High bitrate setting for aac codec

[yilun-zhangs]

@jwcullen

  // Set bitrate based on the equation recommended by the documentation.
  RETURN_IF_NOT_OK(AacEncErrorToAbslStatus(
      aacEncoder_SetParam(
          encoder, AACENC_BITRATE,
          3 * num_channels * num_samples_per_frame * output_sample_rate / 2),
      "Failed to configure encoder bitrate."));

This is the bitrate setting reference from iamf-tools(aac_encoder.cc), the aac packet after encoding will contain more redundant data,
which will lead to larger packet size than ffmpeg's.

Following reference is from ffmpeg:

        if (avctx->bit_rate <= 0) {
            if (avctx->profile == AV_PROFILE_AAC_HE_V2) {
                sce = 1;
                cpe = 0;
            }
            avctx->bit_rate = (96*sce + 128*cpe) * avctx->sample_rate / 44;
            if (avctx->profile == AV_PROFILE_AAC_HE ||
                avctx->profile == AV_PROFILE_AAC_HE_V2 ||
                avctx->profile == AV_PROFILE_MPEG2_AAC_HE ||
                s->eld_sbr)
                avctx->bit_rate /= 2;

BWT, IAMF spec specify to use AAC-LC.

[jwcullen]

Good point. Looks wrong. In current build you could try changing bitrate_mode. It matches FDK AAC's API here. So any mode 1-5 should trigger a variable bit rate modes. Which ignores the calculation above.

That default bit rate equation was supposed to come from the library documentation. I see 1.5 bits per frame for AAC-LC (§2.9.1 of doc). But it looks like the calculation is bad. I think it shouldn't have the factor of num_samples_per_frame.

And even so, we could expose per substream bit rate settings. Similar to #11, but also for AAC.