class ALACDecoder(object):
def __init__(self, filename):
self.reader = BitstreamReader(open(filename, "rb"), False)
self.reader.mark()
try:
# locate the "alac" atom
# which is full of required decoding parameters
try:
stsd = self.find_sub_atom(b"moov", b"trak", b"mdia", b"minf",
b"stbl", b"stsd")
except KeyError:
raise ValueError("required stsd atom not found")
(stsd_version, descriptions) = stsd.parse("8u 24p 32u")
(alac1, alac2, self.samples_per_frame, self.bits_per_sample,
self.history_multiplier, self.initial_history, self.maximum_k,
self.channels, self.sample_rate) = stsd.parse(
# ignore much of the stuff in the "high" ALAC atom
"32p 4b 6P 16p 16p 16p 4P 16p 16p 16p 16p 4P" +
# and use the attributes in the "low" ALAC atom instead
"32p 4b 4P 32u 8p 8u 8u 8u 8u 8u 16p 32p 32p 32u")
self.channel_mask = {
1: 0x0004,
2: 0x0003,
3: 0x0007,
4: 0x0107,
5: 0x0037,
6: 0x003F,
7: 0x013F,
8: 0x00FF
}.get(self.channels, 0)
if ((alac1 != b'alac') or (alac2 != b'alac')):
raise ValueError("Invalid alac atom")
# also locate the "mdhd" atom
# which contains the stream's length in PCM frames
self.reader.rewind()
mdhd = self.find_sub_atom(b"moov", b"trak", b"mdia", b"mdhd")
(version, ) = mdhd.parse("8u 24p")
if (version == 0):
(self.total_pcm_frames,
) = mdhd.parse("32p 32p 32p 32u 2P 16p")
elif (version == 1):
(self.total_pcm_frames,
) = mdhd.parse("64p 64p 32p 64U 2P 16p")
else:
raise ValueError("invalid mdhd version")
# finally, set our stream to the "mdat" atom
self.reader.rewind()
(atom_size, atom_name) = self.reader.parse("32u 4b")
while (atom_name != b"mdat"):
self.reader.skip_bytes(atom_size - 8)
(atom_size, atom_name) = self.reader.parse("32u 4b")
finally:
self.reader.unmark()
def find_sub_atom(self, *atom_names):
reader = self.reader
for (last, next_atom) in iter_last(iter(atom_names)):
try:
(length, stream_atom) = reader.parse("32u 4b")
while (stream_atom != next_atom):
reader.skip_bytes(length - 8)
(length, stream_atom) = reader.parse("32u 4b")
if (last):
return reader.substream(length - 8)
else:
reader = reader.substream(length - 8)
except IOError:
raise KeyError(next_atom)
def read(self, pcm_frames):
# if the stream is exhausted, return an empty pcm.FrameList object
if (self.total_pcm_frames == 0):
return empty_framelist(self.channels, self.bits_per_sample)
# otherwise, read one ALAC frameset's worth of frame data
frameset_data = []
frame_channels = self.reader.read(3) + 1
while (frame_channels != 0x8):
frameset_data.extend(self.read_frame(frame_channels))
frame_channels = self.reader.read(3) + 1
self.reader.byte_align()
# reorder the frameset to Wave order, depending on channel count
if ((self.channels == 1) or (self.channels == 2)):
pass
elif (self.channels == 3):
frameset_data = [
frameset_data[1], frameset_data[2], frameset_data[0]
]
elif (self.channels == 4):
frameset_data = [
frameset_data[1], frameset_data[2], frameset_data[0],
frameset_data[3]
]
elif (self.channels == 5):
frameset_data = [
frameset_data[1], frameset_data[2], frameset_data[0],
frameset_data[3], frameset_data[4]
]
elif (self.channels == 6):
frameset_data = [
frameset_data[1], frameset_data[2], frameset_data[0],
frameset_data[5], frameset_data[3], frameset_data[4]
]
elif (self.channels == 7):
frameset_data = [
frameset_data[1], frameset_data[2], frameset_data[0],
frameset_data[6], frameset_data[3], frameset_data[4],
frameset_data[5]
]
elif (self.channels == 8):
frameset_data = [
frameset_data[3], frameset_data[4], frameset_data[0],
frameset_data[7], frameset_data[5], frameset_data[6],
frameset_data[1], frameset_data[2]
]
else:
raise ValueError("unsupported channel count")
framelist = from_channels([
from_list(channel, 1, self.bits_per_sample, True)
for channel in frameset_data
])
# deduct PCM frames from remainder
self.total_pcm_frames -= framelist.frames
# return samples as a pcm.FrameList object
return framelist
def read_frame(self, channel_count):
"""returns a list of PCM sample lists, one per channel"""
# read the ALAC frame header
self.reader.skip(16)
has_sample_count = self.reader.read(1)
uncompressed_lsb_size = self.reader.read(2)
uncompressed = self.reader.read(1)
if (has_sample_count):
sample_count = self.reader.read(32)
else:
sample_count = self.samples_per_frame
if (uncompressed == 1):
# if the frame is uncompressed,
# read the raw, interlaced samples
samples = [
self.reader.read_signed(self.bits_per_sample)
for i in range(sample_count * channel_count)
]
return [samples[i::channel_count] for i in range(channel_count)]
else:
# if the frame is compressed,
# read the interlacing parameters
interlacing_shift = self.reader.read(8)
interlacing_leftweight = self.reader.read(8)
# subframe headers
subframe_headers = [
self.read_subframe_header() for i in range(channel_count)
]
# optional uncompressed LSB values
if (uncompressed_lsb_size > 0):
uncompressed_lsbs = [
self.reader.read(uncompressed_lsb_size * 8)
for i in range(sample_count * channel_count)
]
else:
uncompressed_lsbs = []
sample_size = (self.bits_per_sample - (uncompressed_lsb_size * 8) +
channel_count - 1)
# and residual blocks
residual_blocks = [
self.read_residuals(sample_size, sample_count)
for i in range(channel_count)
]
# calculate subframe samples based on
# subframe header's QLP coefficients and QLP shift-needed
decoded_subframes = [
self.decode_subframe(header[0], header[1], sample_size,
residuals)
for (header,
residuals) in zip(subframe_headers, residual_blocks)
]
# decorrelate channels according interlacing shift and leftweight
decorrelated_channels = self.decorrelate_channels(
decoded_subframes, interlacing_shift, interlacing_leftweight)
# if uncompressed LSB values are present,
# prepend them to each sample of each channel
if (uncompressed_lsb_size > 0):
channels = []
for (i, channel) in enumerate(decorrelated_channels):
assert (len(channel) == len(
uncompressed_lsbs[i::channel_count]))
channels.append([
s << (uncompressed_lsb_size * 8) | l for (s, l) in zip(
channel, uncompressed_lsbs[i::channel_count])
])
return channels
else:
return decorrelated_channels
def read_subframe_header(self):
prediction_type = self.reader.read(4)
qlp_shift_needed = self.reader.read(4)
rice_modifier = self.reader.read(3)
qlp_coefficients = [
self.reader.read_signed(16) for i in range(self.reader.read(5))
]
return (qlp_shift_needed, qlp_coefficients)
def read_residuals(self, sample_size, sample_count):
residuals = []
history = self.initial_history
sign_modifier = 0
i = 0
while (i < sample_count):
# get an unsigned residual based on "history"
# and on "sample_size" as a lst resort
k = min(log2(history // (2**9) + 3), self.maximum_k)
unsigned = self.read_residual(k, sample_size) + sign_modifier
# clear out old sign modifier, if any
sign_modifier = 0
# change unsigned residual to signed residual
if (unsigned & 1):
residuals.append(-((unsigned + 1) // 2))
else:
residuals.append(unsigned // 2)
# update history based on unsigned residual
if (unsigned <= 0xFFFF):
history += ((unsigned * self.history_multiplier) -
((history * self.history_multiplier) >> 9))
else:
history = 0xFFFF
# if history gets too small, we may have a block of 0 samples
# which can be compressed more efficiently
if ((history < 128) and ((i + 1) < sample_count)):
zeroes_k = min(7 - log2(history) + ((history + 16) // 64),
self.maximum_k)
zero_residuals = self.read_residual(zeroes_k, 16)
if (zero_residuals > 0):
residuals.extend([0] * zero_residuals)
i += zero_residuals
history = 0
if (zero_residuals <= 0xFFFF):
sign_modifier = 1
i += 1
return residuals
def read_residual(self, k, sample_size):
msb = self.reader.read_huffman_code(RESIDUAL)
if (msb == -1):
return self.reader.read(sample_size)
elif (k == 0):
return msb
else:
lsb = self.reader.read(k)
if (lsb > 1):
return msb * ((1 << k) - 1) + (lsb - 1)
elif (lsb == 1):
self.reader.unread(1)
return msb * ((1 << k) - 1)
else:
self.reader.unread(0)
return msb * ((1 << k) - 1)
def decode_subframe(self, qlp_shift_needed, qlp_coefficients, sample_size,
residuals):
# first sample is always copied verbatim
samples = [residuals.pop(0)]
if (len(qlp_coefficients) < 31):
# the next "coefficient count" samples
# are applied as differences to the previous
for i in range(len(qlp_coefficients)):
samples.append(
truncate_bits(samples[-1] + residuals.pop(0), sample_size))
# remaining samples are processed much like LPC
for residual in residuals:
base_sample = samples[-len(qlp_coefficients) - 1]
lpc_sum = sum([(s - base_sample) * c
for (s,
c) in zip(samples[-len(qlp_coefficients):],
reversed(qlp_coefficients))])
outval = (1 << (qlp_shift_needed - 1)) + lpc_sum
outval >>= qlp_shift_needed
samples.append(
truncate_bits(outval + residual + base_sample,
sample_size))
buf = samples[-len(qlp_coefficients) - 2:-1]
# error value then adjusts the coefficients table
if (residual > 0):
predictor_num = len(qlp_coefficients) - 1
while ((predictor_num >= 0) and residual > 0):
val = (buf[0] -
buf[len(qlp_coefficients) - predictor_num])
sign = sign_only(val)
qlp_coefficients[predictor_num] -= sign
val *= sign
residual -= ((val >> qlp_shift_needed) *
(len(qlp_coefficients) - predictor_num))
predictor_num -= 1
elif (residual < 0):
# the same as above, but we break if residual goes positive
predictor_num = len(qlp_coefficients) - 1
while ((predictor_num >= 0) and residual < 0):
val = (buf[0] -
buf[len(qlp_coefficients) - predictor_num])
sign = -sign_only(val)
qlp_coefficients[predictor_num] -= sign
val *= sign
residual -= ((val >> qlp_shift_needed) *
(len(qlp_coefficients) - predictor_num))
predictor_num -= 1
else:
# residuals are encoded as simple difference values
for residual in residuals:
samples.append(
truncate_bits(samples[-1] + residual, sample_size))
return samples
def decorrelate_channels(self, channel_data, interlacing_shift,
interlacing_leftweight):
if (len(channel_data) != 2):
return channel_data
elif (interlacing_leftweight == 0):
return channel_data
else:
left = []
right = []
for (ch1, ch2) in zip(*channel_data):
right.append(ch1 - ((ch2 * interlacing_leftweight) //
(2**interlacing_shift)))
left.append(ch2 + right[-1])
return [left, right]
def close(self):
self.reader.close()
def __enter__(self):
return self
def __exit__(self, exc_type, exc_value, traceback):
self.close()
class ALACDecoder(object):
def __init__(self, filename):
self.reader = BitstreamReader(open(filename, "rb"), False)
stream_start = self.reader.getpos()
# locate the "alac" atom
# which is full of required decoding parameters
try:
stsd = self.find_sub_atom(b"moov", b"trak", b"mdia",
b"minf", b"stbl", b"stsd")
except KeyError:
raise ValueError("required stsd atom not found")
(stsd_version, descriptions) = stsd.parse("8u 24p 32u")
(alac1,
alac2,
self.samples_per_frame,
self.bits_per_sample,
self.history_multiplier,
self.initial_history,
self.maximum_k,
self.channels,
self.sample_rate) = stsd.parse(
# ignore much of the stuff in the "high" ALAC atom
"32p 4b 6P 16p 16p 16p 4P 16p 16p 16p 16p 4P" +
# and use the attributes in the "low" ALAC atom instead
"32p 4b 4P 32u 8p 8u 8u 8u 8u 8u 16p 32p 32p 32u")
self.channel_mask = {1: 0x0004,
2: 0x0003,
3: 0x0007,
4: 0x0107,
5: 0x0037,
6: 0x003F,
7: 0x013F,
8: 0x00FF}.get(self.channels, 0)
if (alac1 != b'alac') or (alac2 != b'alac'):
raise ValueError("Invalid alac atom")
# also locate the "mdhd" atom
# which contains the stream's length in PCM frames
self.reader.setpos(stream_start)
mdhd = self.find_sub_atom(b"moov", b"trak", b"mdia", b"mdhd")
(version, ) = mdhd.parse("8u 24p")
if version == 0:
(self.total_pcm_frames,) = mdhd.parse(
"32p 32p 32p 32u 2P 16p")
elif version == 1:
(self.total_pcm_frames,) = mdhd.parse(
"64p 64p 32p 64U 2P 16p")
else:
raise ValueError("invalid mdhd version")
# finally, set our stream to the "mdat" atom
self.reader.setpos(stream_start)
(atom_size, atom_name) = self.reader.parse("32u 4b")
while atom_name != b"mdat":
self.reader.skip_bytes(atom_size - 8)
(atom_size, atom_name) = self.reader.parse("32u 4b")
def find_sub_atom(self, *atom_names):
reader = self.reader
for (last, next_atom) in iter_last(iter(atom_names)):
try:
(length, stream_atom) = reader.parse("32u 4b")
while stream_atom != next_atom:
reader.skip_bytes(length - 8)
(length, stream_atom) = reader.parse("32u 4b")
if last:
return reader.substream(length - 8)
else:
reader = reader.substream(length - 8)
except IOError:
raise KeyError(next_atom)
def read(self, pcm_frames):
# if the stream is exhausted, return an empty pcm.FrameList object
if self.total_pcm_frames == 0:
return empty_framelist(self.channels, self.bits_per_sample)
# otherwise, read one ALAC frameset's worth of frame data
frameset_data = []
frame_channels = self.reader.read(3) + 1
while frame_channels != 0x8:
frameset_data.extend(self.read_frame(frame_channels))
frame_channels = self.reader.read(3) + 1
self.reader.byte_align()
# reorder the frameset to Wave order, depending on channel count
if (self.channels == 1) or (self.channels == 2):
pass
elif self.channels == 3:
frameset_data = [frameset_data[1],
frameset_data[2],
frameset_data[0]]
elif self.channels == 4:
frameset_data = [frameset_data[1],
frameset_data[2],
frameset_data[0],
frameset_data[3]]
elif self.channels == 5:
frameset_data = [frameset_data[1],
frameset_data[2],
frameset_data[0],
frameset_data[3],
frameset_data[4]]
elif self.channels == 6:
frameset_data = [frameset_data[1],
frameset_data[2],
frameset_data[0],
frameset_data[5],
frameset_data[3],
frameset_data[4]]
elif self.channels == 7:
frameset_data = [frameset_data[1],
frameset_data[2],
frameset_data[0],
frameset_data[6],
frameset_data[3],
frameset_data[4],
frameset_data[5]]
elif self.channels == 8:
frameset_data = [frameset_data[3],
frameset_data[4],
frameset_data[0],
frameset_data[7],
frameset_data[5],
frameset_data[6],
frameset_data[1],
frameset_data[2]]
else:
raise ValueError("unsupported channel count")
framelist = from_channels([from_list(channel,
1,
self.bits_per_sample,
True)
for channel in frameset_data])
# deduct PCM frames from remainder
self.total_pcm_frames -= framelist.frames
# return samples as a pcm.FrameList object
return framelist
def read_frame(self, channel_count):
"""returns a list of PCM sample lists, one per channel"""
# read the ALAC frame header
self.reader.skip(16)
has_sample_count = self.reader.read(1)
uncompressed_lsb_size = self.reader.read(2)
uncompressed = self.reader.read(1)
if has_sample_count:
sample_count = self.reader.read(32)
else:
sample_count = self.samples_per_frame
if uncompressed == 1:
# if the frame is uncompressed,
# read the raw, interlaced samples
samples = [self.reader.read_signed(self.bits_per_sample)
for i in range(sample_count * channel_count)]
return [samples[i::channel_count] for i in range(channel_count)]
else:
# if the frame is compressed,
# read the interlacing parameters
interlacing_shift = self.reader.read(8)
interlacing_leftweight = self.reader.read(8)
# subframe headers
subframe_headers = [self.read_subframe_header()
for i in range(channel_count)]
# optional uncompressed LSB values
if uncompressed_lsb_size > 0:
uncompressed_lsbs = [
self.reader.read(uncompressed_lsb_size * 8)
for i in range(sample_count * channel_count)]
else:
uncompressed_lsbs = []
sample_size = (self.bits_per_sample -
(uncompressed_lsb_size * 8) +
channel_count - 1)
# and residual blocks
residual_blocks = [self.read_residuals(sample_size,
sample_count)
for i in range(channel_count)]
# calculate subframe samples based on
# subframe header's QLP coefficients and QLP shift-needed
decoded_subframes = [self.decode_subframe(header[0],
header[1],
sample_size,
residuals)
for (header, residuals) in
zip(subframe_headers,
residual_blocks)]
# decorrelate channels according interlacing shift and leftweight
decorrelated_channels = self.decorrelate_channels(
decoded_subframes,
interlacing_shift,
interlacing_leftweight)
# if uncompressed LSB values are present,
# prepend them to each sample of each channel
if uncompressed_lsb_size > 0:
channels = []
for (i, channel) in enumerate(decorrelated_channels):
assert(len(channel) ==
len(uncompressed_lsbs[i::channel_count]))
channels.append([s << (uncompressed_lsb_size * 8) | l
for (s, l) in
zip(channel,
uncompressed_lsbs[i::channel_count])])
return channels
else:
return decorrelated_channels
def read_subframe_header(self):
prediction_type = self.reader.read(4)
qlp_shift_needed = self.reader.read(4)
rice_modifier = self.reader.read(3)
qlp_coefficients = [self.reader.read_signed(16)
for i in range(self.reader.read(5))]
return (qlp_shift_needed, qlp_coefficients)
def read_residuals(self, sample_size, sample_count):
residuals = []
history = self.initial_history
sign_modifier = 0
i = 0
while i < sample_count:
# get an unsigned residual based on "history"
# and on "sample_size" as a lst resort
k = min(log2(history // (2 ** 9) + 3), self.maximum_k)
unsigned = self.read_residual(k, sample_size) + sign_modifier
# clear out old sign modifier, if any
sign_modifier = 0
# change unsigned residual to signed residual
if unsigned & 1:
residuals.append(-((unsigned + 1) // 2))
else:
residuals.append(unsigned // 2)
# update history based on unsigned residual
if unsigned <= 0xFFFF:
history += ((unsigned * self.history_multiplier) -
((history * self.history_multiplier) >> 9))
else:
history = 0xFFFF
# if history gets too small, we may have a block of 0 samples
# which can be compressed more efficiently
if (history < 128) and ((i + 1) < sample_count):
zeroes_k = min(7 -
log2(history) +
((history + 16) // 64),
self.maximum_k)
zero_residuals = self.read_residual(zeroes_k, 16)
if zero_residuals > 0:
residuals.extend([0] * zero_residuals)
i += zero_residuals
history = 0
if zero_residuals <= 0xFFFF:
sign_modifier = 1
i += 1
return residuals
def read_residual(self, k, sample_size):
msb = self.reader.read_huffman_code(RESIDUAL)
if msb == -1:
return self.reader.read(sample_size)
elif k == 0:
return msb
else:
lsb = self.reader.read(k)
if lsb > 1:
return msb * ((1 << k) - 1) + (lsb - 1)
elif lsb == 1:
self.reader.unread(1)
return msb * ((1 << k) - 1)
else:
self.reader.unread(0)
return msb * ((1 << k) - 1)
def decode_subframe(self, qlp_shift_needed, qlp_coefficients,
sample_size, residuals):
# first sample is always copied verbatim
samples = [residuals.pop(0)]
if len(qlp_coefficients) < 31:
# the next "coefficient count" samples
# are applied as differences to the previous
for i in range(len(qlp_coefficients)):
samples.append(truncate_bits(samples[-1] + residuals.pop(0),
sample_size))
# remaining samples are processed much like LPC
for residual in residuals:
base_sample = samples[-len(qlp_coefficients) - 1]
lpc_sum = sum([(s - base_sample) * c for (s, c) in
zip(samples[-len(qlp_coefficients):],
reversed(qlp_coefficients))])
outval = (1 << (qlp_shift_needed - 1)) + lpc_sum
outval >>= qlp_shift_needed
samples.append(truncate_bits(outval + residual + base_sample,
sample_size))
buf = samples[-len(qlp_coefficients) - 2:-1]
# error value then adjusts the coefficients table
if residual > 0:
predictor_num = len(qlp_coefficients) - 1
while (predictor_num >= 0) and residual > 0:
val = (buf[0] -
buf[len(qlp_coefficients) - predictor_num])
sign = sign_only(val)
qlp_coefficients[predictor_num] -= sign
val *= sign
residual -= ((val >> qlp_shift_needed) *
(len(qlp_coefficients) - predictor_num))
predictor_num -= 1
elif residual < 0:
# the same as above, but we break if residual goes positive
predictor_num = len(qlp_coefficients) - 1
while (predictor_num >= 0) and (residual < 0):
val = (buf[0] -
buf[len(qlp_coefficients) - predictor_num])
sign = -sign_only(val)
qlp_coefficients[predictor_num] -= sign
val *= sign
residual -= ((val >> qlp_shift_needed) *
(len(qlp_coefficients) - predictor_num))
predictor_num -= 1
else:
# residuals are encoded as simple difference values
for residual in residuals:
samples.append(truncate_bits(samples[-1] + residual,
sample_size))
return samples
def decorrelate_channels(self, channel_data,
interlacing_shift, interlacing_leftweight):
if len(channel_data) != 2:
return channel_data
elif interlacing_leftweight == 0:
return channel_data
else:
left = []
right = []
for (ch1, ch2) in zip(*channel_data):
right.append(ch1 - ((ch2 * interlacing_leftweight) //
(2 ** interlacing_shift)))
left.append(ch2 + right[-1])
return [left, right]
def close(self):
self.reader.close()
def __enter__(self):
return self
def __exit__(self, exc_type, exc_value, traceback):
self.close()
