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LAME, CBR, VBR,.... I'm confused!
Immo
post Mar 26 2003, 19:51
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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
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LordSyl
post Mar 27 2003, 19:30
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CBR is shit...always. The entire fact of giving the same bitrate for transients than for silence is...idiotic.
As for quality, when you set CBR mode, you set VQ, CBR (variable quality, constant bitrate) opposed to VBR (constant quality, variable bitrate)

That happens because CBR uses always a fixed bitrate, so it becomes both overkill and lack of bits at the same time: bits that aren't used while encoding are set to null data to fill the specified bitrate, while other times it will run short of bits on harder-to-encode parts.

VBR, however, will decide what shall be encoded and what has to be left, without wasting bits: it can use bitrates as low as 128kbps or as high as 320 if it considers it necessary.

--alt-preset standard, --alt-preset extreme and --alt-preset insane* are code-tweaked: it's impossible to reach them through commandlines [no, do NOT use them!!!] in terms of quality.

*--alt-preset insane is CBR, yes, but it's on the highest bitrate allowed for the mp3 standard, so it won't get short of bits (inside of mp3's specs) , but it also will happily start wasting bits..... ph34r.gif

This post has been edited by Syleth: Mar 27 2003, 19:34
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Posts in this topic
- Immo   LAME, CBR, VBR,.... I'm confused!   Mar 26 2003, 19:51
- - /\/ephaestous   YOU CANT' SAY SOMETHING SOUNDS BETTER BASED ON...   Mar 26 2003, 19:59
- - ff123   I'd say that --alt-preset standard is the pict...   Mar 26 2003, 20:46
- - evereux   QUOTE (/\/ephaestous @ Mar 26 2003 - 06:...   Mar 26 2003, 21:08
- - Pio2001   The left picture must be alt preset standard, beca...   Mar 26 2003, 21:12
- - outscape   i have to agree with /\/ephaestous on this.. ...   Mar 26 2003, 21:18
- - /\/ephaestous   QUOTE (evereux @ Mar 26 2003 - 03:08 PM)QUOTE...   Mar 26 2003, 21:21
- - Immo   Thanks for the replies, folks. All who guessed, g...   Mar 26 2003, 22:11
- - Andavari   QUOTE (Immo @ Mar 26 2003 - 03:11 PM)The poin...   Mar 26 2003, 22:38
- - [proxima]   QUOTE (Pio2001 @ Mar 26 2003 - 09:12 PM)-h -b...   Mar 26 2003, 23:11
- - LordSyl   Please, don't confuse newbies with those stupi...   Mar 26 2003, 23:54
- - Pio2001   Right, right, It started when I was about to write...   Mar 27 2003, 00:30
- - mithrandir   QUOTE (Immo @ Mar 26 2003 - 04:11 PM)A questi...   Mar 27 2003, 01:43
- - NeoRenegade   I just wanted to point out, Immo, that your comman...   Mar 27 2003, 06:46
- - Immo   Good afternoon, thanks for the tip, NeoRenegade, ...   Mar 27 2003, 13:08
- - Pio2001   QUOTE (Immo @ Mar 27 2003 - 03:08 PM)I suppos...   Mar 27 2003, 14:28
- - Immo   Sorry, you're right! What I meant was th...   Mar 27 2003, 15:12
- - Lev   /\/ephaestous: QUOTE YOU CANT' SAY SOMET...   Mar 27 2003, 17:36
- - Gecko   Immo: VBR isn't about preserving HF content. I...   Mar 27 2003, 17:58
- - Immo   Lev: it gets even more ironic when the author make...   Mar 27 2003, 18:39
- - deej_1977   QUOTE (Pio2001 @ Mar 27 2003 - 03:28 PM)Addin...   Mar 27 2003, 18:47
- - LordSyl   CBR is shit...always. The entire fact of giving th...   Mar 27 2003, 19:30
- - sony666   CBR is not "always shit", plz stay serio...   Mar 28 2003, 01:58
- - NeoRenegade   The lame development team works on the encoder and...   Mar 28 2003, 03:29
- - roman   To Immo About your 1st pictures... Can you think a...   Mar 28 2003, 08:38
- - Oge_user   QUOTE (Immo @ Mar 27 2003 - 12:08 PM)I tried ...   Mar 28 2003, 10:52
- - DigitalDictator   sony666 QUOTE If you plan to use your encodes for...   Mar 28 2003, 11:57
- - Immo   Roman: I guess ti prav explaining how VBR acts ...   Mar 28 2003, 19:12
- - Immo   Oge_user:I just tried it, the results weren't ...   Mar 28 2003, 19:25
- - sony666   QUOTE (DigitalDictator @ Mar 28 2003 - 11:57 ...   Mar 28 2003, 23:48


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