[flac-dev] Exact FLAC subset constraints

Wed Jan 8 22:19:12 PST 2014

Any progress?

> I agree. Please keep up informed.

>> FLAC__format_blocksize_is_subset() was introduced by commit #8ab0138
>> (https://gitorious.org/flac/flac/commit/8ab013837d379d3d1fa84eac5420faec41852fd7)
>> over 5 years ago and available in the latest stable release 1.3.0.
>>
>> It would be nice if the official FLAC documentation used common
>> adopted (used) math notation: math intervals (left-open, right-open,
>> open, left-closed, right-closed, closed) e.g. "[0010, 0101] ∪ [1000,
>> 1110]" or simply math sets e.g. "{0010, 0011, 0100, 0101, 1000, 1001,
>> 1010, 1011, 1100, 1101, 1110}" (if number of elements in not too big)
>> instead of current nonstrict notation e.g. "0001-1110" to
>> disambiguate.
>>
>> There are other ambiguities in official FLAC documentation.
>>
>> It would be nice if FLAC project had its own doc wiki.
>>
>> Should I open a bug ticket?
>>
>> Please keep me informed on your investigation.

>>> I think you've found a bug, Bart.
>>>
>>> flac 1.2.1 did not have any FLAC__format_blocksize_is_subset() function, so
>>> the source you're seeing in format.c must be new. If the format
>>> documentation you linked to is correct, then either the specifications are
>>> self-contradictory, or the code is not implementing the subset checks as
>>> described. My interpretation is that blocksize bits of 0110 and 0111 would
>>> not be included in the subset. The whole point of the subset is to have a
>>> small and finite list of values so that embedded decoders do not have to
>>> deal with every possible block size.
>>>
>>> This bears further investigation.

>>>> I mean that the first statement [Subset streams must use one of
>>>> 192/576/1152/2304/4608/256/512/1024/2048/4096 (and 8192/16384 if the
>>>> sample rate is >48kHz).] published on
>>>> https://www.xiph.org/flac/documentation_tools_flac.html#flac_options_blocksize
>>>> page IS NOT EQUAL to second statement [The blocksize bits in the frame
>>>> header must be 0001-1110. The blocksize must be <=16384; if the sample
>>>> rate is <= 48000Hz, the blocksize must be <=4608.] published on
>>>> https://www.xiph.org/flac/format.html#subset page.
>>>>
>>>> What statement (first or second) is right?
>>>>
>>>> 0001-1110 mean 0110 and 0111 too??? (0110 mean "get 8 bit
>>>> (blocksize-1) from end of header", 0111 mean "get 16 bit (blocksize-1)
>>>> from end of header")
>>>>
>>>> Why you don't use STRICT block size checking in
>>>> FLAC__format_blocksize_is_subset() like this:
>>>>
>>>> FLAC_API FLAC__bool FLAC__format_blocksize_is_subset(unsigned
>>>> blocksize, unsigned sample_rate)
>>>> {
>>>>         if(blocksize == 192 || blocksize == 576 || blocksize == 1152
>>>> || blocksize == 2304 || blocksize == 4608 || blocksize == 256 ||
>>>> blocksize == 512 || blocksize == 1024 || blocksize == 2048 ||
>>>> blocksize == 4096 || (sample_rate > 48000 && (blocksize == 8192 ||
>>>> blocksize == 16384)))
>>>>                 return true;
>>>>         else
>>>>                 return false;
>>>> }
>>>>
>>>> instead of
>>>>
>>>> FLAC_API FLAC__bool FLAC__format_blocksize_is_subset(unsigned
>>>> blocksize, unsigned sample_rate)
>>>> {
>>>>         if(blocksize > 16384)
>>>>                 return false;
>>>>         else if(sample_rate <= 48000 && blocksize > 4608)
>>>>                 return false;
>>>>         else
>>>>                 return true;
>>>> }
>>>>
>>>> FLAC__format_blocksize_is_subset from format.c IS NOT EQUAL to my
>>>> code. E.g. FLAC__format_blocksize_is_subset(1536, 44100) from format.c
>>>> returns true, but 1536 is not subset blocksize because 1536 is not one
>>>> of 192/576/1152/2304/4608/256/512/1024/2048/4096!

>>>>> I don't understand what it is you don't get about those blocksizes. For
>>>>> subset streams, the blocksize has to be one of 576/1152/2304/4608 or
>>>>> 256/512/1024/2048/4096 if the samplerate is lower then or equal to
>>>>> 48kHz, if higher, 8192 and 16384 are allowed too. If you use any other
>>>>> blocksize, the stream is not subset compliant.
>>>>>
>>>>> Considering the fixed or variable blocksize stuff, the subset does not
>>>>> restrict that, so using a variable blocksize is technically subset
>>>>> compliant. However, flake does warn when using variable blocksizes that
>>>>> the stream is not subset compliant. The thing is, it's not in the
>>>>> reference encoder, so probably most (hardware) decoders haven't been
>>>>> tested with it. If you want to be safe, you should probably restrict
>>>>> yourself to a fixed-blocksize stream.
>>>>>
>>>>> Finally, ffmpeg level 12 is not subset compliant.

>>>>>> I'm misleading about FLAC subset constraints... Please help me
>>>>>> understand exact FLAC subset limitation.
>>>>>>
>>>>>> From https://www.xiph.org/flac/documentation_tools_flac.html#flac_options_blocksize:
>>>>>>
>>>>>> Subset streams must use one of
>>>>>> 192/576/1152/2304/4608/256/512/1024/2048/4096 (and 8192/16384 if the
>>>>>> sample rate is >48kHz). The reference encoder uses the same block size
>>>>>> for the entire stream.
>>>>>>
>>>>>> From https://www.xiph.org/flac/format.html#subset:
>>>>>>
>>>>>> The blocksize must be <=16384; if the sample rate is <= 48000Hz, the
>>>>>> blocksize must be <=4608.
>>>>>>
>>>>>> Finally, "one of 192/576/1152/2304/4608/256/512/1024/2048/4096" or
>>>>>> just (simply) "<=4608", "one of 8192/16384" or just "<=16384"?
>>>>>>
>>>>>> Maximum (4608) value and any intermediate values are not one of
>>>>>> 192/576/1152/2304/4608/256/512/1024/2048/4096.
>>>>>>
>>>>>> The first statement is more restricted.
>>>>>>
>>>>>> Is the word "must" to be interpreted as described in RFC 2119
>>>>>> (http://www.ietf.org/rfc/rfc2119.txt)?
>>>>>>
>>>>>> The second question:
>>>>>>
>>>>>> From https://www.xiph.org/flac/documentation_format_overview.html:
>>>>>>
>>>>>> The reference encoder uses a single block size for the whole stream
>>>>>> but the FLAC format does not require it.
>>>>>>
>>>>>> Should stream MUST have constant (fixed) block size (if blocksize
>>>>>> satisfies all restrictions from first question) to be subset?
>>>>>>
>>>>>> Is variable block size stream (if blocksize satisfies all restrictions
>>>>>> from first question) be subset?
>>>>>>
>>>>>> The third question:
>>>>>>
>>>>>> Is FLAC files compressed by FFMPEG with "-compression_level 12" switch
>>>>>> are subset?