Circuitry for generating sync signals in FM communication systems including video informatio

Landauer, F. P.

English

NASA Technical Reports Server

A circuit is provided for  receiving each sync signal as 
frequencies in two narrow frequency bands which are 
adjacent to  one another in the frequency spectrum, The 
two bands are transmitted in succession with a common 
transition time for both. The circuit includes a first chan- 
nel which senses the signals in the f i s t  band and provides 
a first activating pulse of a duration which is longer than 
the duration during which signals in the first band are 
received. The circuit also includes a second channel which 
provides a second activating pulse when signals in the 
second band are detected. A sync indicating pulse is 
produced only when the two activating pulses occur in 
itime coincidence. 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is subject 
to the provisions of Sec. 305 of the National Aeronautics 
and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 
42 U.S.C. 2457). 
BACKGROUND OF THE INVENTION 
Field of the invention 
Tbis invention generally relates to data communication 
circuitry and, more particularly, to circuitry for detecting 
synchronizing signals which are used to decipher and/or 
correlate received data. 
Description of the prior art 
Synchronizing signals, often also referred to as sync 
pulses or sync words, are extensively used in data com- 
munication. In practice, they are injected into the stream 
of data transmitted to a receiver, wherein they are detected 
to separate blocks of data and/or to correlate them. For 
example, when video information is transmitted, sync 
pulses are used to align, such as in a horizontal direction, 
the video information contained in the signals between 
sync pulses, in order to produce a composite picture. 
The absence or presence of misalignment or jitter in the 
picture greatly depends on the accurate detection of the 
sync pulses. 
In a frequency modulated (FM )communication sys- 
tem, such as is employed for video communication from 
space, the sync pulses are represented by frequencies 
which fall outside the frequency spectrum of the video 
information to be communicated. Generally, the video 
information falls between the FM carrier and the sync 
frequenices. In the receiver, a tuned RF filter is used to 
sense the presence of signals a t  the sync frequency and 
thereby detect the sync pulse. 
The problem of accurately detecting sync pulses be. 
comes more difficult as the overall system's signal-to-noise 
ratio decreases which is characteristic of a system used 
to communicate data from space. In such a system, noise 
spikes in the narrow band of the sync frequency cause 
ambiguous sync pulse detection, which greatly affects the 
proper deciphering of the data and/or the proper presen- 
tation of the video information. 
OBJECTS AND SUMMARY O F  THE INVENTION 
Accordingly, it is an object of this invention to provide 
a new arrangment for detecting sync signals with greater 
accuracy than possible with prior art arrangements. 
Another object is the provision of a new, relatively sim- 
ple circuit for minimizing the effect of noise on the detec- 
tion of sync signals. 
Yet another object of this invention is to provide new 
means for recovering synchronizing signals in a system 
0 with relatively low signal-to-noise ratio. 
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Still a further object of this invention is the provision 
)f a new method of recovering synchronizing signals in 
L FM video communication system with low signal-to- 
ioise ratio. 
Yet a further object of this invention is to  provide 
neans to  recover synchronizing signals transmitted as 
>art of, and together with FM video information. 
These and other objects of the invention are achieved 
>y transmitting, to a receiver, each sync signal as fre- 
pencies in two narrow frequency bands which are adja- 
m t  to one another in the frequency spectrum. The trail- 
ng edge in the first frequency band, hereafter referred 
o as the sync tip frequency, corresponds in time to the 
leading edge of the second frequency band, hereafter also 
referred to as the porch frequency. The receiver includes 
two narrow band detection channels, tuned to  detect the 
3ync tip and porch frequencies comprising each sync 
pulse. The two channels are interconnected in such a 
manner that only when the two sync-pulse-defining fre- 
quencies are received in the proper time relationship i s  a n  
output pulse, representing a sync pulse, produced. 
The novel features of the invention are set forth with 
particularity in the appended claims. The invention will 
best be understood from the following description when 
read in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a simple diagram of frequency domains, use- 
FIG. 2 is a block diagram of the present invention; and 
FIG. 3 is a diagram of waveforms of signals, pro- 
ful in explaining an aspect of the invention; 
duced in the various elements, shown in FIG. 2. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
The present invention will be described in conjunction 
with an F M  communication system in which video infor- 
mation from black-to-white levels is transmitted at  des- 
ignated frequencies by a R F  carrier. It should be noted 
however that this is to be considered as exemplary and 
not as a limitation upon the invention, which can be uti- 
lized in any FM communication system in which sync 
pulses are employed. 
Referring to FIG. 1, there is illustrated a frequency 
domain of a transmission channel in which it is assumed 
that a carrier frequency fo is used to carry video infor- 
mation of black-to-white levels represented by pulses 12. 
The video information falls in a frequency band between 
f, and fd. In accordance with this invention, each sync 
pulse is transmitted as signals in two specific, very nar- 
row frequency bands near the limit of the transmission 
channel. In FIG. 1, these bands are designated as having 
center frequencies f, and f, and hereafter are referred to 
as the sync tip and porch frequencies, respectively. Thus 
as seen, the video information falls between the F M  car- 
rier f, and the sync frequencies f, and f,. 
For each sync pulse, signals in the sync tip frequency, 
f, are transmitted immediately meceding signals in the 
70 porch frequency f,. This produces a transit& of inter- 
est, represented by line 14 which is common to both fre- 
quencies. That is, the trailing edge of signals in the sync 
https://ntrs.nasa.gov/search.jsp?R=19710001806 2020-03-17T02:35:47+00:00Z
3,535,451 
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tip frequency represented by line 6 corresponds in time Then, when the transition ommon to both frequency 
to the leading edge of signals in the porch frequency, fp bands occurs, the amplifier 35 is provided with signal 26a 
represented by line 17. Briefly stated, the leading edge of to produce transition 30b, so that its output triggers the 
the sync tip frequency is utilized as a time reference for Schmitt trigger 31 to provide the activating signal 31a 
enabling the transition common to both to be used as a to gate 32. Consequently, the gate is provided with two 
svnc time reference. activating simals coincidentallv in time. causing it to 
Reference is now made to FIG. 2 which is a block dia- 
gram of a circuit assumed to be included in a receiver 
to which a dual frequency sync pulse is transmitted. The 
circuit includes an input terminal 20 assumed to be con- 
nected to the receiver's demodulator limiter, so that the 
received signals a t  terminal 2 are of a constant ampli- 
tude. The use of a demodulator and limiter are well known 
in the art of radio communication. 
Input terminal 25 is shown connected to  a band-pass 
filter 21 assumed to be tuned to the frequency band f,. 
Filter 21 is in turn connected to a low-pass filter 22 
through an envelope detector 23. The band-pass filter 21, 
- -  
provide a true output 32a. This output may be ;sed to 
trigger a one shot multivibrator 33, whose output is the 
actual sync pulse. The leading edge of such a pulse will 
only be provided during the transition common to both 
frequency bands, i.e., a t  a time when the trailing edge 
of signals in the frequency band f, and the leading edge 
of signals in the frequency band fp are received. 
It should be noted that since the noise a t  the summing 
point input to the differential amplifier 30 is derived from 
two separate frequency domains f, and f,, assumed to be 
of equal bandwidth but statistically from separate sources, 
~ 
the sum of the noise may be expressed as .\/An2+Bn2, 
envelope detector 23 and IOW-PaSS filter 22 may be thought where A, and B, represent noise in the signals, in the first 
of as forming a first detection channel designed to Pro- 20 and second channels, respectively. The noise sum is 3 db 
vide an output signal at the output of filter 2% when Sig- greater than the separate noise in each signal. However, 
nals in frequency band f, to which filter 21 is tuned, are the peak-to-peak signal is the sum of the two or 6 db 
received. greater than each separate signal. Thus, for a given signal- 
The circuit includes a second band-pass filter 24 which to-noise condition, jitter due to noise VS. rise time is 3 db 
is connected to terminal 20. The filter 24 is tuned to the z5 less than would be the case without the amplifier 30. In 
frequency band fp Filter 24. is connected to a low-Pass brief, if the signal-to-noise ratio with one detection chan- 
filter 26 through an envelope detector 25. The three cir- nel is 1, the two channel detection arrangement increases 
cuits 24, 25 and 26 form a second detection channel the signal-to-noise ratio to 2/2 .  
which provides an output signal upon the detection of There has accordingly been shown and described herein 
signals in the frequency band f,. 30 a novel circuit for detecting sync pulses which are trans- 
The output of filter 22 is supplied to a Schmitt trigger mitted to the circuit as signals in two adjacent frequency 
circuit 28 to trigger or fire a One shot multivibrator 29 domains with a transition common to both domains. It 
whenever the level of the output Signal of filter 22 ex- should be appreciated that those familiar with the art may 
ceeds a given level which occurs only when signals in make modifications and/or substitute equivalents in the 
the sync tip frequency band fe. are ~ ~ i v e d .  The Outputs 35 arrangements as shown without departing from the spirit 
of filters 22 and 26 are supplied to the Plus (+ and minus of the invention. Therefore, all such modifications and/or 
(-) input terminals respectively Of a differential amPli- equivalents are deemed to fall within the scope of the 
fier 30, whose output is connected to a second Schmitt invention as claimed in the appended claims. 
trigger 31. The circuit further includes an AND gate 32 What is claimed is: 
which provides a true output O&' when inputs from the 40 1. In a system in which a sync pulse is transmitted to 
one shot 29 and Schmitt trigger 31 are supplied thereto, a receiver in the form of signals in a first frequency band, 1 
coincidentally in time. followed by signals in a second frequency band, a circuit: 
The operation of the circuit shown in FIG. 2 may best in said receiver for providing an output signal represent 
be explained by referring to FIG. 3 wherein are dia- inn said Sync Dulse, comorising: 
grammed the waveforms or shapes of various pulses or 45 
signals produced by the elements, shown in FIG. 2. The 
signals are designated by the numerals of the signal pro- 
ducing elements followed by the letter a. Thus, signals 
I 
21a a i d  23a represent the butputs of filter 211 and de- 
tector 23 respectively, etc. As seen, the output signals 22a 5o 
and 26a are supplied to the plus (+ ) and minus (-) 
inputs of differential amplifier 30. The output thereof is 
signal 30a, with a transition 30b at which the signal 
changes from a positive to a negative level. This transi- 
tion is sensed by Schmitt trigger 31, which produces pulse 55 
31a. 
Prior thereto, the output of Schmitt trigger 28 pro- 
vides pulse 28a, which triggers the one shot 29 to pro- 
vide pulse 29a. The duration or period of pulse 29a is 
greater than the duration during which the sync tip fre- Bo 
quency i s  supplied. Thus, a short interval exists when 
both the one shot 29 and the Schmitt trigger provide true 
(assumed positive) pulses, Le., 29a and 31a. These ac- 
tuate AND gate 32 to provide the desired sync-indicat- 
In  the absence of signals in either frequency band f, 
or f,, the inputs to amplifier 36) would be noise from the 
two detection channels. The noise may cause Schmitt trig- 
ger 31 to provide gate 32 with an activating signal. How- 
ever, the absence of a signal from the one shot 29 would 70 
prevent the gate 32 from providing a true output. How- 
ever. when a sync pulse is transmitted to, and received 
ing pulse 32a. 65 
first detection channei responsive to the signals in saih 
first frequency band for providing a first signal hav- 
ing leading and trailing edges in response thereto; 
differential means having first and second inputs; 
first means for applying said first signal to the first in- 
put of said differential means; 
second means coupled to said first detection channel 
and responsive to  said first signal for providing a first 
activating signal of a duration which is longer than 
the duration of said first signal; 
a second detection channel responsive to the signals in 
said second frequency band for providing a second 
signal having leading and trailing edges in response 
thereto; 
third means for applying said second signal to the sec- 
ond input of said differential means, whereby the out- 
put of said differential means changes from a first 
peak level to a second peak level when the leading 
edge of said second signal coincides in time with the 
trailing edge of said first signal; 
fourth means coupled to said differential means and re- 
sponsive to the change in the level of the output of 
said differential means for providing a second activat- 
ing signal; and 
gating means for providing a sync-indicating signal only 
when said first and second activating signals are pro- 
I vided in time coincidence. 2. The circuit as recited in claim 1 wherein each of j - -  
by, the receiver, the leading edge of signals in the band 
f, would cause the firing of the one shot 29, and SUP- 
ply the amplifier with the signal 22a from the filter 22. 75 and said gating means is an AND gate. 
said first and second detection channels includes a narrow 1 
bandpass filter, an envelope detector and a lowpass filter, + + 
1 
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6 
4. The circuit as recited in claim 3 wherein said dif- 3. In a receiver for receiving a sync pulse in the form 
of signals in a &st frequency band, followed by signals ferentiating means comprises a differential amplifier for 
in a second frequency band, a circuit for providing an out- providing an output whose amplitude represents the dif- 
put signal representing said sync pulse comprising: ference of the levels of the two signals supplied to said 
a first detection channel to which said signals in said differential means, and said trigger means comprises a 
first frequency band are applied for providing a first Schmitt trigger for providing said second activating pulse 
signal of a preselected level, said first signal having when said second signal is applied to said differential 
leading and trailing edges; amplifier. 
a second detection channel to which signals in said sec- 5. The circuit as recited in claim 4 wherein said pulse 
ond frequency band are applied for providing a sec- generating means includes a one shot multivibrator for 
ond signal of a preselected level, said second signal providing said first activating pulse. 
having leading and trailing edges; 6.  The circuit as recited in claim 5 wherein said gating 
pulse generating means responsive to the leading edge means comprises an AND gate to which the first and sec- 
of said first signal for providing a first activathg ond activating pulses are supplied to  provide a sync pulse 
pulse of a duration which is slightly longer than the 15 indicating signal when supplied with said first and second 
duration during which signals in said first frequency activating pulses coincidentally in time. 
band are received; 
differential means coupled to said first and second de- References Cited 
tection channels and responsive to  said first and sec- UNITED STATES PATENTS 
1,824,635 9/1931 Stocker. ond signals for providing an output with a defined 20 level transition when said first and second signals are 2,401,405 6/1946 Bedford _ _ _ _ _ _ _ _ _ _ _ _  178-69.5 
2,519,911 8/1950 Kuperus. of a said preselected levels; 
2,741,661 4/1956 De France. trigger means coupled to said differential means for 
level transition in the output of said differential 25 
means; and 
gating means coupled to said pulse generating means 
and to said trigger means for providing an output 
signal representative of said sync pulse only when 
said first and second activating pulses are coinci- 30 
5 
providing a second activating pulse in response to the 3,317,669 5/1967 Ohnsorge _ _ _ _ _ _ _ _ _ _  178-69.5 
ROBERT L. GRIFFIN, Primary Examiner 
G. G. STELLAR, Assistant Examiner 
US. C1. X.R. 
dentally supplied thereto. 178-7.3 
MEANS FOR GENERATING A SYNC SIGNAL IN AN FM 
COFJIMUNICATION SYSTEM 
Filed Dec. 21, 1967 2 Sheets-Sheet L 
IG. I 
21 
2 8 ~  F I G .  
EBERICK P. LANOAUER 
INVENTOR. 
BY 
TTORNEYS 
ct, 
I ST AE 
TI0 
MEANS FOR GEKERATING A SYNC SIGNAL IN AN FM 
COMMUNICATION SYSTEM 
0 
Filed Dec. 21, 1967 2 Sheets-Sheet : 
iY 
d w  I o  c 
INVENTOR. 
FREDERICK P. LANDAUER 
ATTORNEYS 


Means for generating a sync signal in an FM communication system Patent

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