of Microprocessors

SOURCE-SIGNAL ANALYSIS 583
PULSE PERIOD = SIGNAL PERIOD
BLANKING
TUNABLE
SWITCH
4-POLE
2-POLE
INPUT
FREOUENCY
~- DOWNSLOPE SSINGLE OUTPUT
SIGNAL"" BUTTERWORTH --10 30-Hz PEAK --10 SHOT ~~ CONVERSION -r- INDICATIDN
LOW-PASS
HIGH-PASS DETECTOR 1
/
CUTOFF = 1.2
FREOUENCY
FAST OR SLOW
DISCHARGE
SINGLE
SHOT
2
iT1ME=BO%
OF PERIOD
ADAPTIVE
CONTROL I-
PROCESSING
Fig. 16-18. Simple time-domain pitch detector. Source: Electronotes Newsletter
# 55, July, 1975.
milliseconds following a transient. Of course, if the fundamental is absent as
in Figs. 16-17C and F, low-pass preprocessing may filter our the entire
signal and leave nothing to process further. Thus, with more sophisticated
pitch detectors, the preprocessing filter cutoff should be fairly high, such as 1
kHz, and used primarily for reducing noise.
Experience with typical instrument waveforms and examination of Fig.
16-17 reveals that in all cases bur one the location of waveform peaks contains
sufficient information for determining periodicity. In fact, a simple positive
peak detector with dynamic threshold performs infinitely better than a zerocrossing
detector. A dynamic threshold is implemented by setting the
threshold for peak detection to some fraction of the amplitude of the last
detected peak. The trick is to make this fraction small enough so that sounds
0f decreasing amplitude, such as Fig. 16-17G, are followed properly yet
high enough to avoid double peaks from waveforms such as Fig. 16-17F.
When double peaks are unavoidable, the detector can be blanked for some
fraction of the currently measured period. Peaks would not be detected
during the blanking interval. Of course, any peak detector scheme will fail
on the waveform in Fig. 16-17D, which has two equally spaced positive
peaks of equal amplitude. However, Fig. 16-170 has only one negative peak
so perhaps two detectors, one for positive and one for negative peaks, would
work better. Where there is disagreement between the two, the one teporting
the longest period would be selected for output.
Let's examine a fairly simple pitch detector based on peak deteerion 2
that has been reasonably successful as an analog implementation and should
2This pitch detector was designed by B.A. Hutchins and was described in Elecrronotes
53-55.