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Патент USA US3042756

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July 3, 1962
P. A. WlGLEY ETAL
3,042,745
HIGH LEVEL COLOUR DEMODULATION SYSTEM'
Filed Nov. 20, 1958
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INVENTORs
PATRICK A. WIGLEY
RONALD LISTER
BY
_
‘a WA. 1?.
AGENT
July 3, 1962
P. A. WIGLEY EI‘AL
3,042,746
HIGH LEVEL COLOUR DEMODULATION SYSTEM
Filed Nov. 20, 1958
2 Sheets-Sheet 2
R-Y DEMODULATOQ
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EQUALIZING _CIRCU|T
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INVENTORS.
PATRICK A. WIGLEY
RONALD LISTER
BY
M 16.395556“;
ilnitcd States
3,042,746
lice
Patented July 3, 1962
1
2
The aforesaid method suffers from a limitation in that
combination of the I-signals and Q-signals to form the
3,642,746
Patrick Augustus Wigley and Ronald Lister, Scarborough,
IHGH LEVEL COLOUR DEMGDULATIUN SYSTEM
colour-diiference signals, by means of so-called matrix--v
networks, involves great loss of signal voltage. Hence,
Ontario, Canada, assignors to North American Philips
Company, Inc., New York, N.Y., a corporation of
the colour signals or the colour-difference signals have to
be subjected to additional ampli?cation which, beside
additional valves, involves in general that the directvcur
Delaware
Filed Nov. 20, 1958, Ser. No. 775,233
rent component'has to be introduced into the ampli?ed
Claims priority, application Canada Nov. 20, 1957 v
4 Claims. (Cl. 178—5.4)
signals.
10
The present invention relates to receivers for use in a
system for transmitting colour television signals in which
the transmitted signal comprises a signal component es
sentially relating to the brightness of a scene and further
comprises a signal component consisting of an auxiliary
carrier-wave modulated in quadrature with two signals of
-
'
In order to avoid this disadvantage, recourse has been
had to high level demodulators which are fundamentally
based on the principle of anode-detection. The reference
signal required in synchronous detection, the frequency
of which signal corresponds to the frequency of the aux
iliary carrier-wave and which has a particular phase rela--~
tion with regard to the auxiliary carrier-wave, is applied
different ‘bandwidths, each of which consists of a given
to the grid of a triode valve, to the anode of which the
combination of signals relating to the respective colour
modulated auxiliary carrier-wave is applied. An appro
priate choice of said phase relation yields demodulation
of the desired signal whilst, however, involving a con
components of the scene, which combinations are such as
to obtain by linear operations colour di?erence signals ~
from the two signals of di?erent bandwidths.
The term “colour difference signals” is to be under
stood to mean signals constituting the ditierence between
a signal relating to a given colour component of the
scene, that is the green, the red or the blue light com-'
ponent of the scene, and the signal component substan
tially relating to the brightness of vthe scene.
In a known system of the aforesaid type, the signal of
smaller bandwidth, the so-called Q-signal, is limited to
500 kc./-s., while the signal of larger bandwidth, the so-' 30
called I-signal, is limited to 1500 kc./s. The quadrature
component of the auxiliary carrier, which is modulated
siderable conversion-ampli?cation.
High level demodulation occurs in the direction of the
(R—Y)—and (B—-Y)-axes and, as the case maybe,
moreover in the direction of the (Gi—Y)-axis. The
(G—Y)-signal, instead of being obtained by means of a
separate demodulator, is also obtainable by a suitable corri
bination of the (R—Y)-signal and the (B-Y) signal:
When using a tricolour reproducing device comprising
three electron guns, combination of the colour-difference
signals with the luminance signal may occur in the con
trol circuits of these guns, for example by applying a
with the Q-signal, is double side-band modulated to 500
colour-difference signal to the control grid of such a gun,
kc./s.; the quadrature component modulated with the
I-signal, the frequencies of which range of from 0 kc./s. OD U! and the luminance signal to the cathode of the gun.
The advantage of high level demodulation in the direc
single side-band from 500 kc./s. to 1500 kc./s.
The component substantially relating to the bright
tion of the (R—Y)-axis and vthe (B—Y)-axis and, as
the case may be, the (G~—-Y)-axis, is that the output sig
ness, that is the Y-signal, substantially corresponds to
nals of the demodulators can then be directly supplied,
the luminance signal of a blackeand-white television
without the intermediary of additional ampli?ers, to the
reproducing device.
In the receiver, the I-signal and the Q-signal can be
High level demodulation in the direction of the axis
rewon from the quadrature-modulated auxiliary carrier
of the colour-di?erence signals, has however, a limitation.
wave by means of synchronous detectors. If demodula
In this case, the phase relation of the signal portion the
tion occurs in the direction of the I-ax'is of the auxiliary
frequencies of which are above 500 kc./s. with regard
carrier, the signal part of the I-signal, the frequencies
to the frequencies of the signal parts, the frequencies of
of which are above 500 kc./s., are obtained with the cor
which amount of from 0 kc./s. to 500 kc./s., is no longer
rect phase relation with regard to the signal part of the
correct to such a degree that, in practice, the various
I-signal, the frequencies of which range from 0 kc./s.
colour-difference signals are limited ‘in bandwidth to
to 500 kc./s. However, the amplitude of the ?rst-men
500 kc./ s. in order to avoid undue errors in reproduction.
tioned part is only half the required amplitude. Since
This however, has the disadvantage that the ?ne details
the human eye is relatively insensitive to abrupt colour
are lost in colour-reproduction of the image.
variations, of which the ?rstementioned signal portion is
The present invention relates to receivers in which they
representative, no steps are in general taken for restoring
loss of ?ne details in colour reproduction of the image
the correct amplitude relation.
From the I- and Q-signals thus obtained, the colour 55 is avoided without the advantage of high level demodu
lation need be lost.
'
difference signals-the (R—Y)-signal and the (B—Y)
For this purpose, the receiver, according to the inven
signal, are subsequently formed according to the formula:
tion has the feature that the modulated auxiliary carrier
wave is supplied to at least two synchronous, preferably
system.
_
~
high level demodulators, in which the auxiliary carrier
E'R represents the signal relating to the red light com
ponent of the scene to be reproduced, E’B represents the
signal relating to the blue light component of the scene,
wave is demodulated in the direction of the axis of rela
tively di?erent colour-difference signals with respect- to
frequencies within the frequency range of the signal of
E'Y represents the Y-signal, E’Q represents the Q-‘signal
smaller bandwidth while the auxiliary carrier-wave is
and E’; represents the I-signal. The accents indicate that 65 moreover demodulated with regard to frequencies out;
the required gamma-correction has already been made
side the frequency range of the signal of smaller band
in composing the various signals. Subsequently, E'G-E'Y
width, and the detection result of this last-mentioned de
modulation is supplied to one or more of the colour—dif
can be deduced from E'R—E’Y and E’B—E’Y. E6 is
the signal relating to the green lightecompo‘nen't of the
ference signals with a phase corresponding to the phase of
scene. Combination of the various colour-di?erence 70 the detection result which would be obtained on demodu
signals and the luminance signal ?nally yields colour
lation of the auxiliary carrier-wave in the direction of the
axis of the signal of larger bandwidth.
signals E'R, E'B and E'Q.
3,042,743
;
.
3
4
reference to the following drawings of which FIGURE 1
, shows the colour phase diagram useful in explaining the
invention;
,
t
_
_
5 FIGURE 2 shows a block diagram of a part of a colour
television receiver in accordance with the invention which
is designed to demodulate along the (R--Y) and (B-Y)
FIGURES 3a, 3b, 3c and 3d show the phase and ampli
tude characteristics respectively of the detected (R_—Y)
and (B-—'Y) signals;
a
7
s
.
-
7
It will be noted from Equation 5, with due regard to
operating constants, that the original low" frequency terms
have been recovered, namely sin (at-l-p) and sin (,Bt-I-y).
So far the above analysis has dealt only with double
sideband transmission. For the region of single sideband
transmission, the following applies. Only the lower e1
vsideband is present for which the demodulated output is:
The invention will now be described in detail with
=% sin (wheat-33°) sin (n+0)
10
~
FIGURES 4a, 4b, 4c show the phase and ampli
tude, characteristics of ?lters designed to correct the
characteristics shown in FIGURE 3;
As was done previously, the second harmonic term is
disregarded, hence:
t
.
'
,.
’ FIGURE 5 is a diagram of a basic ?lter adapted to 15
provide the required response characteristics of FIG
URE 4;
_
FIGURE 6 shows a block diagram of a further em
bodiment of the invention wherein the high frequency
chrominance components are detected along the Iéaxis. 20
In order to more clearly describe the present invention,
As before, the low frequency term has been recovered, '
however with‘ an accompanying phase angle given by
a mathematical analysis of demodulation along the
(0—123°). Also the output-is half the double sideband
(R——Y) and (Br-Y) axis is given with respect to the
well-known colour phase diagram, shown in FIGURE 1
Equations 5 and 6, then, represent the general case of
of the system known as the N.T.S.C. system.
25
double and single sideband demodulated outputs. The
The chrominance information of the N.T.S.C. signal
demodulation ‘axis is determined by the phase angle 0.
is given by: 7
output.
‘
t
i
,
.
Consider now the case of demodulating along (R-Y)
'EC:EQ sin (wt+33°)+EI cos (w't+33°)
(3)
and (B-Y) axis; For this 0:90“ and 0°, respectively.
In the (B—Y) direction:
EQ and B1 are the modulating voltages and for purposes
of analysis a single Fourier component will be considered.
The summation of all Fourier components would result in
E1 or EQ, hence the same analysis would apply for all such
components.
Let
V
'
Double sideband output:
62-“ [cos (#33:) sin (11mm; [sin (-330) sin <iii+~m
'
p
'
EQ:eq sin (at-I-p) and EI:e1 sin (,Bt-I-y)
Rewriting Equation 3:
E¢=eq sin (at-l-p) sin (wt-F330) +ei sin (?H-y) cos (wt
7
=0.g39eq sin (alH-o) —.0'52446i Sin (515+?)
35
'
(7) ,
Single sideband output:
é’fsin <iii+i—12a°>=% sin (Bi+1+57°)
40
(s)
In the (R-Y) direction:
Double sideband output:
This is the general formula of a suppressed subcarrier 45
signal with double sideband transmission. The process of
52% [cos 57° sin (ai+p)]+§[sin ‘57° 'sin()3t+'y)]
:gggzcq sin (at-l-p) +9_'_82;32ei sin (ISM-7)
demodulating the above is treated in a simpli?ed manner
‘as being the product of the sidebands and a sin (mt-+0)
.
function, where 0 is the phase angle of the demodulating
frequency compared with the (3-1’) phase.
The demodulated output is:
50
V
.,
7
Single sideband output: "
'
i
-
V
gsisuswy-aso) ’
since
i
1500 kcs.
55
(9)
V
‘
(.10)
V
’
2 ei Sin (?i+'r)=E’r
0
t
and
1500 kcs.
203
60
then Equations 7,, 8, 9 and 10 can be rewritten, disregard
ing the
+cos (0—Bt—'y—33°)—cos (2wt-l-0+?i+'y+33°)]
1
2
Ignoring the second harmonic terms (i.e. 2wt),,sinc6
it is assumed that the demodulator output circuit has no 65
response to this frequency, it follows that the output is:
demodulation constant, as follows- ‘ >
At (B-Y) terminal:
'
i0 Double sideband output:0.8§9E'Q——‘0.544E'I
(ll)
Single sideband output: 0.5E'I|— 123 ° = -0.5E.'I]5 7 °
7
p
At’ the (R-Y) terminal:
(12)
7
Double sideband output:0.544E'Q+0.839E'I
75 Single sideband 'output:0.5E'I]—33 °
(13)
( 14)
3,042,746
5
.
In order to evaluate these latter equations, it is necessary
to rearrange the N.T.S.C. equations for E6, and E’; as
follows:
6
usually employed and thus negative signals-(R—Y)
and —(R—Y) are readily available.
‘In order that the (R—Y) and (B-Y) signals be cor
rected in phase and amplitude, the signals must bepassed
through appropriate equalizing circuits ‘which have the
amplitude and phase characteristics as shown in FIGURE
4. It is possible to obtain close approximations to these
desired characteristics by the use of a ?lter network, a
basic diagram of which is shown in FIGURE 5. The com‘
10 ponents must, of course, be adjusted in each case to give
(B—Y) output=0.839[0.41(E'B—E'Y)
the proper correction for the colour signal concerned.
In carrying out the embodiment of the invention as
+0.4s(E'R~E'Y)']—0.544[_0.27(E'B_E'Y)
shown in FIGURE 2, in order to obtain the (G--Y) sig
+0.74 (ER-FY) ] =0.49(E'B—E’Y) double sideband
nal, the -—(R-—Y) and —(B—-Y) signals can be mixed
The corresponding single sideband output required for
15 prior to equalization thus saving one ?lter network. Alter
this is (see Equation 2)
Substituting in Equations 11 and 13 the following is
obtained
From 11:
nately, the (G-Y) signal may be produced by the equal
ized (R—Y) and (B-—‘Y) signals, consideration being
given to the required phases, i.e. the -—(R—Y) and
—-(B—Y) signals are used to produce (6-1’).
When the proper correction is applied and the ampli
tude of each colour difference signal is adjusted to suit the
e?iciencyof the appropriate gun in the picture tube, a
The corresponding single sideband output required for
colour picture of high quality may be obtained. The
this is (see Equation 1)
Y-signal applied at terminal 15 is fed through an ap
propriate delay-network 10 to the cathodes 11 of the
='O.88(0.96E’I)=0.84E'I
guns of the picture tube.
Summarizing these results
A further embodiment of the invention is shown in the
At the (R—Y) terminal: '
block diagram of FIGURE 6 wherein only the portions of
the colour television receiver, with which the invention is
Double sideband output=0.49(E’B—E'y)
30 concerned, are shown.
Required single sideband output=—0.54E'I
In this embodiment demodulation is again carried out
Single sideband output obtained=—0.5E’I|_5_7l
on the (R—Y) and (B——Y)Tvectors in demodulators 21
At the (R—Y) terminal:
and 22 to which are supplied at terminal 32 the chromi
Double sideband output=O.88(E'R—-E'y)
Required single sideband output=0.84E'I
Single sideband output obtained=0.5E'I[—33°
nance signal and at terminals 33 and 34 the reference
signals with appropriate phase angles. The outputs of
these detectors are supplied to the appropriate red and
blue guns 25 and 26 through low pass ?lters 23 and 24
having a cut-off frequency of S00 kcs. As was shown
Examination of above results show that errors of both
previously, the distortion in the detected signal is contained
amplitude and phase occur in the single sideband region,
40 in the frequencies above 500 kcs. Thus, by use of low
when demodulating along colour difference axis.
pass ?lters this distortion is blocked from the control grids
It will be noted that the amplitude from the (B—Y)
of the red and blue picture tube guns.
~
_
demodulator does not diifer from the desired output by
Similarly the proper proportions of the negative counter
very much (approximately 8%), however a phase cor~
parts of the (R—Y) and (R—Y) signals are passed
rection of 57° is required for the single sideband frequen
through low pass ?lters 23 and 29, having a cut-oif fre
cies. The (R—Y) demodulator will produce an error
quency of 500 kcs., to the control grid 27 of the green gun.
of amplitude (approximately 5 db) and a phase error of
In order that a high quality colour picture result, de
—33°. In this case, both these errors would require cor
‘ tection of the chrominance signal is also carried out along
rection before satisfactory operation could be obtained.
the I axis so that a distortion-free signal is produced. To
FIGURE 3 shows the phase and amplitude character
istics of the detected (R—Y) and (R—Y) signals. The 50 that end the chrominance signal at terminal 32 is also
applied to a demodulator 36, to which is also supplied at
dashed curve shows the desired characteristic.
terminal 37 a reference signal with appropriate phase
Referring now to FIGURE 2, the block diagram shows
angle.v The output of the I demodulator is supplied to
the detection and equalizing sections of a colour television
the red, blue and green ‘gun control grids 25, 26 and 27
receiver designed to demodulate on the (R—Y) and
through bandpass ?lters 38, 39 and 40 which transmit
(B-Y) vectors and compensate for the resultant distor
signals in the range of 500 kcs. to 1500 kcs. The I sig
tion of the higher chrominance frequencies.
nals will be in the correct phase relationship with the
The block diagram of FIGURE '2 comprises an (R—Y)
other colour signals applied to the control ‘grids of the
demodulator 1, a (B—~Y) demodulator 2, to which are
colour tube guns. Correction can easily be made for
supplied atterminal 12 the chrominance signal and at
the discrepancies in amplitude. The Y-signal supplied
terminals 13 and 14 the reference signals with appropriate
at terminal 35 is vfed through an appropriate delay-net
phase angles. Each of the demodulators supplies its out
work 30 to the cathodes 31 of the guns of the picture
put through the medium of appropriate equalizing net
tube.
’
works 3 and 4, to the control grids 5 and 6 of the red and
From the foregoing it will readily be seen that the in
blue guns respectively.
Selected proportions of the detected (R—Y) and
(B—Y) signals are fed through equalizing circuits ‘8 and
9 to the control grid 7 of the green gun so that the follow
ing equations are satis?ed: Signal at (EG-—Ey)—
65 vention discloses a television demodulation and repro
duction system which allows for high quality colour pic
ture production at a‘ reduced cost.
What is claimed is:
g
1. A receiver for color television signals of the type
Double sideband re gion: —0.5l (E'R—E'y)
comprising a ?rst signal substantially relating to the
'
-O.l9(E’B—E'y)
brightness of a scene, and a second signal consisting of
an auxiliary carrier-wave modulated in quadrature with
Single sideband region: -—0.51 (0.96E'I)
and fourth signals, said third signal having -a larger
--O.19(1.1OE';)=—0.70E’1 third
bandwidth than said fourth signal, each of said third
In high level demodulation balanced modulators are 75 and fourth signals consisting of a given combination of
3,0423%
7v
8
,
signals 'relating to respective color components of the
scene, the combinations permitting formation of color
difference signals of different bandwidths by linear Op
7 erations, said receiver comprising ?rst ‘and second syn
chronous demodulators, means applying said second sig
prising equalizing network means‘connected to the out
put of said synchronous demodulator means and having
such phase and amplitude characteristics, ‘for signals in
said frequency band, that the resultant output signals in
said frequency band have, the phase and amplitude that
emodulators, said oscillations having respective phases
would’ occur by demodulation of said second, signal in the
direction of the axis of said third signal.
4. A receiver for, color television signals of- the type
whereby said second signals are demodulated in said ?rst
comprising a ?rst signal substantially relating to the
nal, to said demodulators, means applying oscillations of
the frequency of said auxiliary carrier-wave to said de-'
and second demodulators in the direction of the axes of 10 brightness of a scene, ‘and a second signal consisting of '
?rst and second color difference signals to provide ?rst
and second output signals respectively, and ?rst and sec
an auxiliary carrier-wave modulated in quadrature with
third and fourth signals, said third signal having a larger
bandwidth than said fourth signal, each of said third and
fourth signals consisting of a given combination of sig-'
second demodulators respectively for equalizing said ?rst
and second output signals respectively, said network 15 nals relating to respective color components of the scene,
the combinations permitting formation of color differ
means having phase characteristics providing such a
ence signals of different bandwidths by linear operations,
phase displacement of said output signals, for signals
"said receiver comprising ?rst and second synchronous de
outside of the frequency band of said fourth signals, that
modulator means, means providing ?rst and second ref
the resultant signals outside of said frequency band have
erence oscillations of "the frequency of said auxiliary
the phase that would occur on demodulation of said sec
carrier-wave and having different phases corresponding
ond signal in the direction of the axis of said third signal.
to selected color difference signals, means applying said
2. The receiver of ‘claim 1, in which ,means are pro
?rst reference oscillation ‘and said second signal to said
vided to combine said ?rst and second output signals to
ond equalizing network means connected to said ?rst'and
provide a third output signal corresponding to a third
color difference signal.
j
a
p
‘
.
?rst demodulator means,’ means applying said second
25 reference oscillation and said second signal to said sec
ond demodulator means, and ?rst and second equalizing
3. A receiver for color television signals of the type
network means connected to the outputs'of said ?rst and
comprising a ?rst signal substantially relating to the
second demodulator means respectively, said network
brightness of a scene,’ and a second signal consisting of
means having phase and amplitude characteristics pro
an auxiliary carrier-wave modulated in quadrature with
third and fourth signals, said third signal having a larger 30 viding such a phase displacement and amplitude’ correc
tion for the output signals of said demodulator means,
bandwidth than said fourth signal, each of said third and
for signals in’the frequency band of said third signal
fourth signals consisting of a given combination of sig
exceeding the frequencies of said fourth signal, that the
nals relating to respective 'color components of the scene,
resultant signal output of said network means in said
the combinations permitting formation of‘ color differ
ence signals of different bandwidths by linear operations, 35 frequency band have the phase and amplitude that would
occur by demodulation of said second signal in the direc
said receiver comprising synchronous, demodulator
tion of the axis of said third signal.
means, a source of reference oscillations of the frequency
‘of said auxiliary. carrier-wave and having a phase corre
References Citedrin the ?le of this ‘patent
sponding to the phase of a selected color difference sig
nal, means applying said reference oscillations and sec 40
UNITED STATES PATENTS‘
ond signal to said synchronous demodulator means to
2,831,919
Lockhart ____________ __ Apr. 22, 1958
provide an output signal, and ‘means for correcting the
2,857,457
Richman ____________ __ Oct. 21, 1958
phase and amplitude of said output signals in the fre
Lockhart _________ -______ Oct. 13, 1959
quency band of said third signal which exceeds ‘the fre
2,908,752
quencies of said ‘fourth signal, said correcting means com 45
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