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In mp3 compression or audio in compressed video files, what frequecies are being cut?

post #1 of 7
Thread Starter 
I current have a pair of speakers that will go up to 35khz +/- 3db and considering buying a subwoofer that will go down to 16hz +/- 3db (anechoic measurement).

So would be interested to know what frequencies are missing in compressed materials (both mp3 and common divx video files) or anything else bad because I could potentially have a system that could output a flat response between 20hz-20khz the whole audible frequency and still have more to spare.

How much of that potential would be actually utilised? Assume Mp3 rip at 192 min, 320 is what I usually download at. For vid files, it's usually 45mins for 350mb, though I do dl 1.2gb for 45mins at 720p when I can. I'm assuming less audio compression goes on in the 720p vids?
post #2 of 7
at 192kbps it cuts off at 16khz. 320kbps cuts off at 20khz, the human hearing threshold.
post #3 of 7
Quote:
Originally Posted by The-One View Post
I current have a pair of speakers that will go up to 35khz +/- 3db and considering buying a subwoofer that will go down to 16hz +/- 3db (anechoic measurement).
There is little musical life below 20hz and nothing you can hear above 25K at the very most optimistic if you have exceptional hearing so you pretty much have it covered. 44.1 khz CD does not extend above 22.05Khz and is rolling off from 20khz


Quote:
So would be interested to know what frequencies are missing in compressed materials (both mp3 and common divx video files) or anything else bad because I could potentially have a system that could output a flat response between 20hz-20khz the whole audible frequency and still have more to spare.
Mp3 tends to have a low pass filter anywhere from 16khz to 20khz. However the decoder may roll-off even lower if it does not detect any significant energy at the higher frequencies. You can set the lowpass yourself or even bypass it altogether but if the source is CD then you basically have a best case of 20 - 20K.

On the other hand it is highly unlkely you would even hear a lowpass filter that cut in at 18K or 20K.
post #4 of 7
MP3s get most of their compression not by cutting off things at fixed locations but removing sound based on psychoacoustics - that is, removing sounds that would not be easily noticed by the ear/brain on a moment-by-moment basis.


Quote:
Originally Posted by nick_charles View Post
There is little musical life below 20hz and nothing you can hear above 25K at the very most optimistic if you have exceptional hearing so you pretty much have it covered. 44.1 khz CD does not extend above 22.05Khz and is rolling off from 20khz
I keep hearing this so I thought I'd comment. Since we're talking about speakers, sounds below 20hz still do have an impact, since they can be perceived by means other than the ears. Just because there are no discernable "notes", it doesn't mean there's no point to them being there. There is a tactile aspect to music (some music, at least) that is still important.
post #5 of 7
Thread Starter 
Thx for the replies so far.

So what are the psycoacoustic things that gets cut out?

Ok, so the low pass filter is from 16khz upwards.

But what about a high pass filter? Ie, how much bass is retained in compressed files? I know most music do not have <20hz occuring at a useful loudness, though some music definitely do. But a lot of films etc should have ultra low bass, and the audio track on those is compressed, could I miss a lot of very low frequencies? I do not hope to hear below 20hz, only for the feeling of tactile pressure or vibrations in the room/air. I'd be fairly dissappointed if a high pass was applied at 20hz standard as that would mean I would get little benefit from a 16hz tuned subwoofer.
post #6 of 7
Quote:
Originally Posted by b0dhi View Post
MP3s get most of their compression not by cutting off things at fixed locations but removing sound based on psychoacoustics - that is, removing sounds that would not be easily noticed by the ear/brain on a moment-by-moment basis.
They do both. They first apply a lowpass, then process the remaining with psycho-acoustics.

But downloaded mp3 can have any lowpass, whatever bitrate they are. It is not uncommon for some people to prevent completely the encoder from lowpassing, which produces mp3 files with high frequency content up to 22 kHz, and poor overall quality, since frequencies above 16 kHz have little to no impact on sound quality, while they need a lot of room to be encoded into mp3.

Here are the default lowpass values for LAME : LAME - Hydrogenaudio Knowledgebase

mp3 encodes ultra-low frequencies without problem.

Quote:
Originally Posted by b0dhi View Post
Since we're talking about speakers, sounds below 20hz still do have an impact.
That's right. Though I personally perceive them in my ears as oscillations against my eardrum, rather than physical vibrations. The frequencies that I feel I perceive most as physical vibrations are rather in the 100-300 Hz range.

CD featuring frequencies below 20 Hz are very rare. But when they do, it is very interesting to reproduce them. Frequencies above 20 kHz seem much less interesting.
post #7 of 7
16-20KHz, depending on encoder and settings.
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