opl.py

#!/usr/bin/env python2
'''
Short script to make OPL music (in DOSBox DRO files) less loud.
'''
from struct import *
from collections import namedtuple
import sys
import argparse

def namedunpack(data, names, format):
    '''
    Unpacks into a named tuple.
    '''
    str = namedtuple('struct', names)
    data = str(*unpack(format, data))
    return data

CARRIERS = (0x43, 0x44, 0x45, 0x4B, 0x4C, 0x4D, 0x53, 0x54, 0x55)
LEVEL_TO_ALGORITHM = {
    0x40: 0xC0, 0x41: 0xC0, 0x42: 0xC1, 0x43: 0xC1, 0x44: 0xC2, 0x45: 0xC2,
    0x48: 0xC3, 0x49: 0xC3, 0x4A: 0xC4, 0x4B: 0xC4, 0x4C: 0xC5, 0x4D: 0xC5,
    0x50: 0xC6, 0x51: 0xC6, 0x52: 0xC7, 0x53: 0xC7, 0x54: 0xC8, 0x55: 0xC8
}

class OPL2Quieter(object):
    
    def __init__(self, quiet_function):
        self.quiet_function = quiet_function
        self.registers = [0x00 for i in xrange(0x100)]
    
    def write(self, register, value):
        if 0x40 <= register <= 0x55:  # if it is a level register
            # algorithm bit is the LSB of algorithm/feedback reg
            algorithm = self.registers[LEVEL_TO_ALGORITHM[register]] & 0x01
            
            if algorithm == 0x00:
                # FM synthesis, only modify the level if it's a carrier
                modify_level = True if register in CARRIERS else False
            else:
                # AM synthesis, modify level for both operators
                modify_level = True
            
            if modify_level:
                # keep the top two bits (the KSL value) separate
                # for purposes of calculation
                ksl = value & 0xC0
                # perform the volume modification on the lower six bits
                level = self.quiet_function(value & 0x3F)
                value = ksl | (level & 0x3F)
        
        self.registers[register] = value
        return register, value

parser = argparse.ArgumentParser(
    description='Reduces the volume of a DOSBox DRO file.'
)
parser.add_argument('infile', help='input file', type=str)
parser.add_argument('outfile', help='output file', type=str)
args = parser.parse_args()

infile = open(args.infile, 'rb')
outfile = open(args.outfile, 'wb')

chunk = infile.read(26)
data = namedunpack(chunk, ['cSignature', 'iVersionMajor', 'iVersionMinor', 'iLengthPairs', 'iLengthMS', 'iHardwareType', 'iFormat', 'iCompression', 'iShortDelayCode', 'iLongDelayCode', 'iCodemapLength'], '8sHHIIBBBBBB')
outfile.write(chunk)

if data.cSignature != 'DBRAWOPL':
    parser.error('input file is not a DOSBox DRO file.')
    sys.exit(1)

# Copy over the codemap table, which has a variable length.
chunk = infile.read(data.iCodemapLength)
outfile.write(chunk)

pairsTotal = data.iLengthPairs
pairsRead = 0
quieter = OPL2Quieter(lambda level: max(level - 5, 0))
while pairsRead < pairsTotal:
    # Read the next OPL bytes from the file
    chunk = infile.read(2)
    pairsRead += 1
    if not chunk: break
    reg, val = unpack('BB', chunk)
    reg, val = quieter.write(reg, val)
    outfile.write(pack('BB', reg, val))

infile.close()
outfile.close()