I am working on HE AAC decoder. Input files are aac(with sbr extension) parsed from .mp4 files. Now since .mp4 files have an AudioSpecificConfig() for SBR which have bits for sbr_present_flag and bs_samplerate_mode, where can I find find bs_samplerate_mode / bDownSampleSbr within an aac + sbr file.

I need more information about sbr header for implicit signaling within aac files.

Please help.

If my understanding is right, that's what I implement: the double of the AAC frequency, when you detect SBR present.

Correct, but how will you detect whether SBR present or not when you have no AudioSpecificConfig() specifically for sbr in the bistream which does gives following info
{
sbr_present_flag;
bs_samplerate_mode;
bs_reserved;
}
This is the case when you have an aac file not a .mp4 for decoding. So where can we find the bits representing sbr_present_flag and bs_samplerate_mode.