jmvalin: (opus)

Three years ago Opus got rated higher than HE-AAC and Vorbis in a 64 kb/s listening test. Now, the results of the recent 96 kb/s listening test are in and Opus got the best ratings, ahead of AAC-LC and Vorbis. Also interesting, Opus at 96 kb/s sounded better than MP3 at 128 kb/s.

Full plot

jmvalin: (Default)
Monty has just finished a very interesting CELT demo that covers most of the techniques used in CELT and their history. It also includes a large number of audio samples, including comparisons with Vorbis and various flavours of AAC. CELT has come a long long way in the past three years and even in the past three months, quality has gone up significantly, to the point where it sounds better than Vorbis on many (most?) samples and even comparable to HE-AAC at 64 kb/s. The target is to freeze the bit-stream early January for integration within the Opus codec, but there may still be a few quality improvements we can make before that -- not to mention all the encoder-side improvements we can make even after the bit-stream is frozen.
jmvalin: (Default)
Recently, I was curious about how CELT and Vorbis differ in the way the allocate bits. Now, CELT's bit allocation is really explicit with a fixed number of bits per band. This is not quite the case of Vorbis, so a comparison isn't straightforward. What I've done is I've ran some audio (mono version of the audio I used in my previous post) through Vorbis and measured the SNR as a function of frequency. By dividing the SNR by 6 db/bit, I can get the (approximate) bit allocation. The result (smoothed a bit) is shown below for encoding quality -1 to 10.



Now, these are the curves currently used by CELT for its bit allocation:




Among the differences are:
1) The Vorbis allocation lines for different rates are nearly parallel, meaning that starting from a certain allocation, bits are added/removed nearly uniformly when changing the bit-rate
2) Vorbis allocates a lot of bits to very low frequencies, and then there is a sharp drop-off around 400 Hz.
3) In the mid-high range, the Vorbis allocation is much flatter than CELT

Now I tend to trust that the Vorbis allocation has been decently tuned, so the question is whether the differences in allocation are due to fundamental differences between Vorbis and CELT or just to bad tuning of CELT so far. I suspect there's a bit of both. I've actually created an exp_vorbis_tuning branch to find out. I just took the Vorbis data and turned that into CELT bit allocation data just to see what it would do. I expected something terrible, but it actually sounds quite decent. In some circumstances, it sounds a bit worse than the original CELT tuning, but I think in other cases it actually sounds better. More investigation needed...

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jmvalin

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