[WIP] Spectral Band Replication for HE-AAC v1

[devongovett]

Spectral Band Replication was added to the AAC-LC profile to create HE-AAC v1. It effectively allows cutting the sample rate in half by reconstructing the high-frequency bands using harmonics from the lower/mid frequencies and some small side data (~2kbps). This greatly increases the compression ratio while retaining decent quality at low bit rates. Here is a high level overview, and here is the MPEG/ISO spec.

This is a work in progress, but it kinda works on a few files I tested. Definitely needs more testing though. I'm sure there are tons of bugs.

TODO:

• Implement non-coupled and single channel decoding.
• Clean up the code and remove debug logging
• Optimize performance. It's really slow right now due to memory allocations during decoding.
• Polyfills for typed array methods not supported cross browser (e.g. sort and fill)?
• Generate some tables at initialization time instead of hardcoding them to reduce code size.
• Test against more files

[devongovett]

This is quite a bit faster now. It used ~20% CPU on my iMac before (decoding in real time), and now uses ~3-4%, which isn't much higher than simple AAC-LC decoding. Implemented optimizations by pre-allocating memory, and using flattened multidimensional typed arrays rather than arrays of (arrays of...) typed arrays. Made a babel plugin to make this easier: https://github.com/devongovett/babel-plugin-multidimensional-array.

That brought CPU usage down to ~10%. The other optimization was switching the QMF filter banks to use IMDCT instead of multiplying by a matrix. This is based on a procedure to implement the filterbank on the DCT-IV described in Hsu, H.W. et al. "A Complex Quadrature Mirror Filterbanks for MPEG-4 HE-AAC", and implementation in ffmpeg. http://sites.google.com/site/wenchiehlee1020/Papers/ComplexFB_AES_121paper144.pdf

[jimmywarting]

Progress?