[FFmpeg-devel] [Ffmpeg-devel] [PATCH] GSM-MS decoder and encoder

[Baptiste Coudurier]

Michel Bardiaux wrote:
> Baptiste Coudurier wrote:
>> Hi,
>>
>> Michel Bardiaux wrote:
>>> Baptiste Coudurier wrote:
>>>> Michael Niedermayer wrote:
>>>>> Hi
>>>>>
>>>>> On Mon, Feb 19, 2007 at 11:59:14AM +0100, Michel Bardiaux wrote:
>>>>>> Michael Niedermayer wrote:
>>>>>>> Hi
>>>>>>>
>>>>>>> On Sat, Feb 17, 2007 at 03:42:52PM +0100, Michel Bardiaux wrote:
>>>>>>>> Michael Niedermayer wrote:
>>>>>>>>> Hi
>>>>>> [snip]
>>>>>>>>> is 13000 exactly correct isnt it 13200 for CODEC_ID_GSM?
>>>>>>>> Yes and no. The last 4 bits of each frame are not actually used, they
>>>>>>>> come on stage only because we (both libgsm and lavc) use a
>>>>>>>> byte-oriented
>>>>>>>> API, and files made of bytes. They should be considered as container
>>>>>>>> overhead in TOAST files, hence not part of the codec bitrate.
>>>>>>> but with this argumentation we would have to subtract the padding
>>>>>>> bits from
>>>>>>> mpeg from the bitrate too and thats something we dont 
>>>>>> I think you're talking codec-level padding here, but I wrote of
>>>>>> container overhead.
>>>>>>
>>>>>>> also it would cause
>>>>>>> problems for containers which expect the bitrate well to be the
>>>>>>> bitrate of
>>>>>>> what they get, not to be slightly less due to some padding bits
>>>>>>> they dont
>>>>>>> know about ...
>>>>>> AFAIK currently only some MS containers (AVI, WAV) accept GSM, and
>>>>>> then only MS-GSM, which does not have the problem. The only
>>>>>> container for non-MS-GSM is TOAST (currently not implemented) and
>>>>>> that one knows about the 4 extra bytes.
>>>>> well a grep for gsm shows a hit in aiff.c so it seems it is supported
>>>>> in a
>>>>> non toast format currently and as there is no gsm specific code in it
>>>>> i would
>>>>> guess it will end with 13200 as bitrate
>>>>>
>>>>> anyway i wont fight about this appl the patch, i will remove the
>>>>> 13000 check
>>>>> when it breaks something
>>>>>
>>>> I was fixing agsm in mov, and this bitrate check cause init to fail for
>>>> decoding (encoding shares the function), IMHO we don't care about
>>>> bitrate when decoding.
>>>>
>>>> Can I remove it ?
>>>>
>>> Oops, forgot I am maintainer for libgsm!
>>>
>>> Since bitrate is also irrelevant for encoding, what about this?
>>>
>>>
>>> ------------------------------------------------------------------------
>>>
>>> Index: libavcodec/libgsm.c
>>> ===================================================================
>>> --- libavcodec/libgsm.c	(revision 12579)
>>> +++ libavcodec/libgsm.c	(working copy)
>>> @@ -36,8 +36,10 @@
>>>  #define GSM_FRAME_SIZE 160
>>>  
>>>  static av_cold int libgsm_init(AVCodecContext *avctx) {
>>> -    if (avctx->channels > 1 || avctx->sample_rate != 8000 || avctx->bit_rate != 13000)
>>> +    if (avctx->channels > 1 || avctx->sample_rate != 8000)
>>>          return -1;
>>> +    if (avctx->bit_rate != 13000)
>>> +        av_log(avctx, AV_LOG_WARNING, "Bit rate %d instead of expected 13000 - ignored\n");
>>>  
>> Well, Im not really ok to annoy user with a false message, when decoding
>> this is irrelevant.
> 
> It is not false. IMO a file with any bitrate other than 13000 is 
> corrupted - well, at least it is not strictly according to specs. Yes, 
> this is debatable, but to debate it we need facts, that is a collection 
> of files with that problem. Printing the message will inform us about that.

In wav/avi maybe, but this is not the case of aiff/mov, which does not
supply bitrates, so this will annoy them everytime.

Now IMHO, strictly speaking, a wav header with bit_rate field set to 0
does not necessarly mean that gsm bitstream is corrupted, so again I
think this is irrelevant.

You could maybe check that bit_rate is actually != 0 && != 13000

>> What is this check good for, since bitrate is irrelevant ?
>>
> 
> Remove the message on encoding, and very soon we will have someone 
> complaining that he used -acodec libgsm -ab 26000 but the output file is 
> too small and WMP says it is 13kbits...
> 

Well, Im not against removing it for encoding, I'd like it removed for
decoding :>

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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