[FFmpeg-devel] [PATCH] als in mp4

[Baptiste Coudurier]

Hi Jai,

On 4/9/2009 12:06 AM, Jai Menon wrote:
> On Wed, Apr 8, 2009 at 10:38 AM, Baptiste Coudurier
> <baptiste.coudurier at gmail.com> wrote:
>> Hi Jai,
>>
>> On 4/6/2009 5:12 AM, Jai Menon wrote:
>>> On Fri, Apr 3, 2009 at 11:28 PM, Baptiste Coudurier
>>>
>>> [...]
>>>
>>> diff --git a/libavcodec/mpeg4audio.c b/libavcodec/mpeg4audio.c
>>> index 9c7e02b..b391609 100644
>>> --- a/libavcodec/mpeg4audio.c
>>> +++ b/libavcodec/mpeg4audio.c
>>> @@ -55,6 +55,8 @@ int ff_mpeg4audio_get_config(MPEG4AudioConfig *c, const uint8_t *buf, int buf_si
>>>      c->object_type = get_object_type(&gb);
>>>      c->sample_rate = get_sample_rate(&gb, &c->sampling_index);
>>>      c->chan_config = get_bits(&gb, 4);
>>> +    c->absolute_channels = 0;
>>> +    c->bits_per_sample   = 0;
>>>      c->sbr = -1;
>>>      if (c->object_type == 5) {
>>>          c->ext_object_type = c->object_type;
>>> @@ -80,5 +82,11 @@ int ff_mpeg4audio_get_config(MPEG4AudioConfig *c, const uint8_t *buf, int buf_si
>>>                  get_bits1(&gb); // skip 1 bit
>>>          }
>>>      }
>>> +    if (c->object_type == AOT_ALS) {
>>> +        c->absolute_channels = 1;
>>> +        c->sample_rate       = AV_RB32(buf + 10);
>>> +        c->chan_config       = AV_RB16(buf + 18) + 1;
>>> +        c->bits_per_sample   = (((buf[20] >> 2) & 0x07) + 1) << 3;
>>> +    }
>> There is something I don't understand according to the code above,
>> position might not be fixed depending on several conditions.
>>
>> How can you read at fixed position in "buf" ?
>> It would depends on what has been read.
> 
> very sorry, i didn't quite get what you mean. extradata is the
> audiospecificconfig object and the code parses required values from
> the alspecificconfig, after skipping an additional 5 bits of padding
> (Table 1.15 ? Syntax of AudioSpecificConfig - 14496-3 sp01).

Well, first you need to ensure buf size is > 20.
Let me try to explain better:

Code will read more bits depending on some values, for example
get_sample_rate, if escape is used, will read 24 bits more.

I wonder why then, the sample rate value for ALS would be at fixed
buf[10] position since it will depends on wether escape is used or not.

>> Futhermore, I don't get why ALS does not use correctly sample rate like
>> others AOT. Can you please explain/quote specs ?
> 
> i could remove the samplerate and bits_per_coded_sample part. however,
> setting no. of channels this way is correct.
> Table 1.19 ? Channel Configuration - 14496-3 sp01
> 0	-	-	defined in AOT related SpecificConfig
> So i think that part of the hunk is correct.

I don't have latest specs unfortunately, but if specs says that sample
rate is set in ALSSpecific config, then it's fine. Though I wonder why
since sample rate can be anything fitting in 24 bits using escape...

Are the new fields really necessary for now ? They could be added later.

-- 
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