Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
09
Title of the Invention
Televlsion signal receiving apparatus
~ackground of the Invention
1. Field of the Invention:
The present invention relates to an improvement of
a television signal receiving apparatus. This invention
particularly concerns an improvement of a television signal
receiving apparatus for receiving a television signal which is
made by a frequency modulation by a video signal.
2. Background o~ the Invention:
In the receiving of a television signal, which is
made by a frequency modulation by a video s~gnal, there has
been a problem that black impulse noise and white impulse noise are likely to
be produced in the reproduced picture, especially when the
power of the received television signal is weak. Hitherto, no
, effective measure for decreasing such noises on the reproduced
picture has been proposed.
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Summary of the Invention
The present invention, therefore, purports to provide
an improved television signal receiving apparatus capable of
reproducing picture of an improved quality.- The television
signal receiving apparatu~ of the prasent invention is suitable
for receiving a television signal which is made by frequency
modulation by a video signal. Accordingly, the apparatus in
accordance with the present invention is suitable for use in a
television signal relaying or transmission system, and in
panticular, in 2 satellite television broadcasting system.
Brief Description of the Drawing
FIG. 1 is a circuit diagram showing general construction
of a known television signal receiver for elucidating principle
of the present invention.
FIG. 2 is a waveform diagram showing a waveform of a
video signal.
FIG. 3 is a graph showing a general relation between
noises of the picture and shifting of the center frequency of
lo a band-pass filter, of a television signal receiving apparatus.
FIG. 4(a) is a vector diagram for elucidating an
effect of noise to the video signal.
FIG. 4(b) and FIG. 4(c) are time charts to elucidate
generation of a noise in the video signal.
FIG. 5 is a graph showing relations between frequency
shifts ~FBo of the center frequency of a band-pass filter of a
television signal receiving apparatus and impairment scales of
the reproduced picture.
- FIG. 6 is a circuit diagram showing an example of
television signal receiving apparatus, embodying the present
invention.
FIG. 7(a) is an example of a band-pass filter in the
television signal receiving apparatus.
FIG. 7(b) is a graph showing relation between the center
frequency and an AGC voltage of the apparatus of ~IG. 6.
FIG. 8 is a graph showing experimental data of noises
of the picture and shifting of the center frequency of a band-pass
filter, of one example of the present invention.
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Description of the Preferred Embodiment
. _
First, principle of the present invention is
elucidated with reference to FIGs. 1 to 5. FIG. 1 shows a
general construction of a known satellite television signal
receiver, wherein the output terminal of a parabolic antenaa l
disposed outdoor is connected to the input terminal of an
SHF-UHF converter 2, which converts the SHF television si~nal
into a UHF television signal and gives it through a UHF cable
9 to the input terminal of a UHF-VHF converter, i.e., a second
frequency converter 3 in an indoor unit 8. The second frequency
converter 3, utilizing the output signal of the local oscillator
319 converts the UHF signal into a signal Fo of a second inter-
mediate frequency (herein after¦r-IF) and gives it throu~h a
band-pass filter 4 to a second intermediate frequency amplifier
(herein after ~-IF amplifier) 5. The ~-IF amplifier 5 is a
known automatic-gain-controlled amplifier and amplifies ~-IF
signal of FM signal and gives the signal to a known FM detector
6 comprising a known limiter and a discriminator. ~he F~ detector 6
detects the~-IF signal and issues detected multiple signal
consisting of a video signal and a sound subcarrier signal to
an output terminal 7.
In the abovementioned construction, according to
known Carson's rule of FM modulation, the following equations
hold:
FBo = Fo .......... (1)
and ~ = 2(~F + fm) .......... (2~ wherein,
r. ~ 3~
,, ~,.
~ 9
Fo is the second intermediate frequency(~-IF),
f`m is the maximum frequency component included in the video signal,
~F is the maximum frequency shift of FM signal,
FBo is the center frequency of the band-pass filter 4,
B is the bandwidth of the FM signal.
In the conventional system, the television signal
receiving apparatuses for receiving the FM television signal
have ~een designed in a manner to fulfil the conditions of the
abovementioned equations.
However, the present inventor e~pirically found that the
noises in the reproduced picture are more decreased when the centre
frequency F~o is shifted from ~-IF Fo, which is a frequency
corresponding to the center level of the video signal, to the
side of another frequency corresponding to an instaneous frequency
of white-signal level.
Hereinafter, the principle of the present invention
is elucidated further in detail. The television carrier is
modulated by a modulating signal consisting of a picture signal
component and a synchronization signal component as shown in
FIG. 2. And, the ratio of the maximum frequency deviation "a" (,
corresponding to a change from blac~ level to white level,)
for the video signal component and the frequency deviation "b"
for the synchroni7ation signal component is selected to be
7 : 3. As shown in FIC. 2, the instantaneous levels of the
video signal correspond to instantaneous frequencies of the
Fl~, signal of ~ -IF. In the conventional recelving apparatus,
,-~,.
11;~ 9
the center frequency FBo of the band-pass filter 4 is selected at
the frequency th~t is the center of the sum of frequency deviations
of a + b. Accordingly, the filter center frequency FBo lies,
not at the center of the video frequency deviation "a", but rather
nearing to the side corresponding to black level, by a shift of
frequency of 3 X a.
In receivin~ s~t~llite broadcasting, the input SHF
signal is not sufficiently strong, and therefore, the receiving
of the frequency-m.odulated F~ signal is made at the vicinity of
lo its FM threshold level, namely at the condition where carrier/
noise r2tio is small. As is well known, in such receiving of
the FM signal at the vicinity of the threshold level, t.here
are problems of triangulzr noise and impulse noise which are
peculiar to FM. Especially, at the vicinity of the threshold
level, the impulse noise rapidly increases as the power of the
input signal becomes smaller. On the picture, the influence of
the impulse noises are divided into generations of black impulse
noises and white impulse noises. The relation between the
shifting ~F~o of the center frequency FBo of the band-pass
filter 4 and numbers of black impulse noise, white impulse
noise and their total are shown in FIG. 3. As shown in FIG. 3,
at the zero frequency shift (~F30 = O), where the conventional
television receiving apparatus put the central frequency, the
black impulse noise is superior to the white impulse noise.
On the other hand, number of both the black impulse noise and the
white impulse noise are relatively small, and also the total
_ .
,~
thereof is miminum at the frequency shift of ~FBo = c, where
the center frequency FBo of the band-pass filter is selected
at the center of the frequencies corresponding to the white
level and black level of the video signal, as shown by FIG. 2.
The inven.tor em~irically found that the best result
in decreasing the noises in~picture is obtained at a further
shift to FBo = d of the center frequency FBo of the band-pass
filter. Namely, picture of lowest noise is obtainable by select-
ing the center frequency FBo to such a frequency that is nearer
to the frequency corresponding to the white level, exceeding the
center frequency c of the frequency range "a" for the picture
signal. This empirical fact would be able to be elucidated that
average human eye would feel the black impulse noises stronger
than white impulse noises.
The reason of generation and nature of the impulse
noises induced in the video signal can be elucidated as follows:
The genuine video signal and noise can be shown by
a vector diagram as shown in FIG. 4(a), wherein ~ represents a
vector of the~genuine video signal, N represents that of a noise
such as a thermal noise in the receiving apparatus and S
represents that of composite vector of the vectors ~ and ~. As
the time passes, the thermal noise vector changes its length
and direction, for example, to that shown by the dotted line
arrow D. Accordingly, the composite vector changes from S to
S as shown in FIG. 4(a), and during the change that tip of the
vector moves around the origin point of the vector D by changing
the vector direction by 2~ . Since the output of the FM
detector 6 is proportional to time differential of the instan-
teneous phase of carrier, the abovementioned 2~ change of the
composite vector produces an impulse noise shown by FIG. 4(c)
and the impulse forms black or white noise.
In order to suppress such impulse noises, hitherto
have been proposed FM feedback demodulation system, phase lock
demodulation system and multiple loop feedback FM demodulation
system. However, such systems require very complicated
lo constructions, and therefore make the receiving apparatus too
much expensive for home appliances such as satellite direct
receiving apparatus of home use.
Next, picture impairment scale due to factors other
than the impulse noise characteristic, for example, differential
gain and differential phase of the video signal, residual buz
noise and distorsion of synchronizing signal has the characteristics
as shown by a curve e of FIG. 5 which shows relation between
the center frequency shift and the impairment scale. As shown
by the curve e, the picture impairment scale becomes worse as
the shift of the central frequency of the band-pass filter ~ from
the center frequency of the frequency ranges a + b of FIC. 2
increases. This is caused by known group delay characteristic
of a band-pass filter. Curves f,g and h show total picture
impairment characteristics including the abovementioned impulse
noise characteristic, wherein the parameter of the curves e to
h is the input video signal power intensity. As shown by these
'
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curves, as the input television signal power becomes smaller, the
impairment scale becomes worse, and further, the condition for
obtaining minimum impairment scale shifts from the condition of
~F30 = o to the condition of ~FBo - c or ~FBo = d.
Summarizing the above, the shift of the center frequency
should be greater towards the white level frequency as the input
television signal is smaller, and for a sufficiently large input
television signal the condition of ~FBo = O of the conventional
apparatus is acceptable.
Accordingly, in an apparatus for receiving a television
signal which is made by a frequency-modulation by a video signal,
the improvement in accordance with the present invention
is that
a band-pass filter provided in the receiver for
selectively taking out the video signal has a center frequency
which is selected between an instantaneous frequency correspond-
ing to the center level of the video signal and another
instantaneous frequency corresponding to white-signal level.
A preferred embodiment of the present invention is
elucidated with reference to FIGs. 6 and 7.
In the circuit shown in FIG. 6, the output terminal
of a parabolic ~ntenna 1 disposed outdoor is connected to the
input terminal of an SHF-UHF converter 2, which con~erts the
SHF FM television signal into a UHF FM television signal and
gives it through a UHF cable 9 to an input ter~inal of a UHF-
VHF converter, i.e., a second frequency converter 3 in an indoor
unit 8. The second frequency converter 3, utilizing the output
signal ~f- the local oscillator 31, converts the UHF signal into
a signal Fo of a second intermediate frequency (~ -IF) and
gives it through a band-pass filter 4 to a second intermediate
frequency amplifier (~-IF amplifier) 5. The ~-IF amplifier 5
is a known automatic-gain-controlled amplifier and amplifies
~ -IF signal of FM signal and gives the signal to a known FM
detector 6 comprising a known limiter and a discriminator. T~e
FM detector 6 detects the ~-IF signal and issues detecte~
multiplex signal consisting of a video signal and a sound
subcarrier signal to the output terminal 7. The ~-IF amplifier
5 gives a signal, which is corresponding to the input signal
power level, through a connection 18 to a voltage control circuit
17. Then, the voltage control circuit 17 feeds control voltayes
to the band-pass filter 4 and to the local oscillator 31, so
that the band-pass filter 4 changes its center frequency and
the local oscillator 31 changes its oscillating frequency.
FIG. 7(a) shows one example of a circuit construction of a known
band-pass filter 4, where variable capacitance diodes Cv are
connected in series to capacitors of eakh of tank circuits,and
a bias voltage is impressed to the variable capacitance diodes
Cv from a ~ias voltage teminal Va. The band-pass filter 4
shows the characteristic curve as shown by FIG. 7(b), where
abscissa indicates the voltage impressed across their electrodeS
and ordinate indicates the change of the center frequency FBo.
By means of the voltage change at the connection 18, the
characteristic of the band-pass filter 4 is control~ed in a manner
that the frequency shift of the center frequency is made
_ g _
,-
!
~ )9
responding to the intensity of the input television signal in
the hatched area in FIG. 5~ The oscillation frequency of the
local oscillator 31 of the second frequency converter 3 is
also controlled by other variable capacitance elements in a
manner that the selection can be made easily by application
of suitable control voltage by the voltage control circuit 17
FIG. 8 shows curves of measured relations between the
frequency shift ~Bo of the center frequency FBo of the ~and-
pass filter 4 of an embodiment of FIG. 6 and counts of ~hite
impulse noise, black impulse noise and total thereof. Tn the
embodiment, the bandwidth "a" of the video signal is 12MH7 and
the bandwidth 3~ of the band-pass filter 4 is 2~MHz. The
parameter is the input power "Pi" of the television signal.
As the curves show, the best performance with respect to
noise is obtainable when the frequency shift ~F~o is between
1 to 1.5 MHz towards white level side. By the shifting of the
centre frequency FBo, the C/N (Carrier/Noise) figure is improved to
a value equivalent to that when the apparatus receives a television
signal of a power 1 to 2 dB stronger t~an actually receiving.
As has been elucidated in detail, by selecting the
center frequency of the band-pass filter in a specified manner,
the receiving apparatus in accordance with the present inventlon
is capable of reproducing the picture with an improved quality.
Therefore lowest input television signal for enabling direct
receiving of satellite broadcast is 1 to 2 dB lower than that of
the conventional apparatus. This enables use of smaller
.i
,, 1 0
parabolic antenna and use of receiving sets with a higher noise
figure, and hence will enable mass production of cheaper
receiving apparatus than the conventional one.
, ~
