total_value = 0
    interval = float(sample_rate) / BASE_SAMPLE_RATE
    index = 0
    while index < sample_count:
        sample = unpacked_samples[int(index)]
        total_value += sample

        resampled_samples.append(sample)
        index += interval

    sample_bins = sorted(set(resampled_samples))

    return resampled_samples, sample_bins


def main():
    ap = argparse.ArgumentParser()
    ap.add_argument('mode')
    ap.add_argument('filenames', nargs='*')
    args = ap.parse_args()

    method = {
        'wav': convert_to_wav,
        'pcm': convert_to_pcm,
    }.get(args.mode, None)

    if method == None:
        raise Exception, "Unknown conversion method!"

    method(args.filenames)

if __name__ == "__main__":
    main()