a response to a growing rumor...
a response to a growing rumor...
Feb 12 2002, 00:36
Joined: 26-August 02
From: Nottingham, UK
Member No.: 1
Normally I wouldn't attempt to address an issue in this manner, but since it is getting a bit out of hand, and usually on boards I'm not participating in (or have little desire to participate in), I'll try and address it officially, once, in the place where it should be the most relevant.
The matter I'm discussing is related to the --alt-presets and their handling of the "stereo image".
There have been some completely unsubstantiated reports and rampant speculation going on in a few threads which I will list below:
3. http://www.hydrogenaudio.org/forums/showth...s=&threadid=759 (I simply hadn't gotten around to responding to this thread though its on this board).
At any rate, I'll try to make a few points as clearly as I can.
1. All of the --alt-preset VBR modes are tuned for "stereo image".
2. All of the vbr presets provide better sound quality via joint stereo than LAME on it's own with joint stereo, and in some cases should even sound better than with --nssafejoint, while at the same time providing a lower bitrate.
3. The --alt-presets do not, by design, make any sacrifice in regards to stereo image to keep bitrate down. Anyone who tells you this has no idea what they are talking about. I should know since I actually wrote the code and designed the presets.
4. An extremely high degree of stereo frames is not always needed to achieve good sound quality. I challenge anyone who believes that --alt-preset standard has poor stereo seperation, on a common basis (as a few unsubstantiated claims imply), to provide me with direct evidence of this.
5. Joint stereo is needed even at bitrates of 320kbps to achieve the best sound quality in some critical cases. Forcing stereo on everything up to 320kbps and then forcing joint stereo does not fix the problem (as user implies in one of those threads). I've tried this before.
6. There seems to be a misconception that all that the --alt-presets improve on are pre-echo. This is sorely mistaken. Indeed they do improve on pre-echo and impulse handling to a fairly large degree, but they also improve upon:
- joint stereo handling (serioustrouble is a prime example)
- dropout prevention (2nd_vent_clip is a prime example)
- fluttering (gekkou is a prime example)
- knocking (velvet is a prime example)
- ringing (bloodline is a prime example)
- noise pumping (piano, rach_original, etc, are examples)
- rasping (present with noise shaping 2 on some clips like fatboy, or on clean vocals sometimes. Mostly eliminated, even on the most critical samples, with --alt-presets)
And that's just the stuff I can think of off the top of my head.
Now, that's not to say the --alt-presets are perfect. I certainly know they aren't. But they also don't have some massive flaw in regards to stereo image which is present to the degree some people imply. In fact, the only case I've seen which I put any credence in is the few isolated cases which Wombat has found (and provided samples for I might add). I will eventually attempt to address these few samples, but note that these are exceptional cases, not common cases, and as far as I can tell, they are completely unrelated to the other complaints being made. This is especially so since Wombat doesn't describe the artifact as being a collapse of the stereo field (which isn't your typical joint stereo artifact in LAME anyway...).
At any rate, I'm always looking to improve things if I can, but claims must be substantiated which includes providing abx results (which are then verified by other parties) and providing test samples, preferrably multiple ones if you are implying a problem with general behavior.
Not to come across arrogant, but for the most part, I'm the only one who truly understands the workings behind the --alt-preset specific tunings. Not even the other developers have followed my work (though that's by their choice, not mine). The code is available for all to see, but so far I have not seen anyone attempt to reimplement my modifications or to discuss them with me on a technical level. So unless you see someone who is closely related to the work I've done (ie, they have participated in testing, JohnV for example) stating something, or you see me stating something directly about the presets, then chances are whoever is discussing the presets doesn't have the full picture. This is especially true when people begin discussing how the --alt-presets work internally or technically, and especially in relation to joint stereo.
If you see a discussion on another board about these issues, please point people to this thread. If you have a question, please ask me here, you'll likely get a much more correct answer in addition to helping to keep questions about this issue centralized and concise (which will help when the FAQs are created). Speculation is not only wasteful, but it also helps to propogate misinformation such as the old "joint stereo is bad" line of thinking.
Feb 15 2002, 23:08
Joined: 12-October 01
From: the great wide open
Member No.: 277
during playing around with mp3gain, I found following:
- a mp3 song as source encoded with --alt-preset extreme or dm xtreme, containing very much only stereo frames, but this is not important.
1. maximize this song lossless ! with mp3gain, so that it does not clip.
Re-Encode it eg. with alt-extreme
2. lower the gain of the source.
Reencode it with same preset.
Result after reencoding:
1. the maximized song will contain mostly joint stereo frames.
2. the same song with lossless lower gain/volume will contain less js frames, more stereo frames !
Looking with Encspot:
Both songs have after reencoding nearly same bitrate, nearly same bitrate distribution.
Of course only nearly, but nearly exact the same....
That would be well....
But: the distribution of js and ms is very different.
In alt preset extreme (i tested) there has been achieved that all songs result into a comparable average bitrate aorund 256 kbit, independent from gain.
But this was achieved by a compromise in stereo/joint stereo frames.
Loud songs have much joint stereo,
quiet songs much more pure stereo frames.
Even if it is the very same song just changed with lossless gains...
Of course I understand why it is like it is:
the gain change is lossless for one mp3, but reencoding works via wave.
And the 2 resulting waves of same song in my test are very equal, but have different gain....
And encoding these waves is different, because of ath and so on, , so if the quiet song gets encoded with alt extreme, that means target bitrate 256 kbit, and there is plenty space for quiet songs, encoder thinks: quiet parts are under ath....
so encoder offers bitrate for stereo instead of js.
relative ath is different, quiet parts of loud song are listenable...
so much bitrate is required....
so use of js instead of stereo,
I think, that's it.
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