Nov. 11th, 2008

jmvalin: (Default)
It's been a while since the last time I discusses CELT, so at last, here's an update. A while ago, I was working on a low-complexity "profile" of CELT. The idea is to disable the use of the pitch predictor, which is quite costly in terms of complexity. To help speed things up, I also changed the allocator to do the conversion from bits to pulses one band at a time instead of doing it jointly for all bands at once. This decreases the complexity, while making the allocation a bit less optimal -- in theory. In practice, it means that for higher rates where bands require a large number of bits, the encoding can actually be more efficient because no bits are wasted. Because of that, I was able to replace all 64-bit arithmetic from CELT by 32-bit splits. On top of that, Timothy (derf) managed to -- again -- save some computation in the pulse encoding. The result is that in low-complexity mode, it takes about 1% CPU to encode and decode a 44.1 kHz mono stream at 64 kbit/s (on my 2 GHz box).

Here's what lies ahead now. I'd like to slowly work towards freezing the bit-stream. But there's a few things I want to do before even thinking about a freeze:

- Dynamic bit allocation
Right now, the bit allocation in each band remains about the same for every frame. I'd like to change that and allow more bits in the regions of the spectrum that are hard to encode at any given time. It's not as easy as it looks because: 1) you need to figure out the best allocation based on psychoacoustics and 2) You need to *encode* the allocation information compactly enough that it doesn't waste all you saved from the dynamic allocation. So far, my attempts at 1) haven't been very successful.

- Folding decision
To prevent "birdie" artefacts, we use a certain amount of spectral folding that acts as a noise floor. In most cases, this improves quality, but for very tonal signals (e.g. glockenspiel), it transforms a pure tone into noise, which is annoying. So I'd like to be able to turn that feature on or off based on the data, but again, it's not simple.

- Stereo coupling
CELT already does stereo. It does it by encoding the energy independently for each channel and doing (sort of) M-S encoding of the "residual". This works, but probably doesn't save much compared to using two mono streams. So I want to see how it can be improved. There's already some (disabled) code to do intensity stereo, but maybe there's more that can be done.

Of course, I only have a vague idea of how to do the three things I listed above, so suggestions are welcome.

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