What's a lowpass?

[chepe36]

Could somebody tell me what lowwpass filter means because i have found it in several programs like lame and headac3he.

[MaTTeR]

Direct quote from the HeadAC3he FAQ.

"_When should I use the lowpass filter?
The lowpass filter shouldn't be applied at all, as
a) there is no need for it for properly mastered ac3
b) with a DPL2/CircleSurround/Logic7 amp, the rear channel will sound worse. "

Not exactly the definition your looking for but I think you get the idea. Please read the FAQ's in the future so I and other people don't feel like were wasting our time.

[chepe36]

Thak you but i had alredy read the faq and thats not what im looking for, i need the definition i need to know whats the diference between aplying or not a lowpass filter an at wich freq and how does it works.

[pacohaas]

lowpass looks like this on a positive frequency graph: ___---

it switches from zero at the cutoff freq, and i believe HeadAC3he's is 7kHz. So that means it "zero's" all frequencies below 7kHz (bassy stuff) and leaves everything above it untouched. If it were something like 10Hz, then I'd say go ahead and use it cause below that you can't hear for sure and better to use encoding bits on frequencies you can hear, but as suggested, don't use this lowpass unless you know that the frequencies are useless, or you feel like testing it for whatever reason.

Anyhow, i believe LAME's encoder has a pseudo ~10Hz lowpass built in to it's encoder because one way mp3's work is to get rid of sound that the human ear can't hear.

[tangent]

You got it the wrong way round. A lowpass filter removes high frequency contents (allowing low frequency contents to pass through). There are a few reasons to use a lowpass filter, CDs and CD Players use a lowpass filter at 20kHz to avoid aliasing. MP3 encoders lowpass lower than that because most people cannot hear beyond 18.5kHz pure tones, and it's usually lower than that for harmonics in the presence of many other tones (e.g. in music). Encoding the high frequencies take up a lot of bits and therefore affect the quality of the low frequency tones (which are more audible), so these encoders lowpass to save the low tones.

[pacohaas]

hehe, and after all that thinking I think I confused myself. oh well, just think of what I said as a highpass chepe36.

positive lowpass looks like this: -----_____

and you can definately hear frequencies above 7kHz, so you don't want to use this filter.

[Antti]

>Anyhow, i believe LAME's encoder has a pseudo ~10Hz lowpass built >in to it's encoder because one way mp3's work is to get rid of >sound that the human ear can't hear.

Lame has an adjustable lowpass filter. Default settings vary by bitrate and encoding mode, but you can always override them with
--lowpass option. Lpf is very useful for encoding low bitrates (for me, that is <=128), because encoder has more bits to be used for more audible frequencies. Besides, I've tested a long ago that I can't hear frequencies >17.5 kHz. And almost nobody hears frequencies >19.5 kHz.

[DarkAvenger]

Hmm, I just get that the question in the FAQ is not precise. YOu have to distinguish between low-pass for the rear channels on downmix (what the FAQ is referring to) and "global" low-pass. BTW this rear low-pass is applied by azid as a workaround...

High time for me to update the FAQ..

[Beave]

Hearing high frequencies is dependent on the age. The younger you are the more high frequencies you can hear. They say, for babies it is even higher than 20 Khz. If you are older, there is not much left to hear over 16 or even 15 Khz.

But the thing is that high frequencies have some sort "mirrors" in the deeper hearable frequencies. That's why even CD player lowpass to avoid artifacts in the lower frequencies.
I think for movie sound it's not so important to have the very highest kHz in it, cause if you use low MP3, it is one of the first places you hear artifacts.

Personly I prefer lowcuts at 16 to 17 kHz for movies with MP3 around 110 kbit. If I don't directly compare to the original I even don't notice.

btw. --r3mix uses a lowpass of 19.5 kHz for lame!