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Re: Pitch shifting

Subject: Re: Pitch shifting
From: Lang Elliott <>
Date: Fri, 18 Jun 2004 13:25:41 -0400
Rich:

Please check my math and my reasoning; maybe I'm way off here:

Before you sped up your recording, the maximum time arrival differences
between the two mikes would be 150 feet divided by 1100 feet/sec equal 0.136
seconds or 136 milliseconds. That's a substantial delay as far as the human
ear is concerned, but this is the delay that would occur if the jet noise
originated straight out to one side, along the axis of the two spaced omni
mikes.

When you play the recording back at 16X normal speed (= 16 minutes shrinks
to 1 minute), this is equivalent to making sound travel faster between the
two mikes so that the human ear would hear a maximum delay between channels
of only 8.5 milliseconds, which is equivalent to how long it takes sound to
travel 150/16 = 9.37 feet.

It might be that by reducing the signal delays between the ears, you
actually improve stereo imaging by bringing the time delays between the two
channels closer to what we normally experience. Also, the pitch of the jet
has been raised into a frequency band that the human ear is optimized to
interpret. I'll bet that both the reduction in time delay and the increase
in pitch greatly improves the stereo experience for the human listener.

Human ears are about 7 inches apart. How long does it take sound to travel 7
inches? Well, 1100 feet/second = 13,200 inches per second. That means that
sound can travel 7 inches in 7/13,200 of a second = 0.5 milliseconds.

Isn't it amazing that the human ear can detect such tiny arrival time
differences at the two ears, and use this information to effetively
determine where sounds originate in space?

When you use spaced omni mikes, you are accentuating time delays between the
two channels. Intensity differences would be nonexistent for distant sources
such as the jet. And phase differences would bear no relation to what the
human ear encounters naturally. It follows that the time delays is what our
ears would primarily use to image the sound sources.

Just babbling . . .

Lang


Yes it is that stretch and pitch change that I can not do well but I
guess it can not be done well at extremes.  My thinking was that
Meteors are fairly short events and in order to make them audible I
needed to pitch shift without time compress at about X100.

I can do the lockstep by changing the sample rate on playback very
easy as well in Audition.

To test out Robs thoughts about bass image location and to determine
if it was just wishfull thinking with severe lobes getting in the
way, I tried it.

Linked is a high altitude Jet that I suspect was over 7 miles high.
Mics are about 150' spaced on a very quiet Canadian lakeshore.  Time
compression and pitch shifted to check the stereo image of the bass
segment X16. That means that a 16 min file was converted to 1 min
and the pitches were raised x16 as well.

It works, there is a stereo bass image on a distant object when
recorded with wide spaced omnis.

265kb download
http://home.comcast.net/~richpeet/jet.mp3

Rich Peet

PS Rob, I am getting your e-mails but when I send to you it bounces.

--- In  Aaron Ximm <>
wrote:
> > I wish to pitch shift up in freq 20 to 100 times.
> > My editor, Audition, can not do it well at these extremes.
> 
> Hi Rich,
> 
> Do you want to to vary speed as well as pitch, or *only* the pitch?
> 
> As you probably know it's a lot easier to find algos that sound
good doing
> varispeed (both in lockstep) than one or the other alone (ie, time
> stretching without altering pitch or vice versa)...
> 
> If you can live with varispeed, the package I use (Samplitude) has
a great
> mode in which you can grab sound clips and stretch/compress them
directly
> in the multitrack editing view, even be listening back to find a
good
> sweet spot as you do... ie, it's all realtime processing, not
offline...
> 
> I say varispeed as the individual time/pitch stetching is I believe
> limited to 2 octaves up/down.... :/
> 
>  best,
>   aaron
> 
>   
>   http://www.quietamerican.org



"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg 


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