Telex DOC02-DN9848 Application Notes, Audio Precision, 2nd order allpass Q=1 @ 300, 1K, 10K + Q=6

Application Notes

Audio Precision				1st order allpass @ 300, 1K, 10K					01/19/01 16:00:02
	+150									+150
	+100									+100
	+50									+50
d	+0									+0	d
e											e
g											g
	-50									-50
	-100									-100
	-150									-150
	20	50	100	200	500	1k	2k	5k	10k	20k
						Hz

Color	Line Style	Thick	Data	Axis
Green	Solid	1	Anlr.Phas e	Left
Yellow	Solid	1	Anlr.Phas e	Right
Yellow	Solid	1	Anlr.Phas e	Left

phas e.at1

Response of the All Pass Filter for 1st Order phase shift at frequencies of 300Hz, 1kHz and 10kHz

The response of the filter for a 2nd order shift is shown below with the Q control set to 1, at frequencies of 300Hz, 1kHz, and 10kHz, and also Q set to 6 (max) and 0.4 (min) at 1kHz. As can be seen, the filter response now tends from 360 at low frequencies to 0 at high frequencies, and Q controls the rate at which the phase changes (i.e. the slope) around the transition point. With low Q, the phase changes gradually across the whole frequency range. With high Q, the phase changes rapidly in the transition area, and is unchanging at 360°/0° over the remainder of the frequency spectrum. Hence, the 2nd order all-pass provides the user with the additional control of shaping the phase shift ‘window’.

Audio Precision		2nd order allpass Q=1 @ 300, 1K, 10K + Q=6						Q=0.4	01/19/01 16:09:11
					@ 1K
	+400									+400
	+300									+300
d	+200									+200	d
e	+100									+100	e
g											g
	+0									+0
	-100									-100
	-200	50	100	200	500	1k	2k	5k	10k	-200
	20									20k
					Hz

Color	Line Style	Thick	Data	Axis
Green	Solid	1	Anlr.Phase	Left
Yellow	Solid	1	Anlr.Phase	Right
Yellow	Solid	1	Anlr.Phase	Left

phase.at1

Response of the all pass filter for 2nd order phase shift with Q=1 at frequencies of 300Hz, 1kHz and 10kHz and also with Q=6 at (max) and Q=0.4 (min) at 1kHz

Note: Both filters are all-pass with a flat amplitude response between 20Hz and 20kHz; only the phase response changes with frequency.

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