Note: Descriptions are shown in the official language in which they were submitted.
03
This inveTItioll relates to electrical multi-channel signal trans-
miSsiOIl SyStellls, itl which different signals are transmitted by carriers (i.e.
carrier waves) witll different frequencies.
In practice, when transmitting such signals from one place to an-
other, whether through the atmosphere and/or along wires, it is usually nec-
essary to pass the carriers, with their signals, through non-linear electri-
cal devices, which cause intermodulation products ~often known as beats) to
be produced by interaction between the carriers. Such non-linear electrical
devices may for example be amplifiers. These intermodulation products may
interfere with the signals carried by other carriers, thereby rendering clear
reception of the signals impossible.
One well known example of a transmission system in which such prob-
lems arise is in the system known as cable television, in which television
programs being transmitted through the atmosphere are picked up by a sophis-
ticated antenna at a central station and, after suitable processing including
amplification, are transmitted by cable to individual subscribers. The ad-
vantage of such a system is that the subscribers can receive television pro-
grams from places which are too far away from the subscriber to enable the
programs to be picked up by a simple antenna of the kind normally found on a
domestic television receiver. Processing of the signals and their carriers
at the central station, and perhaps also at sub-stations, includes their
treatment by non-linear electrical devices, such as certain kinds of ampli-
fiers, which cause intermodulation products to be produced, with consequent
likelihood of interference, as previously mentioned.
The problem could be avoided if the frequencies of the various car-
rier waves were allocated in such a manner that the intermdoulation products
were not of such frequencies as to produce unwanted interference with other
carrier frequencies.
.~ - 1 -
603
For example, if each carrier frequency was an integral multiple of the dif-
ference in frequency between each carrier in the series, the significant
intermodulation products would be at the frequencies of other carriers, and
hence would not be detected as modulation by the signal reception equipment.
This is known as a harmonically related carr-er system. However, in many
existing transmission systems of this type, for example systems for trans-
mitting radio and television programs, the carrier frequencies have been es-
tablished for many years, and it is not practically possible to change the
carrier frequencies at the original transmitting station.
It has been previously proposed to avoid the problem found with
existing transmission systems by receiving the carriers, and of course the
signals carried thereby, on their original frequencies at a central station,
and re-transmitting the signals on harmonically related carriers, which are
obtained from a common frequency source. However, certain difficulties are
imposed by this method. For example, domestic television receivers can only
be tuned to certain selected channels, that is to say to only receive car-
riers and their signals at certain selected frequencies, since they only
have a main tuner with pre-set positions corresponding to the frequencies of
the carriers that the television set is intended to receive, and it is not
practically possible to manufacture a main tuner with pre-set positions which
correspond exactly to the carrier frequencies, since in practice such fre-
quencies vary from time to time about a fixed value. The receivers are
therefore also provided with a fine tuner, which can be adjusted to provide
fine tuning when the main tuner has been adjusted to a pre-set position.
The main tuner is set for adjustment to the frequencies at which the various
carriers are originally transmitted, such frequencies having been estab-
lished many years ago, as previously mentioned. If the signals, that is
L~
03
to say the television l-rograms, are re-tr<~nsmitted on h~rmonically related
carriers, it is fo~u~d th.lt it is not possible to satisfactorily tune all dom-
estic television receivers to the new carrier frequencies, because the new
carrier frequencies are so far apart from the original carrier frequencies,
for which the main tuner is pre-set, that the fine tuner does not have a suf-
ficiently wide range of adjustment to enable the new carrier frequencies to
be received.
Another way of overcoming the problem would be to eliminate non-
linear electrical devices, such as single-ended amplifiers, i.e. non push-
pull amplifiers, with which many existing cable television systems aTeequipped. Though it is possible to avoid the problem by replacing the single-
ended amplifiers by amplifiers of the push-pull kind, the cost of such a pro-
cedure is undesirably high.
It is therefore an object of the invention to provide a transmis-
sion system in which the carrier frequencies, while not harmonically related,
are judiciously selected to reduce signal interference by intermodulation
products to a tolerable level.
According to the invention, each carrier frequency is expressed as
an integral multiple of a constant which is a measure of the frequency spacing
plus an amount (which may be positive or negative) selected such that the sig-
nificant intermodulation products are coincident in frequency with other car-
rier frequencies. The amount may be an integral multiple of another constant,
or may be one of two constants with their sum or difference. In the former
case, the values of the constants may be found by selecting two carrier fre-
quencies, and solving the resultant equations accordingly. In the later
case, the values of the constants may be found by selecting three carrier
frequencies, and solving the resulting equations accordingly. The values of
the constants may then be used as the frequencies of oscillators
i03
in an appropriate synthesizer to produce the other carrier frequencies.
The invention enables the new frequencies to be selected in
such a manner as to produce relatively small shifts between the original
and new carrier frequencies.
The invention is especially useful in cable television systems
which pick up and transmit television programs and possibly other kinds
of signals in the standard low, mid and high television carrier bands,
in that it enables second order intermodulation products to be controlled
in such a manner that the mid band channels can be used to a greater
extent than before.
The two or three selected frequencies, from which the other
frequencies are synthesized, may be original channel frequencies. This
enables these frequencies to be phase locked to off-air carriers where
ambient pick up may produce a interference problem.
It is possible with a multi-channel band to lock some channels,
but not others, so as to obtain an optimum arrangement in which, on the
one hand the problem of ambient reception is minimized, and on the other
hand the problem of shift between original and new frequencies is mini-
mized.
Thus, in accordance with one aspect of the present invention
there is provided a cable television system for reducing second order
intermodulation products caused by interaction between carriers in non-
linear electrical devices in the system wherein the carrier frequencies
of the channels shown in the first column of the following table are
re-assigned as expressed in analytical form in the fourth column of the
table, the standard frequencies given in the second column being express-
able in analytical form as shown in the third column of the table where
x = 6 and y = 0.25, and the values of x and y for the re-assigned frequ-
encies being found by selecting frequencies for two channels, setting
them equal to the corresponding expressions in the fourth column to form
two simultaneous equations, and solving for x and y:
--4--
1144~03
TABLE
Channel Freq. MHz Standard New Assignment
_
2 55.25 9x + 5y 9x + Sy
3 61.25 lOx + 5y lOx + 5y
4 67.25 llx + 5y llx + 5y
77.25 13x - 3y 13x - 3y
6 83.25 14x - 3y 14x - 3y
C 133.~5 22x + 5y 22x + 2y
D 139.25 23x + 5y 23x + 2y
E 145.25 24x + 5y 24x + 2y
F 151.25 25x + Sy 25x + 2y
G 157.25 26x + 5y 26x + 2y
H 163.25 27x + 5y 27x + 2y
I 169.25 28x + 5y 28x + 2y
7 175.25 29x + 5y 29x + 7y
8 181.25 30x + 5y 30x + 7y
9 187.25 31x + 5y 31x + 7y
193.25 32x + 5y 32x + 7y
11 199.25 33x ~ 5y 33x + 7y
12 205.25 34x + 5y 34x + 7y
13 211.25 35x + 5y 35x + 7y
said system comprising two oscillator means for generating the frequencies
of said two channels, means for combining the two generated frequencies
to produce a third frequency, a harmonic generator means for generating
a channel spacing frequency and harmonics thereof, and means for combin-
ing and filtering frequencies from the two oscillator means, said third
frequency and the harmonic generator means to derive other re-assigned
channel frequencies.
In accordance with another aspect of the invention ~here is
provided a cable television system for reducing second order intermodu-
lation products caused by interaction between carriers in non-linear
electrical devices in the system wherein the carrier frequencies of the
channels shown in the first column of the following table are re-assigned
so that they have the frequencies shown in the third column and are
expressable in analytical form as shown in the second column and where x,
-4a-
11'~4f~03
u and v are constants having the values 6, 0.25 and 1.125, respectively:
TABLE
Channel Equation Re-assigned Frequency
Frequency ~MHz) Deviation ~KHz)
2 9x + v55.125 -125
3 lOx + v61.125 -125
4 llx + v67.1~5 -125
13x + u - v 77.1~5 -125
6 14x + u ~ v 8~ 5 -125
C 22x + u132.250 -1000
D 23x + u138.250 -1000
E 24x + u144.250 -1000
F 25x + u150.250 -1000
G 26x + u156.250 -1000
H 27x + u162.250 -1000
I 28x + u168.250 -1000
7 29x ~ u + v 175.375 +125
8 30x + u + v 181.375 +125
9 31x + u + v 187.375 +125
32x + u + v 193.375 +125
11 33x + ~ + v 199.375 +125
12 34x + u + v ~05.375 +125
13 35x + u + v 211.375 +125
said system comprising first, second and third oscillators for generating
first, second and third re-assigned channel frequencies, ~eans for com~
bining said first frequency with said second frequency and said second
frequency with said third frequency to produce fourth and fifth frequencies,
means for producing harmonics of said fifth frequency, and means for
combining and filtering the first, second, third, fourth and fifth frequ-
encies and harmonics to produce the other re-assigned channel frequencies.
In accordance with a further aspect of the invention there is
provided a cable television system for reducing second order inter du-
lation products caused by interaction between carriers in non-linear
electrical devices in the system wherein the carrier frequencies of the
channels shown in the first column of the following table are re-assigned
-4b-
11~4~;03
so that they are expressable in analytical form as shown in the fourth
column, the standard frequencies given in the second column being ex-
pressable in analytical form as shown in the third column of the table
where x = 6 and y = 0.25, the values of x, u and v for the re-assigned
frequencies being found by selecting frequencies for three channels,
setting them equal to the corresponding expressions in the fifth column
to form three simultaneous equations, and solving the equations for x,
u and v:
TABLE
Channel Freq. MHz Standard Re-assigned Equation
Frequency
(~Z )
2 55;25 9x + 5y 55.125 9x + v
3 61.25 lOx + 5y 61.125 lOx + v
4 67.25 llx + 5y 67.125 llx + v
77.25 13x - 3y 77.125 13x + u - v
6 83.25 14x - 3y 83.125 14x + u - v
C 133.25 22x + 5y 132.250 22x + u
D 139.25 23x + 5y 138.250 23x + u
E 145.25 24x + 5y 144.250 24x + u
F 151.25 25x + 5y 150.250 25x + u
G 157.25 26x + 5y 156.250 26x + u
H 163.25 27x + 5y 162.250 27x + u
I 169.25 28x + 5y 168.250 . 28x + u
7 175.25 29x + 5y 175.375 29x + u + v
8 181.25 30x + 5y 181.375 30x + u + v
9 187.25 31x + 5y 187.375 31x + u + v
193.25 32x + 5y 193.375 32x + u + v
11 199.25 33x + 5y 199.375 33x + u + v
12 205.25 34x + 5y 205.375 34x + u + v
13 211.25 35x + 5y 211.375 35x + u + v
said system comprising first, se~ond and third oscillators for generating
first, second and third channel frequencies, means for combining said
first frequency with said second frequency and said second frequency with
said third frequency to produce fourth and fifth frequencies, means for
producing harmonics of said fifth frequency, and means for combining and
-4c-
. `
03
filtering the first, second, third, fourth and fifth frequencies and
harmonics to produce the other re-~ssigned channel frequencies.
Embodiments of the invention will now be described, by way of
example, in which:
Figure 1 is a block diagram showing how the carrier frequencies
are converted, according to one embodiment of the invention, where no phase
lock to off-air signals is required;
Figure 2 is a block diagram showing how the carrier frequencies
are converted, according to a second embodiment of the invention, where
off-air phase
-4d-
~,
~144603
lock is required;
Figure 3 is a block diagram showing a typical circuit
for phase-locking the oscillators to selected
off-air frequencies;
Fi~ure 4 is a block diagram showing a typical circuit
for phase-locking a signal tc its reference
carrier, and
Figure 5 is a block diagram showing a typical cable
television transmission system.
As indicated previously, the invention is especially
useful with cable television systems. Table 1 which follows
shows in the first two columns the standard frequencies in meg-
ahertz of the television channels in the low low band, the high
low band, the mid band and the high band. The third column
shows the channel frequency in standard analyti~ form and the
fourth column the assigned channel frequencies in accordance
with the invention also in analytic fonm. ~he derivation of
these forms will be more ~ully explained later.
T A B L E
Channel Freq. MHz Standard New Asslgnment
( 2 55.25 9x + 5y 9x + 5y
Low Low ( 3 61.25 lOx + 5ylOx ~ 5y
Band ( 4 67.25 llx + 5yllx + 5y
High Low ( 5 77.25 13x - 3y 13x - 3y
Band ( 6 83.25 14x - 3y 14x - 3y
( C 133.25 22x + 5y22x + 2y
( D 139.25 23x + 5y23x + 2y
E 145.25 24x + 5y24x + 2y
~id Band ~ F 151.25 25x + 5y25x + 2y
( G 157.25 26x + 5y26x + 2y
( H 163.25 27x ~ 5y27x + 2y
( I 169.25 28x + 5y28x + 2y
( 7 175.25 29x + 5y29x + 7y
( 8 181.25 30x + 5y30x + 7y
( 9 187.25 31x + 5y31x ~ 7y
~igh Band (10 193.25 32x + 5y32x + 7y
3~ (11 199.25 33x + 5y33x + 7y
(12 205.25 34x + 5y34x + 7y
(13 211.25 35x + 5y35x + 7y
~0
~ #1089 P/2 CA ~ 5 ~
;03
For the purposes of this ~pecification, each group of
channels is defined as a 'set', and the term "adjacent" refers
to channels within a set.
Channels A and B are not shown in the mid band, because
they are not used as television channels in Canada, since these
channels are reserved for other purposes. However, the invention
I
2~
#1089 P/2 CA - 5 a -
. ~ ,
03
is equally applicable to SUc}l cll~nels.
Many e~istillg cable television systems were constructed to process
only the low b;~nd ~u~d higll band cll~lnels, and as previously mentioned, many
existing systems were equipped with broad band single-ended amplifiers. When
the standard low and high band television carriers are applied to such ampli-
fiers, intermodulation products are produced which appear in the mid band.
In the ?ast, this has not been a significant problem, since the mid band was
not used for domestic television purposes. However, now the mid band is
being so used, the incorporation of mid band channels into existing cable
television systems results in the intermdoulation products mentioned becoming
a significant problem. Further problems are also introduced because the am-
plification of mid band carriers by single-ended amplifiers also produces un-
desirable intermodulation products in the lower and high bands. Such inter-
modulation products produce horizontal or vertical lines superimposed on a
television picture, and these are clearly undesirable.
In its application to cable television, the present invention in-
corporates an appreciation that the most undesirable intermodulation products
produced by the single-ended amplifiers are caused by second order sum and
difference products between the various bands, and according to the invention
various carrier frequencies can be re-assigned to substantially eliminate the
effect of such intermodulation products by ensuring that the frequencies of
these products coincide with various re-assigned carrier frequencies, so that
they are not detected as modulation in the receiving equipment.
Referring again to table 1, the various channel frequencies are ex-
pressed in analytical form in the third column, according to the inVeDtiOn,
in terms of constants x and y, where x equals 6 and y equals 0.25. With the
standard frequencies
~ _,
603
expressed in this form, it can be seen how amplifications by
single-en~ed amplifiers of low and high band carriers produces
undesirable intermodulation products in the mid band, and that
amplification of additional mid-band carriers produces inter-
modulation products in the low and high bands. For example, the
sum product of channel 4 and 6 carriers has a sum of 25x + 2y,
which produces an undesirable intermodulation product within
channel F, i.e., a 3y or 0.75 M~z beat will occur. Also, chan-
nel 10 and channel D carriers have a difference product, which
produces an undesirable intermodulation product, a 1.25 MHz beat,
with channel 2 carrier.
According to the invention, the channel frequencies are
re-assigned by varying the integral multiple of the y constant
of the analytical expressions to the values as shown in column
4 to give the desired frequency congruence and substantially
solve the problems. It will now be seen that the channel 4
and channel 6 carriers now have a sum which is equal to channel
F carrier and that the channel 10 and channel D carriers now
have a frequency difference which is equal to the channel 2
carrier.
The values of x and y can be found by solving two of
the carrier equations simultaneously. This provides a great
deal of flexibility,since ~he products of the equations can
be selected to produce the least amount of shift from the stan-
dard carrier frequencies, or they can be chosen to allow lock-
ing onto off-air carriers, where ambient reception of these
carriers by a domestic receiver may result in interference
problems. This may happen when a transmitter on a particular
channel i8 SO near a receiver that the receiver receives the
off-air signal from the transm~tter as well as the signal re-
ceived over the cable system. Tn this latter case, the fre-
~1089 P/2 CA - 7 -
f~
03
quency of the particular channel concerned may be used as the
frequency of one of the basic oscillators. ~ further basic
oscillator is of course required, owing to the presence of the
two variables, i.e., x and y, in the analystical expressions.
For example, if it is necessary to lock onto channel 6
and channel ll,the frequencies of x and y can be found as follows:
#1089 P/2 CA - 7 a -
11 ~4f~03
Channel 11 = 3-x + 7y = 1(~
Ch~ cl ~ = 14~ - 3y = 83.26 MH~
therefore, x equals 5.9929 MHz
y equals 0.2135 MHz
These values of x and y are then substituted in the re-assigned an-
alytical expressions shown in column 4 of table 1, and the resulting re-
assigned carrier frequencies are shown in the second table which follows:-
TABLE 2
ChannelStandard Re-assigned Frequency
Frequency (MHz) Frequency (MHz~Deviation (KHz)
2 55.25 55.0036 246
3 61.25 60.9965 253
4 67.25 66.9893 260
77.25 ~7.2671 17
6 83.26 83.26 none
C 133.25 132.2708 979
D 139.25 138.2635 986
E 145.2~ 144.2564 993
F 151.25 150.2495 1,~00
G 157.25 156.2422 1,007
H 163.25 162.2351 1,015
I 169.25 168.22~0 1,021
7 175.25 175.2884 38.5
8 181.25 181.2813 31.3
9 187.25 187.2742 24.2
193.25 193.2671 17.1
11 199.26 199.26 none
12 205.25 205.2529 2.9
13 211.25 211.2457 4.2
This second table also shows the frequency shift in column 4, that
is to say the difference between the original carrier frequency and the re-
assigned carrier frequency, for each channel. In the high and low bands,
the shift is well within the fine tuning range of standard domestic re-
ceivers. A slightly greater shift is provided in the mid band channels, but
this is not a practical problem since at the present time most domestic re-
ceivers are not equipped with mid band tuners, i.e. are not equipped to re-
ceive mid band channels, and consequently will in any event have to be used
with an external converter which can incorporate its own fine tuner adequate
to meet the shift. The external converter will for example simply convert
the selected mid band channel into
l l`t~
the carrier frequency of one of the channels which the receiver
is equipped to receive, with the cable transmission on the chan-
nel frequency having of course been filtered out.
A further way of expressing the channel freguencies in
analytical form, according to a second embodiment of the inven-
tion, is shown in the second column of table 3, which follows,
where no phase-locking to off-air signals is required, even
smaller frequency shifts can be produced off the standard car-
riers.
T A B L E 3
=
Re-assigned Frequency
Channel Equation Frequency (MHz) Deviation (RHz)
-
2 9x + v 55.125 -125
3 lOx ~ v 61.125 -125
4 llx ~ v 67.125 -125
13x ~ u - v 77.125 '-125
~ 14x + u - v 83.125 -125
C 22x + u 132.250 -1000
D 23x + u 138.250 -1000
E 24x + u 144.2S0 -1000
F 25x + u 150.250 -1000
G 26x + u 156.250 -1000
H 27x + u 162.250 -1000
I 28x ~ u 168.250 -1000
7 29x + u + v 175.375 +125
8 30x + u + v 181.375 ~125
9 31x ~ u + v 187.375 ~125
32x + u + v 193.375 +125
11 33x + u + v 199.375 +125
12 34x 1 u + v 205.375 +125
13 35x + u + v 211.375 +125
In this case, each carrier frequency is shown expressed
in re-assigned form in terms of three constants, namely, x, u
and v. If x, u and v are given values of 6MHz, 0.25 ~Hz, and
1.125 M~z, respectively, second-order intermodulation may be
avoided with a carirer shift of only 125 KHz in the standard
twelve television channels. It can eas~ly ~e seen that the
frequency shifts for the low and high bands are a maximum of
125 KHz in contrast to the much higher maximum which occurs with
channel 4 in the arrangement shown in the second table.
~ lOB9 P/2 CA - 9 -
~ ~.
Figure 1 shows how the various re-assigned channel frequen-
cies can be produced in a master generator fro~ the two initial
oscillators, in conjunction with a further oscillator generating
a carrier frequency of 6 MHz, which is a typical channel spacing.
The outputs of the oscillators 10,12 are selected to be
the re-assigned frequencies of channel 3 and channel E, 61.125 MHz
and 144.250 MHz, respectively, and are combined in mixer 14 to
produce a pair of frequencies, namely, the sum and the difference
of the two frequencies mixed, thereby producing frequencies of
205.375 MHz and ~-~.125 MHz respectively. These new frequencies
are passed through band pass filter~ 16,18, each of which pass a
respective one of these two frequencies, filter 16 passing only
the frequency of 83.125 MHz and filter 18 passing only the fre-
quency of 205.375 MHz. It will be seen that these are the re-
assigned frequencies of channels 6 and 12 respectively.
The output from the third oscillator 20 is passed through
a harmonic generator 22 which produces frequencies of 6 MHz, 12
MHz and 18 MHz. The other channel frequencies are producea by
appropriate mixing and filtering using the two oscillators 10,12,
the frequencies produced from filters 16,18 and the various fre-
quencies from the harmonic generator 22. As shown in Figure 1,
four more mixers 24,26,28 and 29 are provided, as well as appro-
priate filters 30-40 which filter out all except the re~uired
channel frequency in each case.
For example, mixer 24 is fed a 61.125 MHz signal from oscil-
lator 10 and the signals from the harmonic oscillator 22. Of
the various frequencies produced, only two of these frequencies
are actually required, namely, 61.125 MHæ - 6 MHz, which i~ chan-
nel 2, 55.125 MHz, and 61.125 MHz ~ 6 MHz, which is channel 4,
67.125 M~z. Filter 30 therefore filters out all frequencie~ ex-
cept 55.125 MHz for channel 2, and filter 31 filter~ out all
frequencies exceFt 67.125 NHz for channel 4. The other channel
~1089 P/2 CA - 10 -
''.~.~
6Q3
frequencies are provided in a similar manner. It will be seen
that what is produced is a series of combs with a particular
mathematical relationship, the combs being formed by channels
2 to 4, channels 5 and 6, channels C to G, and channels 9 to 13.
By use of three constants in the analytical expressions,
it is possible to phase-lock to three off-air channels. For
example, it may be desirable to lock to channel 6, channel 10,
and channel 13.
In this case:
channel 6 = 83.25 MHz = 14x + u - v
channel 10 = 193.26 MHz = 32x + u + v
channel 13 = 211.24 MHz = 35x + u + v
Solving these three equations:
x = 5.99333 MHz
u = 0.408333 MHz
v = 1.065 MHz
Using these values, the re-assigned channel frequencies
are calculated, as shown in table 4 which follows, the frequen-
cy shift also being shown:
~1089 P/2 CA - 11 -
`
11~4~03
TABLE 4
Channel Equation Re-assigned Frequency
~requency (MHz) Deviation (MHz)
2 9x + v 55.005000 - .245000
3 lOx + v 60.998333 - .251666
4 llx + v 66.991666 - .258333
13x + u - v 77.256666 + .006666
6 14x + u - v 83.250000 .000000
C 22x + u 132.261666 - .988333
D 23x + u 138.255000 - .995000
E 24x + u 144.248333 - 1.001666
F 25x + u 150.241666 - 1.008333
G 26x + u 156.235000 - 1.015000
27x + u 162.228333 - 1.021666
I 28x ~ u 168.221666 - 1.028333
7 29x + u + v 175.280000 + .030000
8 30x + u + v 181.273333 + .023333
9 31x ~ u + v 187.266666 + .016666
32x + u + v 193.260000 + .010000
11 33x + u + v 199.253333 + .003333
12 34x ~ u + v 205.246666 - .003333
13 35x + u ~ v 211.240000 - .010000
The manner in which the various frequencies shown in table 4 are
obtained in a master generator is shown in Figure 2. Again, three oscillators
are provided, in this instance oscillator 52 is provided with a frequency of
83.25 MHz, oscillator 54 with a frequency of 193.26 MHz, and oscillator 56
with a frequency of 211.24 MHz, namely the frequencies of channel 6, channel
10 and channel 13. The frequencies from oscillators 52, 54 are combined in
mixer 58, and the difference is filtered out by the filter 60, leaving the
other frequency of 110.01 MHz. This is halved in divider 62, ~hich gives a
frequency of 55.005 MHz, which is the frequency of channel 2.
Similarily, the difference between oscillators 54 and 56 is pro-
vided by mixer 64 and filter 66, divided by three in divider 68, and filtered
by filter 70, to produce a frequency of 5.9933 MHz, which is the wanted chan-
nel spacing "x". The output from filter 70 is passed into a harmonic gener-
ator 72 which supplies frequencies of 5.9933 MH2, 11.9~66 MHz and 17.98 MHz.
As before, these various frequencies are appropriately mixed and filtered in
the manner indicated in Figure 2 to produce the various channel
frequencies. It is not believed necessary to individuall~ indicate each mix-
er and filter by reference numerals in this instance.
The off-air locking is achieved by slaving the three basic oscilla-
tors 54, 52, and 56 to the respective three off-air carriers. This is done
by phase comparison. Phase comparators generally operate at low frequency,
and the high frequencies used in the television channels are therefore con-
verted to low frequencies, for phase comparison, a typical circuit being
shown in Figure 3.
The output from a local oscillator 74 is mixed with the off-air
frequency in a mixer 75, and is mixed with the output from the respective
basic oscillator in a mixer 78. The outputs from the mixers 75, 78 (with the
"sum" products filtered out) are fed to a phase comparator 80, which operates
on the balanced mixer principle and produces a correction voltage for the
basic oscillator which pulls the basic oscillator into place. As shown, the
off-air frequency is that of channel 6 to which the basic oscillator 52 is
slaved. Similar circuits are provided for the oscillators 54, 56, which are
slaved to off-air channels 1 and 13.
Similarily to locking the three basic oscillators to the three off-
air signals, it is now possible to lock all signals to be distributed to
their respective carriers, by feeding an appropriate correction voltage to
the appropriate local oscillator of the converter in the processor at the
head end of the cable television system. A suitable circuit is shown in Fig-
ure 4.
Again, fre~uency reduction is achieved by subtracting the frequency
of a local oscillator 82 from the reference signal frequency, and from the
sample from the processer 84 at the head end of the cable television system
in mixers 86, 88 respectively, with phase comparison being made by a phase
comparator 90, the correction voltage from which is fed to the respective
local oscillator in the processer.
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To complete the descriptioll of the embodiment of the invention, the
application to a standard cable television transmission system will now be
briefly described~ with reference to Figure 5.
Figure 5 shows a head end 92 which receives television signals from
various transmitters operating on various frequencies by means of respective
antennae 94. The cablevision station may for example also have its own chan-
nel, represented by camera 96. From the head end 92, a trunk line 98 with
amplifiers 100 is split into various lines as required to supply the signals
to domestic television receivers in subscribers' homes, one home 102 with a
television receiver 104 being shown.
In the head end, the television signals of the various channels,
which are of course in the radio fre~uency (RF) band, are each converted from
the original frequencies to the re-assigned frequencies according to the in-
vention, and then are sent out along the trunk lines 98, after the usual
stages of modulation, RF amplification, RF conversion and signal combination.
It will be understood that the invention is not limited to the
transmission of signals by cable, nor to the transmission of radio or tele-
vision programs. Further, various alternatives to the described embodiments,
within the scope of the invention, will be apparent to the man skilled in the
art, the scope of the invention being defined by the appended claims.
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i .,
