I am trying to write a parser to extract information from the following FLAC file:
$ hd audio.flac | head -n 6
00000000 66 4c 61 43 00 00 00 22 12 00 12 00 00 00 00 00 |fLaC..."........|
00000010 00 00 0a c4 42 f0 00 78 9f 30 00 00 00 00 00 00 |....B..x.0......|
00000020 00 00 00 00 00 00 00 00 00 00 84 00 02 64 1f 00 |.............d..|
00000030 00 00 47 53 74 72 65 61 6d 65 72 20 65 6e 63 6f |..GStreamer enco|
00000040 64 65 64 20 76 6f 72 62 69 73 63 6f 6d 6d 65 6e |ded vorbiscommen|
00000050 74 10 00 00 00 12 00 00 00 54 49 54 4c 45 3d 52 |t........TITLE=R|
Now, according to the specification, the format should be as follow (numbers are in bits):
<32> "fLaC", the FLAC stream marker in ASCII
<16> The minimum block size (in samples) used in the stream.
<16> The maximum block size (in samples) used in the stream.
<24> The minimum frame size (in bytes) used in the stream.
<24> The maximum frame size (in bytes) used in the stream.
<20> Sample rate in Hz.
<3> (number of channels)-1. FLAC supports from 1 to 8 channels
<5> (bits per sample)-1. FLAC supports from 4 to 32 bits per sample.
<36> Total samples in stream.
<128> MD5 signature of the unencoded audio data.
So, I start to write my parser and, while testing, get very strange results. So I test with a "real" metadata extractor:
$ metaflac --list audio.flac
METADATA block #0
type: 0 (STREAMINFO)
is last: false
length: 34
minimum blocksize: 4608 samples
maximum blocksize: 4608 samples
minimum framesize: 0 bytes
maximum framesize: 0 bytes
sample_rate: 44100 Hz
channels: 2
bits-per-sample: 16
total samples: 7905072
MD5 signature: 00000000000000000000000000000000
From the numbers, I can deduce the following:
66 4c 61 43 00 00 00 22 12 00 12 00 00 00 00 00
~~~~~~~~~~~ ~~~~~~~~~~~ ~~~~~ ~~~~~ ~~~~~~~~ ~~
^ ^ ^ ^ ^ ^
| | | | | |
| | | | | + Etc.
| | | | + Minimum frame size
| | | + Maximum block size
| | + Minimum block size
| + What is that ?!?
+ FLAC stream marker
Where does those 32 bits come from? I see they represent the length of the header, but isn't it against the standard to put it there (Taking into account that we already know the length: (32+16+16+24+20+3+5+36+128)/8)?
The 0x22 (34) is indeed the header block size in bytes as part of the METADATA_BLOCK_HEADER which follows the fLaC
marker in the stream. Of the first 8 bits (00), bit 7 indicates that there are more metadatablocks to follow, the next 7 bits indicate that it's a STREAMINFO
block. The following 3 bytes (00 00 22) is the length of the contents of the block;
16 + 16 + 24 + 24 + 20 + 3 + 5 + 36 + 128 = 272 bits
272 bits / 8 = 34 (0x22) bytes.