jmvalin: (Default)
Here's a bit more info on the CELT experimental codec I just released. First, the goals. For the past two years, Monty and I have been discussing what the next generation free audio codec would be. Monty's goal is to basically be better than Vorbis in terms of quality vs compression. My main goal, on the other hand is to have a high-quality codec with very low latency, even if it means being less efficient. So, we're trying to combine both into the Ghost codecs. Whether that'll succeed or we have to go with two separate codecs is still an open question. For now, I'm working on CELT, which I hope to be both a low-latency codec, and a noise encoder to be used in a lower bit-rate Ghost codec like Monty wants.

Below is an overview of how CELT works:


It may look a bit hairy, but it's actually a relatively simple idea. The four main ideas are:


  • We use a lapped transform (here an MDCT) on very short windows (128-256 samples)

  • The spectrum is divided in bands and the energy in each band is encoded and kept constant

  • We use a time-domain pitch predictor, with frequency-domain gains

  • The residual is encoded using a pulse codebook



I'll address each of these (and more) in later posts.
jmvalin: (Default)
Speex 1.2beta3 has been tagged and will be up on the website shortly. There should even be Windows builds this time thanks to Alexander Chemeris. I'm expecting the next release to be named 1.2rc1. There's still a few things to address before 1.2, but I'm hoping the libspeexdsp API will be complete for rc1. Stay tuned.

There's another releasing coming up: a new Code-Excited Lapped Transform (CELT) codec prototype. This codec is (for now at least) meant to compete with neither Vorbis, nor Speex. Instead, the primary idea is to reduce latency to a minimum -- currently around 8 ms (compared to ~25 ms for Speex and ~100 ms for Vorbis). Of course, this comes with a price in terms of efficiency, but I'm already surprised the price isn't bigger. I've been mainly focusing on speech, but unlike Speex, I'm hoping this one will handle music as well. For the curious, I've put a 56 kbps CBR music file (original). This is still very experimental and everything is still likely to change, including the exact goals. I'm still trying to figure out how to put psychoacoustics into that. Stay tuned for the release of version 0.0.1 (or should I use a negative version number to make it clearer it's experimental?).

CELT is based on a paper I submitted to ICASSP and which I'm hoping will be accepted so I can make it available to everyone. The only difference is that the ICASSP paper was based on the FFT (non critically sampled), whereas this version is based on the MDCT. One part that is already published though is Timothy's explanation of the pulse codebook encoding along with some source code. Now, here's a challenge. Who can beat the algorithm on Timothy's page? Simply stated, the idea is to enumerate all combinations of M pulses in a vector of dimension N, knowing that pulses have unit magnitude and a sign, but all pulses at the same position need to have the same sign.

Updated: The full source for CELT is available at: http://downloads.us.xiph.org/releases/celt/celt-0.0.1.tar.gz or through git at http://git.xiph.org/celt.git

Updated again:: Speex 1.2beta3 is out.

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jmvalin

June 2017

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