[FFmpeg-devel] [PATCH] AAC Decoder round 4

[Robert Swain]

2008/7/30 Robert Swain <robert.swain at gmail.com>:
> 2008/7/27 Michael Niedermayer <michaelni at gmx.at>:
>> On Fri, Jul 18, 2008 at 03:13:38PM +0100, Robert Swain wrote:
>>> Index: libavcodec/aac.c
>>> ===================================================================
>>> --- libavcodec/aac.c  (revision 0)
>>> +++ libavcodec/aac.c  (revision 0)

[...]

>>> +/**
>>> + * Set up ProgramConfig, but based on a default channel configuration
>>> + * as specified in table 1.17.
>>> + */
>>> +static int program_config_element_default(AACContext *ac, int channels)
>>> +{
>>> +    ProgramConfig pcs;
>>> +
>>> +    memset(&pcs, 0, sizeof(ProgramConfig));
>>> +
>>> +    /* Pre-mixed down-mix outputs are not available. */
>>> +    pcs.mono_mixdown_tag   = -1;
>>> +    pcs.stereo_mixdown_tag = -1;
>>> +
>>> +    if(channels < 1 || channels > 7) {
>>> +        av_log(ac->avccontext, AV_LOG_ERROR, "invalid default channel configuration (%d channels)\n",
>>> +               channels);
>>> +        return -1;
>>> +    }
>>> +
>>
>>> +    /* default channel configurations:
>>> +     *
>>> +     * 1ch : front center (mono)
>>> +     * 2ch : L + R (stereo)
>>> +     * 3ch : front center + L + R
>>> +     * 4ch : front center + L + R + back center
>>> +     * 5ch : front center + L + R + back stereo
>>> +     * 6ch : front center + L + R + back stereo + LFE
>>> +     * 7ch : front center + L + R + outer front left + outer front right + back stereo + LFE
>>> +     */
>>> +
>>> +    if(channels != 2)
>>> +        pcs.che_type[ID_SCE][0] = AAC_CHANNEL_FRONT; // front center (or mono)
>>> +    if(channels > 1)
>>> +        pcs.che_type[ID_CPE][0] = AAC_CHANNEL_FRONT; // L + R (or stereo)
>>> +    if(channels == 4)
>>> +        pcs.che_type[ID_SCE][1] = AAC_CHANNEL_BACK;  // back center
>>> +    if(channels > 4)
>>> +        pcs.che_type[ID_CPE][(channels == 7) + 1]
>>> +                                = AAC_CHANNEL_BACK;  // back stereo
>>> +    if(channels > 5)
>>> +        pcs.che_type[ID_LFE][0] = AAC_CHANNEL_LFE;   // LFE
>>> +    if(channels == 7)
>>> +        pcs.che_type[ID_CPE][1] = AAC_CHANNEL_FRONT; // outer front left + outer front right
>>
>> i tried to simplify this:
>>
>> switch(channels){
>>    case 7: pcs.che_type[ID_CPE][                  1] = AAC_CHANNEL_FRONT; // outer front left + outer front right
>>    case 6: pcs.che_type[ID_LFE][                  0] = AAC_CHANNEL_LFE;   // LFE
>>    case 5: pcs.che_type[ID_CPE][(channels == 7) + 1] = AAC_CHANNEL_BACK;  // back stereo
>>
>> but then i realized your 7 ch case really uses 8 channels...
>
> Mmm, I never noticed that before. Strange. I'll have to have a look to
> see what the spec says about program configurations.

Strange as it is, I checked this and table 1.17 does indeed state that
channels == 7 defines the configuration for 7+1 channels.

Rob