System monitors a communication channel for proper circuit parameters and energizes an alarm if these parameters do not fall within allowable limits. It comprises a monitor-signal transmitter at the transmitting end of the channel and a monitor-signal receiver at the receiving end

Smith, G. P.

English

NASA Technical Reports Server

February 1967	 Brief 67-10028 
NASA TECH BRIEF 
NASA Tech 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. 
Monitor Assures Availability and Quality of Communication Channels 
MONITOR-SIGNAL TRANSMITTER MONITOR-SIGNAL RECEIVER
Communication Communication Channel Bandpass Equipment
Isolation and __
f%.___ J
Filter
f 200-3200 CPS Coupling Amplifier 
Uppe Frequency 
Race 
Isolation &
Impulse Impulse Noise	 Noise 
- - —I Coupling Amplifier frer	 Communication Counter	 I 
Channel war Frequency L -------- 
Communication B	 dwidth 3 KC an 0 DBM	 200-3200 CPS
Receiver	
--- 
C,	 Bandwidth
Available Commercially 
1 Proportional DC Signal L 3 DB Above 1 Received Noise Lend Noise  Level .
Power Ft	 I 200CPSI
D etector Noise Alarm 
I i 
-Anrphher & OSC _______ Modulator I I F2 Moniter Signal I Level F3f
-
Level
_____
_
urn, Detector 
I Ft 
Detector I	
Signal F
1 CPS 
OSC Phase Delay 
-
Distortion Alarm  
1 CPS OSC Output F3 
Level Control
 Amplitude 
3200 CPS Carner..B 
Modulated at 1 CPS
Power 
Amphfier
I	 F 3200 CPS I - Frequency Response Alarm 
& Modulator  OSC F, Level Comparator 
Ft = Lower Frequency Monitor Signal Frequency  
F2 = Upper Frequency Monitor Signal Frequency Translation Alarm 
F3	 DC From = Subaudio Frequency used to Modulate Fr & F2 
50% Modulation with Idle Channel & 100% with Busy Channel) 	 Automatic Detector Frequency Control
 
The problem: quality monitoring devices are commercially 	 avail-
To devise a monitoring system that continuously able and in general use for telegraph and low speed 
monitors a communication channel for proper circuit digital circuits, they cannot monitor channels carrying 
parameters and energizes an alarm if these param- voice or other signals that contain	 many	 random 
eters do not fall within allowable limits. The system frequencies. Also, they cannot monitor all the critical 
must assure the availability and quality of a communi- parameters of voice and high speed data circuits. 
cation channel, whether in use or idle. It must deter- The solution: 
mine when an idle channel has been interrupted or has A monitor system that comprises a monitor-signal 
deteriorated below usable standards, and it must mon- transmitter at the transmitting end of the channel 
itor channel	 quality	 during	 circuit	 use.	 Although and a monitor-signal receiver at the receiving end: 
(continued overlcal)
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights. 
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the monitor-signal transmitter generates two ampli-
tude-modulated signals that are within the communi-
cations channel frequency band, one at the low 
frequency end and the other at the high-frequency 
end. The monitor-signal receiver detects the trans-
mitted monitor signal and measures its power level, 
phase delay, frequency amplitude response, and de-
gree of modulation. An alarm sounds if any of these 
measurements do not fall within normal limits. 
How it's done: 
Transmitter oscillators generate the upper and 
lower frequencies, F 1 and F2, which are used to 
monitor channel quality. The 200- and 3200-cps os-
cillators are typical frequencies only. F 1 and F 2 are 
amplitude modulated by F3, the output of the sub-
audio frequency oscillator. A communication signal 
detector controls the modulating output of this oscil-
lator. During transmissions, the detector increases the 
output (F 3 ) of the oscillator. This increased output is 
detected at the receiver, thereby establishing that the 
communication channel is in use. 
The power amplifiers and modulators isolate the 
oscillators from outside influence and permit ampli- 
tude modulation without frequency modulation. The 
isolation and coupling amplifier isolates the ampli-
tude modulated monitor signals from the communica-
tions equipment and couples the monitor signal to the 
communications channel. It also provides for manual 
adjustment of the transmitted monitor-signal level. 
At the monitor-signal receiver, the two monitor 
signals are separated and demodulated by the upper 
frequency and lower frequency receivers. The de-
modulated outputs of each receiver (F 1 and F 3 ; F2 
and F 3 ) are fed to the monitor-signal detectors and 
comparators. 
The monitor-signal level detector adds the average 
power level of the upper and lower monitor fre-
quencies (F 1 and F 2) to indicate communications 
channel signal level and also channel failure (zero-
level condition). It also detects when the channel is in 
use by measuring the power level of the subaudio 
frequency modulation (50%, idle; 100%, in use). When 
this increase in modulation is detected, the level de-
tector feeds a dc signal to the noise level detector, 
indicating the channel is in use. 
The noise detector actuates its alarm at levels 
slightly greater than normal idle circuit noise. When 
the communication channel is in use, the dc signal 
from the monitor-signal level detector reduces the 
gain of the noise detector to prevent the communica-
tion signal from actuating the alarm.
The phase comparator compares the phase angle 
between the subaudio frequency outputs (F 3 ) of the 
upper and lower frequency receivers. These outputs 
are the same frequency, but one has been transmitted 
by the communication channel as a frequency below 
the signal band and the other as a frequency above 
it. Since these upper and lower frequencies started 
with their modulation in phase, from the same sub-
audio frequency oscillator, any phase difference at the 
receiver is a measure of the phase-delay distortion of 
the commercial channel. 
The monitor-signal level comparator compares the 
received power level of the upper- and lower-
frequency monitor signals (F1 and F2) and thus meas-
ures frequency distortion. 
The frequency translation detector operates from 
the AFC error signal generated in the upper- and 
lower-frequency receivers. An alarm is sounded whn 
this error signal exceeds a predetermined amount. 
The bandpass filter removes the monitor-signal 
frequencies from the communication channel to pre-
vent their interfering with channel use. This filter will 
not be necessary if the level of the monitor signal can 
be low enough that it is comparable to the normal 
channel noise. 
Notes: 
I. The channel quality monitor can be used in voice 
and high-speed data channels to assure circuit qual-
ity and channel availability. It is especially useful 
in high priority voice channels which do not carry 
traffic most of the time, but which must be opera-
tional and available immediately. In data circuits, 
it could signal substandard channel conditions that 
would ordinarily result in list time and data. 
2. Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Kennedy Space Center 
Kennedy Space Center, Florida 32899 
Reference: B67-10028 
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: George P. Smith
of the RCA Service Company
under contract to
Kennedy Space Center
(KSC-66-38) 
Brief 67-10028	 Category 01


Monitor assures availability and quality of communication channels

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