0 but it is recommended to have a preroll config
ISO IEC 23003-3 (USAC):
configLen is preroll frames related
Config element length, and out of band
UsacConfig may be not the same as the one for preroll frames, it is recommended to have the preroll configuration beside the preroll frames.
ISO IEC 23003-3:
Configure or update the USAC encoder in order to put a
Config element in the
AudioPreRoll element, then reencode the content.