On Mon, 19 Feb 2007 10:34:50 -0800, Richard Clark
wrote:
On Mon, 19 Feb 2007 14:06:30 GMT, (Bob Masta)
wrote:
FFT size is 1024 points, so you won't be able to use
this for tuning your guitar, if that's what you had in mind.
Hi Bob,
Why not? 1024 points (bins) has enough resolution to shake out every
note on a Hawaiian slide guitar. The only care is selecting the
sampling rate and most FFT packages should be able to resolve
exceedingly fine.
The problem is that the line resolution of an FFT is the sample
rate divided by the number of time points. So with 44100 Hz
sample rate and 1024 points you get a bit over 43 Hz per line.
So the first non-DC spectral line would be 43 Hz, which is about
a low F on a bass guitar, and the very next line would be the F
an octave above that... you'd miss an entire octave!
The standard musical note frequencies (semitones) differ
from each other by about 6% (12th root of 2, since there are
12 notes in an "octave"). But you need much better
resolution than this for tuning, typically a "cent" or so,
namely 1/100 of a semitone or .06%. At 43 Hz that
works out to about 0.026 Hz. A 64K-point FFT
at 44100 Hz will have a native resolution of 0.67 Hz,
which still isn't good enough... but at 64K samples,
you are looking at 1.48 seconds of sound. If the
pitch changes over that interval, the spectral peak
will be smeared even further.
If you attempt to beat this game by going to lower
sample rates, note that frequency resolution is
still proportional to the inverse of the total sampling
interval. So if you tried to use the 1K FFT but sample at
44100 / 64 = 689 Hz to get the same (inadequate)
resolution as the above 64K FFT, you'd still need 1.48
seconds of sound. Sound cards don't sample that
low intrinsically (typically 8K lower limit, 4K on old
Sound Blasters), but you might manage using sample
rate conversion (either Windows or sound card built-in).
I haven't tried this, but I suspect that it would not be a satisfying
experience due to the s-l-o-w time response.
Best regards,
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!