[Ffmpeg-devel] [DRAFT] Add fact chunk to non-PCM wav

[Michel Bardiaux]

Michel Bardiaux wrote:
> Michael Niedermayer wrote:
>> Hi
>>
>> On Thu, Nov 09, 2006 at 01:53:08PM +0100, Michel Bardiaux wrote:
>>> Michael Niedermayer wrote:
>>>> Hi
>>>>
>>>> On Thu, Nov 09, 2006 at 11:36:13AM +0100, Michel Bardiaux wrote:
>>>>> Michael has reported that
>>>>>
>>>>>>> interresting, accoridng to microsofts excelent and unambigous 
>>>>>>> documentation
>>>>>>> (not kidding ...)
>>>>>>>    Fact Chunk
>>>>>>> This chunk is required for all WAVE formats other than 
>>>>>>> WAVE_FORMAT_PCM. It stores file
>>>>>>> dependent information about the contents of the WAVE data. It 
>>>>>>> currently specifies the time length of the
>>>>>>> data in samples.
>>>>>>>
>>>>>>> so this must not be under CODEC_ID_MSGSM, also it must be a 
>>>>>>> seperate patch
>>>>>>> as its not CODEC_ID_MSGSM specific
>>>>> I took this literally, hence CODEC_ID_PCM_ALAW and 
>>>>> CODEC_ID_PCM_MULAW will get a fact chunk too.
>>>> :)
>>>>
>>>> now just take the largest pts minus the smallest pts of any packet 
>>>> stored
>>>> and convert that by using AVStream.time_base and 
>>>> AVCodecContext.sample_rate
>>>> to the number of samples and store that in the fact chunk
>>> Right, the patch does not make sense without the final update of the 
>>> chunk, and with a *general* formula, not the one I had specialised 
>>> for MSGSM. I'm rather rusty on the handling of timestamps, so this 
>>> might take some time 
>>
>> in the write packet function
>> assert(avpacket->pts != AV_NOPTS_VALUE);
>> context->maxpts= FFMAX(context->maxpts, avpacket->pts);
>> context->minpts= FFMIN(context->minpts, avpacket->pts);
>>
>> and in write_trailer
>> number_of_sample= av_rescale(context->maxpts - context->minpts, 
>> avctx->sample_rate * (int64_t)avstream->time_base.num, 
>> avstream->time_base.den);
>>
>> untested of course but it should work approximately that way ...
> 
> Thanks, it works, with a little correction: the duration of the last 
> packet must be added to the difference in pts. I assumed the 
> pkt->duration is in the same unit as the timestamps.
> 
>>
>>>> then run the regression tests and send a patch which updates the 
>>>> checksums
>>> Not clear how to proceed here, do you mean a subsequent patch, or in 
>>> the same?
> 
> I havent forgotten to make test, and of course the changes breaks 
> regression since it adds 12 bytes to every non-PCM wav. I will post the 
> final version with new checksums when the rest is approved.
> 
Oops, forgot the attachment.

-- 
Michel Bardiaux
R&D Director
T +32 [0] 2 790 29 41
F +32 [0] 2 790 29 02
E mailto:mbardiaux at mediaxim.be

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