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--aps -b192 with 128/160 frames?, How did this happen?
getID3()
post Oct 7 2003, 23:00
Post #1


getID3() developer


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Posts: 252
Joined: 20-September 02
From: Kingston, ON
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I came across a sample file that I'm having trouble figuring out how it was encoded. According to the LAME tag, it's encoded with LAME v3.92 and while v3.92 doesn't actually store the preset used, according to the rest of the info in the LAME tag it appears to be encoded using --alt-preset standard (vbr_quality = 78; vbr_method = vbr-old / vbr-rh; noise_shaping = 2; stereo_mode = joint; ath_type = 4; lowpass = 19000). The problem is that the LAME tag also says that a minimum bitrate of 192kbps was specified, and yet the actual bitrates are distributed like this (which looks like normal --aps):
QUOTE
128 -> 4
160 -> 963
192 -> 2658
224 -> 817
256 -> 246
320 -> 65
I haven't been able to replicate how the LAME tag has minimum bitrate of 192 but 128 and 160 frames appear in the file. I've tried:
--alt-preset standard -b 192
--alt-preset 192
--abr 192


Any ideas anyone?


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getID3() = PHP audio & video metadata parser: http://getid3.sourceforge.net
Current version: v1.7.0 (released January 19, 2004)
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getID3()
post Oct 8 2003, 16:55
Post #2


getID3() developer


Group: Developer
Posts: 252
Joined: 20-September 02
From: Kingston, ON
Member No.: 3413



Aha! Thank you magic75! I just tested with EAC and LAME 3.92 and I could replicate my problem. I ripped a WAV using EAC and then compressed it with --alt-preset standard as the Additional Command Line Options and first 160 then 224 in the bitrate dropdown. Unlike what cOCo said, my files were NOT identical (224kbps vs. 230kbps average on the sample track I used). And the appropriate number (160 or 224) shows in the minimum-bitrate field of the LAME tag. Hmm - I just tried it once more with 192 this time, and the 192 and 224 versions come out with an identical bitrate distribution.

I've experimented a little with command-line parameter order, and this is what I found:
CODE
                                Minimum Bitrate      
       command line             LAMEtag  actual
--alt-preset standard -b192 -h  192kbps  192kbps
--alt-preset standard -h -b192  192kbps  192kbps
-b192 --alt-preset standard -h  192kbps  128kbps
-b192 --alt-preset standard     192kbps  128kbps
-b192 -h --alt-preset standard  192kbps  128kbps
-h -b192 --alt-preset standard  192kbps  128kbps
So to interpret that:
  • If -b??? is specified, it's stored in the LAME tag all the time
  • If -b??? is specified before --alt-preset standard (and I assume other presets too, but I haven't tested) it's ignored and the preset's minimum of 128 is used
  • If -b??? is specified after --alt-preset standard (and I assume other presets too, but I haven't tested) it's respected and the preset's minimum of 128 is overidden by whatever you specify
I'm not sure if this should really be called a bug, but I'd definitely classify it as unexpected behaviour. I think the result should be the same whether you say --alt-preset standard -b192 or -b192 --alt-preset standard. But what is a bug is the fact that the minimum bitrate as stored in the LAME tag is not neccesarily the same one that LAME used when comrpessing!

Can someone look into the LAME source and fix this please? smile.gif


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getID3() = PHP audio & video metadata parser: http://getid3.sourceforge.net
Current version: v1.7.0 (released January 19, 2004)
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