When several microphones are fed into a common system, sound originating at any given source results in poor articulation. Introduction of an automatic microphone priority control suppresses echo and reverberation

Draper, M. S.

English

NASA Technical Reports Server

November
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1968	 Brief 68-10529 
NASA TECH BRIEF 
NASA Teèh Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Improved Communication System for Large Operations Center 
Priority Separation 
A problem generally exists when using several 
microphones fed into a common system. Sound 
originating at any given signal source and picked up 
by multiple microphones, provides an effect similar 
to multiple sources picked up by a single microphone. 
This effect results in poor articulation (speech clarity) 
at the output (speaker) because of the varying phase 
shift at different frequencies and distances. 
This problem is largely overcome by introduction 
into the system of an automatic microphone priority
control. Priority is established by an increase of 
sound level in any single microphone, which causes 
a simultaneous gain reduction in the amplifiers con-
nected to the remaining microphones. This action 
suppresses echo and reverberation as a result of the 
recovery delay incorporated in the circuitry. 
The figure illustrates one application of the tech-
nique. Priority control distributors (control select) 
provide negative bias voltage to the input stage of 
each priority control amplifier. When this voltage 
(continued overleaf) 
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is increased, the gain of the input stage of the affected 
priority control amplifier is reduced. There are 12 
priority control distributors in the system. Each dis-
tributor input is connected to the dc control output 
voltage of one of the priority control amplifiers. This 
output is routed through an isolation diode to 8 or 9 
of the 12 priority control amplifier input stages. 
The purpose of the signal distribution network is 
to mix and distribute the signal picked up by the 
microphones through the amplifiers and speakers at 
levels relative to the distance from each microphone 
to each of the speakers. Input to the signal distribu-
tion network is made from the output of the 12 
priority control amplifiers, while output of the network 
is connected to the inputs of 16 speaker power ampli-
fiers. The gain between any given, microphone and 
each speaker is controlled: by the signal distribution 
network and preset at slightly less than the acoustic 
attenuation between them. 
Notes: 
1. Hum-bucking coils are installed in all micro-
phones to reduce magnetic field interference 
caused by current-carrying media in the environ-
ment.
2. The increased audio communication capability 
inherent in this system should give added effec-
tiveness to centralized management functions in 
complex operations. 
3. Documentation is available from: 
Clearinghouse for Federal Scientific and 
Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: B68-10529 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Melvin S. Draper
of The Boeing Company
under contract to
Marshall Space Flight Center
(MFS-15016) 
Brief 68-10529	 Category 01


Improved communication system for large operations center

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