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Re: High frequency recording

Subject: Re: High frequency recording
From: "Avocet" madl74
Date: Mon Nov 8, 2010 11:22 am ((PST))
> I could be wrong here - but from what I remember of my sound
> engineering course the sample rates only allow you to sample
> frequencies of HALF their stated value.

Gus,

That's the "Nyquist Theorem" which states that any frequencies above
half the sampling rate would "reflect" downwards at a difference
frequency. In other words sampling at 44.1, a sound of 35Hz will
appear as a sound at 9.1KHz. You wouldn't know whether it was 9.1KHz
or 35KHz. They therefore need a sharp LPF filter on the input at about
20 or 21KHz. These are built into digitisers.

Theoretically you can sample a 20KHz input at 44.1Ks/s, but it will
only have two samples per cycle, so it won't give a sensible 20KHz
back. With music, anything up there is hash, but wildlife calls need a
better reproduction than that.

Repeating myself, check that anything sold as as audio device may
sample at high rates to give a high quality but could well be limited
to around 20 or 25 KHz. Check the spec. :-)

I've picked up unidentified rodents most likely shrews or voles at
around 15HKHz sampled at 44.1, but all I can really derive is an
approximate frequency. Slowed down, the calls are very gravelly.

David

David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce







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