[vorbis] Difficult sample for vorbis w/ audible artifacts
dibrom at moscow.com
Fri Jun 15 11:17:56 PDT 2001
I came across an interesting test sample the other day while trying to
compress some of my music and trying out different encoders. You can grab
the sample in question from here:
drone_clip.zip is a 9 second clip of the track where the artifact is most
apparent, drone_short.pac is a 1 minute clip of track. The sample is from
the song "Drone" from "Autechre - Peel Session". The longer clip is
compressed with LPAC which can be found here:
Basically, what happens is when encoding this file at lower bitrates with
vorbis beta 4 there is some very bad pre-echo on the sharp attacks. I have
been told that pre-echo handling is still being worked on though so maybe
there is not much that can be done yet. I was told something about wavelets
being implemented in the future, possibly after 1.0, and that this might
make a difference? The majority of the pre-echo does go away as the bitrate
increases but its still not perfect. The other artifact doesn't go away no
matter what the bitrate is (I tried -b 350). It sounds like low frequency
noise or vibration, or maybe some sort of "squeak" on the bass pulses.
I have also tested this sample with both mp3 and AAC and they both did quite
poorly, the only codec that seems to be able to handle this perfectly is
mp+/mpc. Vorbis is in my opinion the second best though on this sample
though, by quite a bit. I was quite surprised to see how well it actually
did after most others failed. Anyway, maybe one of the developers can take
a look at this sample and see if there is anyway to improve vorbis to handle
this better or something like that. I have heard of some other low
frequency artifacts on heavy bass notes with vorbis in the past but I'm not
sure if they were supposed to have been addressed yet or not, and I'm not
positive this case is caused by the same thing either.
There has already been some discussion about this clip on the r3mix.net
forum for those interested, though it has mainly been centered around mp3.
There is some mention of testing it with vorbis in there however:
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