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Re: Bat detector (and aliasing)

Subject: Re: Bat detector (and aliasing)
From: "Raimund Specht" animalsounds
Date: Mon Nov 20, 2006 3:12 am ((PST))
Hi Gianni,

Thanks for correcting my fault. Yes, I actually meant that the usable
frequency  range is limited to 60...70 kHz (because a 120 kHz signal
component would cause an aliasing product at 72 kHz. One could remove
the potential aliasing artifacts by low-pass filtering the recorded
sound files with a cut-off frequency of about 60 kHz.


> Hi Raimund, I would correct the latest statement.
> According to the frequency response of the 722 and its a-a filters
> (see you'll get
> frequencies above 60 kHz attenuated (-5 dB at 80 kHz, -10 dB at
> 92kHz) whilst the risk of getting aliasing is limited to frequencies
> above 96 kHz.
> Frequencies above 96 kHz (maybe harmonics of echolocation signals
> below 96 kHz) will be attenuated according to the frequency response
> (dark line) and reflected back to the range below 96.
> The pink line going from 96 kHz (-12dB) back to 50 kHz (-70dB) shows
> the maximum level of aliased components that may occur for input
> frequencies higher than 96 kHz if recorded at 0 dB!
> Thus if you record a 120kHz tone at 0 dB (unlikely to happen) you get
> an aliased component of 72kHz at -30dB, but if you get the same 120
> kHz at, say, -30 dB (more realistic) you get the aliasing at -60 dB.
> The aliasing risk could be further attenuated by the frequency
> response of the microphone and of the amplifier you use. If the
> global frequency response of the ultrasonic receiver you're using
> falls down above 100 kHz the aliasing risk could become negligible.
> Further, aliasing of sweeps is clearly identifiable: the first-fold
> alias of an upsweep is a downsweep. By the way, with the equipment we
> are talking about multi-fold aliasing will never happen; this risk is
> limited to AD converters without anti-aliasing circuitry like those
> of, for example, most NIDAQ boards. The widely used NI DAQ 6062
> requires additional expensive filters to avoid aliasing, or the use
> of very high sampling rate to largely exceed the bandwidth of the
> signals to be recorded.
> Gianni
> --------------------------------------------------------------
> Gianni Pavan
> Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
> Universita' degli Studi di Pavia
> Via Taramelli 24, 27100 PAVIA, ITALIA
> Tel         +39-0382-987874
> Fax        +39-02-700-32921
> Web
> Email      

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