Lexicon Prime Time 93 - Hum Filtering; Comb Filters

the change in tone quality.
CASE 1
CASE 2
FIG. 3.9 COMB FILTERS
HUM FILTERING
FREQUENCY SWEEP
SAME SWEEP DELAYED .
SUM OF TWO
FREQUENCY SWEEP
SAME SWEEP DELAYED
AND INVERTED
SUM OF TWO
The above discussion leads to an interesting conclusion that a single fre-
quency and all of its harmonics can be eliminated in the nulls of a comb
filter. This is particularly useful for eliminating hum from a source
since hum can be the line frequency plus all of its harmonics (therefore
high pass
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hum filters .. are not entirely satisfactory). To eliminate hum
from a 60Hz line an inverted delay of 16.7 msec can be added to a delay of
0 at the same amplitude. This creates nulls at 60Hz and all of its harmonics.
The comb filter will change the timbre of the source, however. Since the
perceived change in timbre depends
upon the delay time and the source material
used, a
different delay setting might be more suitable for a given source.
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