Functions to convert between ReplayGain and SoundCheck.

soundcheck.py

import math
import struct


# The following is from http://id3.org/iTunes%20Normalization%20settings

# The iTunNORM tag consists of 5 value pairs. These 10 values are encoded as
# ASCII Hex values of 8 characters each inside the tag (plus a space as 
# prefix).
 
# The tag can be found in MP3, AIFF, AAC and Apple Lossless files.
 
# The relevant information is what is encoded in these 5 value pairs. The 
# first value of each pair is for the left audio channel, the second value of 
# each pair is for the right channel.
 
# 0/1: Volume adjustment in milliWatt/dBm
# 2/3: Same as 0/1, but not based on 1/1000 Watt but 1/2500 Watt
# 4/5: Not sure, but always the same values for songs that only differs in 
#      volume - so maybe some statistical values.
# 6/7: The peak value (maximum sample) as absolute (positive) value; 
#      therefore up to 32768 (for songs using 16-Bit samples).
# 8/9: Not sure, same as for 4/5: same values for songs that only differs in
#      volume.
# iTunes is choosing the maximum value of the both first pairs (of the first 
# 4 values) to adjust the whole song.


def sc2rg(soundcheck):
    """Convert a SoundCheck tag to ReplayGain values"""
    # SoundCheck tags consist of 10 numbers, each represented by 8 characters
    # of ASCII hex preceded by a space.
    try:
        soundcheck = soundcheck.replace(' ', '').decode('hex')
        soundcheck = struct.unpack('!iiiiiiiiii', soundcheck)
    except:
        # SoundCheck isn't in the format we expect, so return default values
        return 0.0, 0.0
    # SoundCheck stores absolute calculated/measured RMS value in an unknown
    # unit. We need to find the ratio of this measurement compared to a 
    # reference value of 1000 to get our gain in dB. We play it safe by using
    # the larger of the two values (i.e., the most attenuation).
    gain = math.log10((max(*soundcheck[:2]) or 1000) / 1000.0) * -10
    # SoundCheck stores peak values as the actual value of the sample, and
    # again separately for the left and right channels. We need to convert
    # this to a percentage of full scale, which is 32768 for a 16 bit sample.
    # Once again, we play it safe by using the larger of the two values.
    peak = max(soundcheck[6:8]) / 32768.0
    return round(gain, 2), round(peak, 6)


def rg2sc(gain, peak):
    """Convert ReplayGain values to a SoundCheck tag"""
    if not isinstance(gain, float):
        gain = float(gain.lower().strip(' db'))
    # SoundCheck stores the peak value as the actual value of the sample,
    # rather than the percentage of full scale that RG uses, so we do
    # a simple conversion assuming 16 bit samples.
    peak = float(peak) * 32768.0
    # SoundCheck stores absolute RMS values in some unknown units rather than 
    # the dB values RG uses. We can calculate these absolute values from the 
    # gain ratio using a reference value of 1000 units. We also enforce the 
    # maximum value here, which is equivalent to about -18.2dB.
    g1 = min(round((10 ** (gain / -10)) * 1000), 65534)
    # Same as above, except our reference level is 2500 units.
    g2 = min(round((10 ** (gain / -10)) * 2500), 65534)
    # The purpose of these values are unknown, but they also seem to be
    # unused so we just use 0
    uk = 0
    values = (g1, g1, g2, g2, uk, uk, peak, peak, uk, uk)
    soundcheck = (u' %08X' * 10) % values
    return soundcheck