dBpower LAME 320 CBR - "fast" encode fewer artifacts - why? Options

[superbu]

I'm not sure if this is in the proper forum, although I did do ABX listening tests, which I'll post below. But all the first one shows it that two different 320kb LAME encodes were ABX-able.

At any rate, I can't figure something out: While comparing 320kb LAME encodes of the popular killer sample "Show Me Your Spine," encoded using dBpoweramp, I found that the well-known "sandpaper scratching" artifact that is prominent in the first few seconds of the song was much more noticeable when I selected "Slow (High Quality)" than when I selected "Fast (Low Quality)." To my ears, the "Fast (Low Quality)" setting produced a result much closer to transparency (though definitely NOT transparent), while with the "Slow (High Quality)" setting the artifacts were VERY obvious.

I can't prove this other than to post the results of comparing those two compressed samples, where I scored 12/12 in differentiating them. There was a faint "cripsy" artifact at a particular point in the "slow" encode that was not present in the "fast" encode, and I'm also hearing that "scratching" sound more prominently in the Slow encode. Again, I scored 12/12:

foo_abx 1.3.4 report
foobar2000 v1.1.2
2011/11/21 00:56:55

File A: L:\Music Folder\1 MP3 Files for flash drive\Show_Me_Your_Spine__Sample (dB 320 quality fast).mp3
File B: L:\Music Folder\1 MP3 Files for flash drive\Show_Me_Your_Spine__Sample (dB 320 quality slow).mp3

00:56:55 : Test started.
00:59:27 : 01/01 50.0%
01:02:12 : 02/02 25.0%
01:04:19 : 03/03 12.5%
01:05:31 : 04/04 6.3%
01:07:33 : 05/05 3.1%
01:08:38 : 06/06 1.6%
01:09:28 : 07/07 0.8%
01:10:02 : 08/08 0.4%
01:10:46 : 09/09 0.2%
01:11:23 : 10/10 0.1%
01:12:09 : 11/11 0.0%
01:12:55 : 12/12 0.0%
01:12:59 : Test finished.

----------
Total: 12/12 (0.0%)

Since I could also ABX the "fast" version 11/11 times with the original WAV, I didn't bother to ABX the "slow" encode with the WAV:

foo_abx 1.3.4 report
foobar2000 v1.1.2
2011/11/21 01:18:16

File A: C:\Users\superbu\Music\1 WAV Files For Itunes Import\Show_Me_Your_Spine__Sample (TRUE WAV).wav
File B: L:\Music Folder Backup\1 MP3 Files for flash drive\Show_Me_Your_Spine__Sample (db 320 quality fast).mp3

01:18:16 : Test started.
01:19:37 : 01/01 50.0%
01:20:24 : 02/02 25.0%
01:21:04 : 03/03 12.5%
01:21:36 : 04/04 6.3%
01:22:11 : 05/05 3.1%
01:23:00 : 06/06 1.6%
01:23:27 : 07/07 0.8%
01:24:33 : 08/08 0.4%
01:25:35 : 09/09 0.2%
01:26:04 : 10/10 0.1%
01:26:47 : 11/11 0.0%
01:26:52 : Test finished.

----------
Total: 11/11 (0.0%)

Anyway, can someone explain to me how it's possible that the "fast (low quality)" encode could have less prominent artifacts than the "slow (high quality)" encode? I thought the slow encode was supposed to be of better quality.

This post has been edited by superbu: Nov 22 2011, 05:12