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Topic: What is lowpass width preset in LAME ? (Read 6420 times) previous topic - next topic
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What is lowpass width preset in LAME ?

I've been searching but to no avail. I am having trouble understanding what this means.

http://lame.cvs.sourceforge.net/viewvc/lam...detailed.html#m
Here it says Specify the width in Hz of the decaying curve of the lowpass.
I dont understand this. I did try encoding .mp3 without lowpass width and with lowpass width - 14khz and the difference was that in Plot Spectrum in Audacity the file with lowpass width was only going till 19.5 khz and the file without it was going up to 20 khz. So fi someone can explain this I would be thankfull.

What is lowpass width preset in LAME ?

Reply #1 –
I assume its how rapidly the low pass cuts on.

What is lowpass width preset in LAME ?

Reply #2 –
Why are you messing with these settings? The defaults are tuned for best quality.

What is lowpass width preset in LAME ?

Reply #3 –
Quote from: saratoga on
I assume its how rapidly the low pass cuts on.

Yes, ok, I've searched a bit more and came to the same conclusion. Can someone tell me what number goes here:
--lowpass-width
What am I suppose to put in? If I have a 320 CBR file that means it's lowpass is 20.5 kHz. So if I put in --lowpass-width 19.5 that would mean it will start to fade at that point and fade out totally at 20.5 ?
Also it says the minimum number must be at least 75% of the total band which would in this case be 15.5 kHz ?

Did I get it right?

Also would you recommend using this because that way the high frequencies would fade out more naturally and produce better sound compared to just cutting off suddenly? I guess that at 20.5 kHz you wouldn't notice the cut but at lower frequencies you might...

What is lowpass width preset in LAME ?

Reply #4 –
Its the width of the lowpass transition band, not the center. If you want to change the center the command is, I believe, --lowpass.

And you aren't supposed to be using these commands at all. If you have to ask do not use them at all.

What is lowpass width preset in LAME ?

Reply #5 –
Quote from: saratoga on
Its the width of the lowpass transition band, not the center. If you want to change the center the command is, I believe, --lowpass.

And you aren't supposed to be using these commands at all. If you have to ask do not use them at all.

So that means if lowpass width is set at 16.5 it will start to fade out there and fade out completely at 20.5 I guess.

I just want to learn what it means exactly. That's all.

What is lowpass width preset in LAME ?

Reply #6 –
blue line: lame --lowpass 20500
red line: lame --lowpass 20500 --lowpass-width 16500



It seems that for '--lowpass 20500 --lowpass-width 16500' the lowpass starts at 20500-16500 = 4000 Hz.

What is lowpass width preset in LAME ?

Reply #7 –
Quote from: belushka on
Can someone tell me what number goes here:
--lowpass-width
What am I suppose to put in?
Nothing because we should all trust the developers to make the right decisions until proven otherwise. In case you didn’t know, without the user supplying a parameter, a sensible default value will be used.

Quote
If I have a 320 CBR file that means it's lowpass is 20.5 kHz. So if I put in --lowpass-width 19.5 that would mean it will start to fade at that point and fade out totally at 20.5 ?
No, as lvqcl has illustrated. The width, intuitively, is the distance from the named cutoff frequency (fc) where attenuation actually begins.

Having said “intuitive”, i’s worth noting that the terminology used here by LAME seems to differ from the standard definition of fc, which is the point at which attenuation has already reached –3 dB: not –40 dB or whatever is the reference above. There’s never actually a point below which there is zero attenuation. How much gets attenuated below fc is, as far as I know, inversely proportional to the steepness (a.k.a. order, number of poles, etc.) of the filter. There are sophisticated mathematical and/or electronic elaborations on this definition of fc, but I’m not on that level yet.

Quote
Also would you recommend using this because that way the high frequencies would fade out more naturally and produce better sound compared to just cutting off suddenly?
No. A properly designed filter will be inaudible around fc (no audible ringing/resonance), regardless of its value. Assuming that standard is met, one ideally wants the steepest filter possible so that, again, attenuation before fc does not encroach into the audible range.

Quote
I guess that at 20.5 kHz you wouldn't notice the cut but at lower frequencies you might...
Again, if someone proves that changes to the default settings are needed, then go ahead, but until then, don’t worry so much!

What is lowpass width preset in LAME ?

Reply #8 –
What are you trying to accomplish? Do you want to make the filter as steep as possible, so that more of the higher frequency content is preserved?

If that's what you're trying to do, what do you think will happen to the quality of the lower, much more audible frequencies if you force the encoder to deal with what is basically random noise in the highest frequencies?

I'm not trying to sound patronizing; it's just a common mistake that we knob-twiddlers keep making, thinking we know better than the LAME developers or that there's some secret combination of settings that will make all the difference.

If there is something else you are trying to do, then please explain; someone here can probably help you figure out the best way to do it.

What is lowpass width preset in LAME ?

Reply #9 –
The only useful application of --lowpass-width I can think of is behavior beyond 16 kHz. The range beyond 16 kHz is not very important but there's a lot of personal preference how to use it. Some people just rely on Lame's default behavior (maybe the best idea), some like to have frequency range go up to 20 kHz, some prefer to give preference to the (much) more audible frequencies below 16 kHz and thus use a lowpass of 16 kHz or a bit above. Some use -Y with the -V levels of -V2 or better to allow for a relatively inaccurate representation of the frequencies above 16 kHz.
--lowpass-width adds another variant for personal preference: have a strong cut-off at, say 20 kHz for instance, but have frequency response gradually going down from, say 16 kHz, or whatever seems appropriate.
lame3995o -Q1.7 --lowpass 17