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United States Patent Office
3 , 535,451
Patented Oct. 20, 1970
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^ 3 535 451
^ANS FOR GENERAllI^G A SYNC SIGNAL IN
an fm communication system
fames E. Webb, Administrator of the National Aero-
nautics and Space Administration, with respect to an
invention of Frederick F, Landaaer, Altadena, Calif*
Filed Bee. 21, 1967, Ser, No. 692,332
Int. Cl. H04n 1 /36; H®4I 27/10
U.S. CL 178 — 69.5 ^ Claims
ABSTRACT OF THE DISCLOSURE
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 first 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
[time coincidence.
ORIGIN OF 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
OBJECTS AND SUMMARY OF 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.
5 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
10 with relatively low signal-to-noise ratio.
Still a further object of this invention is the provision
of a new method of recovering synchronizing signals in
a FM video communication system with low signal-to-
noise ratio.
Yet a further object of this invention is to provide
means to recover synchronizing signals transmitted as
part of, and together with FM video information.
These and other objects of the invention are achieved
by transmitting, to a receiver, each sync signal as fre-
quencies in two narrow frequency bands which are adja-
cent to one another in the frequency spectrum. The trail-
ing edge in the first frequency band, hereafter referred
to as the sync tip frequency, corresponds in time to the
g 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
sync tip and porch frequencies comprising each sync
pulse. The two channels are interconnected in such a
gQ manner that only when the two sync-pulse-defining fre-
quencies are received in the proper time relationship is an
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
gg best be understood from the following description when
read in conjunction with the accompanying drawings.
This 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 at 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simple diagram of frequency domains, use-
ful in explaining an aspect of the invention;
FIG. 2 is a block diagram of the present invention; and
FIG. 3 is a diagram of waveforms of signals, pro-
duced in the various elements, shown in FIG. 2.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
The present invention will be described in conjunction
with an FM communication system in which video infor-
mation from black-to-white levels is transmitted at des-
50 ignated frequencies by a RF 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.
55 Referring to FIG. 1, there is illustrated a frequency
domain of a transmission channel in which it is assumed
that a carrier frequency fc 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
60 /c and /d* 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 /s and /p and hereafter are referred to
65 as the sync tip and porch frequencies, respectively. Thus
as seen, the video information falls between the FM car-
rier /c and the sync frequencies /s and /p.
For each sync pulse, signals in the sync tip frequency,
/s are transmitted immediately preceding signals in the
70 porch frequency /p. This produces a transition of inter-
est, represented by line 14 which is common to both fre-
quencies. That is, the trailing edge of signals in the sync
3,535,451
10
tip frequency represented by line 16 corresponds in time
to the leading edge of signals in the porch frequency, /p
represented by line 17. Briefly stated, the leading edge of
the sync tip frequency is utilized as a time reference for
enabling the transition common to both to be used as a
sync time reference. ^
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 at terminal 20 are of a constant ampli-
tude* The use of a demodulator and limiter are well known
in the art of radio communication.
Input terminal 20 is shown connected to a band-pass
filter 21 assumed to be tuned to the frequency band /g.
Filter 21 is in turn connected to a low-pass filter 22
through an envelope detector 23. The band-pass filter 21,
envelope detector 23 and low-pass filter 22 may be thought
of as forming a first detection channel designed to pro-
vide an output signal at the output of filter 22, when sig-
nals in frequency band fs to which filter 21 is tuned, are
received.
The circuit includes a second band-pass filter 24 which
is connected to terminal 20. The filter 24 is tuned to the gg
frequency band /p. Filter 24 is connected to a low-pass
filter 26 through an envelope detector 25. The three cir-
cuits 24, 25 and 26 form a second detection channel
which provides an output signal upon the detection of
signals in the frequency band /p. 3 q
The output of filter 22 is supplied to a Schmitt trigger
circuit 28 to trigger or fire a one shot multivibrator 29
whenever the level of the output signal of filter 22 ex-
ceeds a given level which occurs only when signals in
the sync tip frequency band /s ^re received. The outputs gg
of filters 22 and 26 are supplied to the plus ( -{- ) and minus
( — ) input terminals respectively of a differential ampli-
fier 30, whose output is connected to a second Schmitt
trigger 31. The circuit further includes an AND gate 32
which provides a true output only when inputs from the
one shot 29 and Schmitt trigger 31 are supplied thereto,
coincidentally in time.
The operation of the circuit shown in FIG. 2 may best
be explained by referring to FIG. 3 wherein are dia-
grammed the waveforms or shapes of various pulses or ^g
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
21a and 23a represent the outputs of filter 21 and de-
tector 23 respectively, etc. As seen, the output signals 22a g^
and 26a are supplied to the plus ( + ) and minus ( — )
inputs of differential amplifier 30. The output thereof is
signal 30a, with a transition 30Z> at which the signal
changes from a positive to a negative level. This transi-
tion is sensed by Schmitt trigger 31, which produces pulse gg
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-
quency is supplied. Thus, a short interval exists when
both the one shot 29 and the Schmitt trigger provide true
(assumed positive) pulses, i.e., 29a and 31a. These ac-
tuate AND gate 32 to provide the desired sync-indicat-
ing pulse 32a. gg
In the absence of signals in either frequency band /s
or /p, the inputs to amplifier 30 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
prevent the gate 32 from providing a true output. How-
ever, when a sync pulse is transmitted to, and received
by, the receiver, the leading edge of signals in the band
/s would cause the firing of the one shot 29, and sup-
ply the amplifier with the signal 22a from the filter 22. 75
Then, when the transition common to both frequency
bands occurs, the amplifier 30 is provided with signal 26a
to produce transition 30Z?, so that its output triggers the
Schmitt trigger 31 to provide the activating signal 31a
to gate 32. Consequently, the gate is provided with two
activating signals coincidentally in time, causing it to
provide a true output 32a. This output may be used 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., at a time when the trailing edge
of signals in the frequency band fs and the leading edge
of signals in the frequency band /p are received.
It should be noted that since the noise at the summing
point input to the differential amplifier 30 is derived from
two separate frequency domains /s and /p> assumed to be
of equal bandwidth but statistically from separate sources,
the sum of the noise may be expressed as
where An and Bn represent noise in the signals, in the first
and second channels, respectively. The noise sum is 3 db
greater than the separate noise in each signal. However,
the peak-to-peak signal is the sum of the two or 6 db
greater than each separate signal. Thus, for a given signal-
to-noise condition, jitter due to noise vs. rise time is 3 db
less than would be the case without the amplifier 30. In
brief, if the signal-to-noise ratio with one detection chan-
nel is 1, the two channel detection arrangement increases
the signal-to-noise ratio to 2/2.
There has accordingly been shown and described herein
a novel circuit for detecting sync pulses which are trans-
mitted to the circuit as signals in two adjacent frequency
domains with a transition common to both domains. It
should be appreciated that those familiar with the art may
make modifications and/or substitute equivalents in the
arrangements as shown without departing from the spirit
of the invention. Therefore, all such modifications and/or
equivalents are deemed to fall within the scope of the
invention as claimed in the appended claims.
What is claimed is: ^
1. In a system in which a sync pulse is transmitted to |
a receiver in the form of signals in a first frequency band,c|
followed by signals in a second frequency band, a circuit!
in said receiver for providing an output signal represent-!
ing said sync pulse, comprising: I
a first detection channel responsive to the signals in said
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-
vided in time coincidence. I
2. The circuit as recited in claim 1 wherein each of |
said first and second detection channels includes a narrow I
bandpass filter, an envelope detector and a lowpass filter, |
and said gating means is an AND gate. |
3,535,451
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3. In a receiver for receiving a sync pulse in the form
of signals in a first frequency band, followed by signals
in a second frequency band, a circuit for providing an out-
put signal representing said sync pulse comprising:
a first detection channel to which said signals in said
first frequency band are applied for providing a first
signal of a preselected level, said first signal having
leading and trailing edges;
a second detection channel to which signals in said sec-
ond frequency band are applied for providing a sec-
ond signal of a preselected level, said second signal
having leading and trailing edges;
pulse generating means responsive to the leading edge
of said first signal for providing a first activating
pulse of a duration which is slightly longer than the 15
duration during which signals in said first frequency
band are received;
differential means coupled to said first and second de-
tection channels and responsive to said first and sec-
ond signals for providing an output with a defined 20
level transition when said first and second signals are
of a said preselected levels;
trigger means coupled to said differential means for
providing a second activating pulse in response to the
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
dentally supplied thereto.
6
4. The circuit as recited in claim 3 wherein said dif-
ferentiating means comprises a differential amplifier for
providing an output whose amplitude represents the dif-
ference of the levels of the two signals supplied to said
differential means, and said trigger means comprises a
Schmitt trigger for providing said second activating pulse
when said second signal is applied to said differential
amplifier.
5. The circuit as recited in claim 4 wherein said pulse
generating means includes a one shot multivibrator for
providing said first activating pulse.
6. The circuit as recited in claim 5 wherein said gating
means comprises an AND gate to which the first and sec-
ond activating pulses are supplied to provide a sync pulse
indicating signal when supplied with said first and second
activating pulses coincidentally in time.
References Cited
UNITED STATES PATENTS
1,824,635
9/1931
Stocker.
2,401,405
6/1946
Bedford
178—69.5
2,519,911
8/1950
Kuperus.
2,741,661
4/1956
De France.
3,317,669
5/1967
Ohnsorge >
178—69.5
ROBERT L. GRIFFIN, Primary Examiner
G. G. STELLAR, Assistant Examiner
U.S. Cl, X.R.
178—7.3
Oct. 20, S®70 JAMES E. WEBB 3,535,451
ADMINISTRATOR OF THE NATIONAL AERONAUTICS
AND SPACE ADMINISTRATION
MEANS FOR GENERATING A SYNC SIGNAL IN AN FM
COMMUNICATION SYSTEM
Filed Dec. 21, 1967
2 Sheets-Sheet J