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Topic: "Moving" frequencies? (Read 4126 times) previous topic - next topic
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"Moving" frequencies?

As some here may know, I run an Internet radio station for dialup modem users, which uses MP3's at the horrendous rate of 24kbps mono.

According to Cool Edit graphs, this does a brutal chop of all frequencies above 6 KHz.

What I'm wondering -- is there some way to "skew" frequencies from (say) 6KHz - 11 KHz into the range of 1KHz-6KHz, mixing with what's already there?  Of course the music wouldn't sound the same, but at least more musical information/detail would be included (unless my thinking is totally off here).

Anyone know if it would be possible to try this?  Or am I asking the impossible?

"Moving" frequencies?

Reply #1
Take a good wave editing program and a music clip, copy it so you have 2 copies, highpass one and lowpass the other. Pitchshift the highpassed one till the frequency range is dropped to the point you want, then mix them together.

I bet it'll sound horrible.

--
GCP

"Moving" frequencies?

Reply #2
Quote
Originally posted by Garf
Take a good wave editing program and a music clip, copy it so you have 2 copies, highpass one and lowpass the other. Pitchshift the highpassed one till the frequency range is dropped to the point you want, then mix them together.

I bet it'll sound horrible.

-- 
GCP

Pitch-shifting wasn't what I had in mind, but it sounds like that would be the only way to do it (makes sense, how else to change certain frequencies to other frequencies?).

Thanks, I don't think I'll try that experiment...

"Moving" frequencies?

Reply #3
You said that you use MP3, but have you tried formats specifically designed for low bitrates, such as VQF or MP3Pro?  Possibly WMA also, although that eats up processor clock cycles pretty badly.  Those might be an easier solution to your problem, unless you're streaming using Shoutcast or something.

"Moving" frequencies?

Reply #4
Quote
Originally posted by silver_cpu
You said that you use MP3, but have you tried formats specifically designed for low bitrates, such as VQF or MP3Pro?  Possibly WMA also, although that eats up processor clock cycles pretty badly.  Those might be an easier solution to your problem, unless you're streaming using Shoutcast or something.

I'm streaming through Live365.com -- unfortunately they offer only MP3 streaming, but the rates are low (100 megs/$6.95 per month, essentially unlimited listeners)... can't really complain.

"Moving" frequencies?

Reply #5
obviously you could pitch shift them down a full octave, or more, and it won't be out of key.  any other pitch shift value will ruin the music completely.  the problem will still be that many more sounds will be in a given frequency range, which will significantly muddy the sound.  i would personally leave it as is.  would be pretty disconcerting to suddenly hear a high flute or a guitar solo suddenly dropping an entire octave without warning (ie. continuous sounds might begin below 6 khz, then go above.  that would sound horrible).  Also say with a guitar chord, some overtones could be over 6 khz in some cases, and these will be dropped an octave which will sound very odd.  well, give it a try if you are curious, I suppose

"Moving" frequencies?

Reply #6
Quote
Originally posted by floyd
obviously you could pitch shift them down a full octave, or more, and it won't be out of key. 


I've just tried it, and the above statement turns completely wrong !

Musical sounds (other than sines) are made from integer multiples of the fundamental frequency.

Example, when a violin plays an A, it will be not only a 440 Hz note, but a sum of 440 Hz, 880, 1320, 1760 etc Hz.

Now if you take the 1320 and 1760 and pitch them down one octave, they'll become 660 and 880 Hz
The 880 Hz won't be out of key, but the 660 will add an E note that couldn't be there in a A-440 Hz note ! :diabolic:

"Moving" frequencies?

Reply #7
ah, i forgot about the instrument being in key with 'itself'.  thanks for pointing that out.

regardless, I don't think its possible acceptably modify music to fit a smaller freq range, without access to editing the original discrete tracks (which obviously isn't possible here)

"Moving" frequencies?

Reply #8
I wonder if the music would sound better, if I used parametric EQ to do a gradual rolloff from 5KHz-6KHz (of the original WAV) and also convert to mono before encoding?  Maybe the MP3 encoder would respond better for some reason... I'll try that & see if there's any difference.

Edit -- tried it, and didn't notice any particular difference.  Doubt if I could ABX the difference between the two clips.

"Moving" frequencies?

Reply #9
Quote
Originally posted by fewtch

I'm streaming through Live365.com -- unfortunately they offer only MP3 streaming


MP3PRO is good for streaming at such low bitrates. I heard that Live365 supports mp3pro. The only app i know, that is capable of encoding in mp3pro with all the features (fake stereo for example) is CoolAudio Pro 2.0 , but it uses the bad fraunhoffer codec.

"Moving" frequencies?

Reply #10
I would definitely do a cut-off of the frequencies before encoding, using Cooledit  or something similar. That way, the encoder isn't wasting bits on information that will be thrown out. Might help a little.
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