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

[Pio2001]

Right, right,
It started when I was about to write "the alt presets are better than any command line", but it occured to me that we were speaking about alt preset CBR, and it didn't make sense.

Immo, the global lowpass needed depends on the hearing ability of the listener. There have been some tests with sweep tones and music lowpassed here recently, to see how high people could hear, but they are affected by the fact that most soundcard resample, and therefore produce low frequency sounds when high frequency ones are fed into them (the "alias" effect).

The lowpass used in the command line and the one used in the alt preset can't be compared, because they were developed at a different time by different people.
Lowpassing is a powerful way to reduce the bitrate of a VBR file, that's why lowpass filters are used in the presets.