[FFmpeg-devel] [PATCH] als in mp4

[Jai Menon]

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).

> 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.

[...]

-- 
Regards,

Jai