Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Counterwave generation control path
Reexamination Certificate
1998-10-01
2001-06-12
Isen, Forester W. (Department: 2743)
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Counterwave generation control path
C381S119000, C381S094700
Reexamination Certificate
active
06246773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to audio signal processors. Illustrative embodiments of the invention relate to audio mixing consoles, especially digital audio mixing consoles.
2. Description of the Prior Art
Consider the illustrative arrangement shown in
FIG. 1. A
stereo musical recording of an orchestra playing classical music is made using a ‘stereo pair’
1
of two identical directional microphones placed as close together as possible in a good listening position.
It is often necessary to increase the loudness of some component of the performance such as a voice or a quiet instrument. In
FIG. 1
a flute F in the orchestra is shown as an example. The established method of doing this is to position a spot microphone
2
as close as possible to the flute and add the output of the microphone to the left and right channels of the stereo signal in some controllable proportions, using a stereo mixing console
3
.
That method creates several problems because the path lengths through the air from the flute to the spot microphone
2
and to the stereo pair
1
are different. The flute signal derived from the spot microphone applied to the left and right stereo channels has a different timing to the same signal derived from the stereo pair.
This, in all cases, creates a filtering effect because at some frequencies the signals add, and others they subtract, due to the phase differences created by the differing air-path lengths. That creates an unwanted comb-filtering effect.
In addition, in general, the flute signal arrives at spot microphone
2
earlier than the flute signal at the stereo pair
1
. The ear responds to the first signal to reach it (not the loudest signal) to fix the position of a stereo audio image. Thus the flute signal derived from the spot microphone can create an incorrectly positioned stereo image. To manually adjust the amplitude and delay of the spot signal is a skilled, difficult, task.
SUMMARY OF THE INVENTION
According to the present invention there is provided an audio signal mixing console comprising: a first input for receiving a first audio signal including first and second components representative of first and second sound sources; a second input for receiving a second audio signal representative of the second sound source; adaptive FIR filter means having a reference input for receiving one of the first and second audio signals as a reference signal, an operand input for receiving the other of the said audio signals as an operand signal, the adaptive filter means including means correlating the operand signal with the reference signal to generate a set of FIR coefficients associated with minimum correlation error between the reference and operand signals; means for processing the operand signal according to the coefficients; and means for combining the first audio signal with the processed operand signal.
Thus by using an adaptive filter to correlate the first signal (e.g the mixed signal from the stereo pair) with the second signal (e.g. the spot signal from the spot microphone), signals which are appropriately matched are automatically produced. The signals may be additively mixed for greater emphasis of the second signal. The signals may be subtracted to cancel the second signal: this is useful to cancel noise signals.
The processing means may be a delay device for delaying the second signal to match the timing of the first signal. The processing means may be an FIR filter which matches the phase and amplitude of the second signal to the second component of the first signal or extracts the second component from the first signal.
In a stereo mixing console having left and right channels, there may be an adaptive filter and a processing means for each channel. Alternatively, and preferably, there may be a processing means for each channel, and one adaptive filter shared by the channels.
REFERENCES:
patent: 5377276 (1994-12-01), Terai et al.
patent: 5724485 (1998-03-01), Rainton
patent: 5917921 (1999-06-01), Sasaki et al.
patent: 2 265 277 (1993-09-01), None
patent: 2 268 026 (1993-12-01), None
patent: 2 279 846 (1995-01-01), None
Frommer William S.
Frommer Lawrence & Haug LLP.
Isen Forester W.
Pendleton Brian Tyrone
Savit Glenn F.
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