Microphone system for producing signals for quadraphonic reproduction

DETAILED DESCRIPTION A primary object of the present invention is to provide a system utilizing a microphone array and an encoding circuit for producing two composite signals equivalent to those required by known quadraphonic systems having a simpler and less expensive microphone array than that used in the system described in the aforementioned copending application.
Another object of this invention is to provide a microphone array-encoding circuit system for producing encoded composite signals equivalent to those required by the RM quadraphonic system to establish the directional position of surround-sound sources.
Briefly, the primary object of the invention is achieved with an array of two gradient microphones and a single omnidirectional microphone supported on a common vertical axis, with the axes of maximum sensitivity of the two gradient microphones oriented at an angle of 90.
degree.
relative to each other and at respective azimuthal angles of 0.
degree.
and 90.
degree.
.
Appropriate fractional portions of the output signal from one of the gradient microphones are combined with appropriate fractional portions of the output signal from the other gradient microphone to produce equivalent gradient patterns displaced at the aforementioned angles of .
+-.
65.
degree.
and .
+-.
165.
degree.
.
These latter equivalent patterns furnish a fractional portion of the output signal (approximately 70%) which is combined with a fractional portion (approximately 30%) of the output signal from the omnidirectional microphone so as to produce the four limacon polar patterns characterized by the normalized limacon equation E = 0.
3 + 0.
7cos.
theta.
.
The four resultant signals are selectively phase-shifted and combined to produce two composite encoded signals of the kind utilized in the SQ quadraphonic sound system, as described in the copending application.
According to another aspect of this invention, the aforementioned array of two gradient microphones and a single omnidirectional microphone is formed of a commercially available microphone which contains four limacon patterns characterized by the equation m + (1-m)cos