LAME, CBR, VBR,.... I'm confused! Options

[Immo]

Hi!

This is my first post on this board, and the reason why I'm posting is to find an explanation for one particular, interesting issue of LAME MP3 compression...

I took a sample from a live instrumental concert with clear high-frequency sounds. I compressed it with two different settings, using LAME 3.91.

-b 192 -m j -h -q 0 (= 192kbps constant, joint stereo, quality-optimised, max. quality) and
--alt-preset standard, which produced a slightly larger file, the bitrate averaging at 200kbps.

The results were both quite good and I couldn't tell the difference in sound - maybe I lack the equipment.

I performed a spectral frequency analysis on each of the results.

Here's what it looked like:


Let's look at the left image: the frequency meter suggests thet there's a sharp cut at 16kHz. Nevertheless, some sounds are given higher frequencies, but then there is another sharp cut just under 19kHz, above which nothing can be seen.

Now the right image: up to about 16kHz the frequencies are generally preserved. Then they gradually become thinner in a varying manner, depending, i suppose, on their importance. There is no sharp cut below ~19.8kHz.

I do not know much abot audio compression - maybe I'm misunderstanding the whole thing - so I'd like you to ask you first: which picture do you think corresponds to which compression method?

Immo

[Immo]

Thanks for the replies, folks.

All who guessed, guessed right about --alt-preset standard - it's the picture on the left. But there's something bugging me.

I really didn't say anything about which sample sounds better - either my ears or my headphones are too bad for that. But REGARDLESS of the audible result it was a surprise for me to see that the lowpass filter, considering the two clearly visible boundaries, is applied much earlier in a VBR algorythm than in CBR. A question arises: how is the advantage of VBR being used? I earlier thought that VBR provides the possibility to preserve a broader range of frequencies where it is needed and cut down on them where it isn't.
As far as I understand, the VBR codec (like any other) "considers" the lower frequencies more important and cuts off the higher ones, which is, of course, a good idea to some extent, as people don't hear them very well. But then why does the CBR codec leave so much of them? It could as well apply a lower lowpass filter if it's such a good idea, but it doesn't. Why? After all, it's the same version of the same encoder, so the applied techniques are surely coordinated.
Doesn't this fact surprise you a bit? Just a little bit???
Or have I just gone mad?

The point is, I don't believe that the marginally higher frequencies preserved in the CBR really destroy the overall sound quality, especially they wouldn't in a VBR codec, where there is a possibility to assign more bits at a particular point...

Regards,
Immo

This post has been edited by Immo: Mar 26 2003, 22:24