Television – Image signal processing circuitry specific to television – Chrominance signal demodulator
Reexamination Certificate
1999-03-31
2002-08-13
Miller, John W. (Department: 2614)
Television
Image signal processing circuitry specific to television
Chrominance signal demodulator
C348S639000, C348S727000, C329S325000, C331S020000
Reexamination Certificate
active
06433837
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a demodulator for a frequency modulated signal, and particularly for a SECAM chrominance signal. The invention is used, for example, in applications for the manufacture of televisions or video signal receivers.
BACKGROUND OF THE INVENTION
According to the SECAM standard, chrominance information in a video signal is transmitted by frequency modulation of chrominance sub-carriers. The video signal comprises a sequence of lines forming a picture. These lines include two successive lines that carry information corresponding to the red component and then corresponding to the blue component respectively, in a three-color image. The sub-carriers on which the information for the red and the blue is coded are centered on separate frequencies that are F
OR
=4.406 MHZ for the red, and F
OB=
4.250 MHZ for the blue.
Chrominance signal demodulators are used to extract information related to the red and blue components from the signal, and more precisely from frequency modulated sub-carriers.
FIG. 1
shows the operating principle for a SECAM chrominance signal demodulator, in a diagrammatic and simplified manner.
The demodulator in
FIG. 1
can be used to demodulate lines containing information related to the red or blue components. The demodulator consists mainly of an oscillator
10
with controlled frequency and a phase comparator
20
. For the purposes of this invention, a phase comparator means a device that has two signal inputs and is capable of outputting a current or a voltage proportional to a phase difference between the two input signals.
The oscillator
10
is a voltage controlled oscillator (VCO) comprising two inputs
12
,
14
called the loop input and the adjustment input respectively, and an output
16
that outputs a signal at a frequency that depends on the voltages applied at the loop and adjustment inputs. The input to the loop
12
of oscillator
10
is connected to an output
26
of the phase comparator
20
, while the output
16
from oscillator
10
is connected to a first input
22
of the phase comparator. A second input
24
of the phase comparator is connected to a two-way switch
28
that selectively connects the second input either to a chrominance channel
30
that transmits the chrominance signal, or to a quartz external reference oscillator
32
that outputs a signal at a reference frequency.
The phase comparator associated with the oscillator forms a PLL (Phase Locked Loop) demodulator. When the two-way switch
28
connects the second input
24
of the phase comparator to the chrominance channel
30
, the demodulator outputs a voltage V proportional to the frequency of the chrominance signal F. The voltage V is used as information representative of the color component (blue or red) corresponding to the signal present on channel
30
.
The voltage V is such that V=kFU where k is a proportionality coefficient and U is a voltage applied to the adjustment input
14
of the oscillator.
The frequency of the output signal from oscillator
10
must be determined very precisely and must remain stable to control the precision of the demodulator output voltage V so that the equipment including the demodulator is disposed will be capable of high fidelity reproduction of the colors. Consequently, the oscillator
10
must be frequently matched to a reference frequency denoted F
ref
, supplied by the external oscillator
32
.
Oscillator
10
is matched by connecting the second input
24
of the phase comparator to the external reference oscillator
32
. During the oscillator matching phase, an on-off switch
40
in an adjustment circuit
42
is closed. The matching phase may then be started, for example, by information that cancels out the video signal frame used to control the two-way switch
28
and the on-off switch
40
. This circuit comprises a transconductor
44
, one input of which is connected to the input of oscillator loop
12
, and the other input is connected to a reference voltage source
46
outputting a voltage V
ref
.
When the voltage at the input to the oscillator loop
10
is not equal to the reference voltage V
ref
, the transconductor outputs a current (positive or negative) that charges or discharges a memory capacitor
48
. Thus, the voltage U of the memory capacitor
48
applied to the oscillator adjustment input
14
is modified to adjust the oscillator output voltage to a value such that the oscillator loop input voltage becomes equal to V
ref
.
During a demodulation phase, the on-off switch
40
is open and the adjusted voltage U is maintained at the terminals of capacitor
48
.
Thus V
ref
=kF
ref
U
let
U
=
V
ref
kF
ref
therefore:
V
=
kFU
=
kF
V
ref
kF
ref
=
V
ref
F
ref
F
=
H
F
where H is the demodulation gain that is very precisely known as the ratio of the reference voltage to the reference frequency.
The voltage U must be kept as stable as possible between two oscillator matching phases, to prevent any deterioration in the color reproduction. Even a minor modification to the adjustment voltage U causes a large modification to the oscillation frequency of oscillator
10
. This modification is particularly large if the gain on the adjustment loop is large. Thus, a leakage current from the memory capacitor
48
could disturb operation of the oscillator.
In order to reduce the influence of leakage currents and to make the oscillator stable between two matching phases, the capacitance of the memory capacitor
48
is chosen to be high. For example, the capacitance of the capacitor is chosen to be between 10 and 100 nF. These capacitance values form an obstacle, n or at least a handicap, for integrating the capacitor with the other demodulator components in the form of an integrated circuit. Thus capacitor
48
is usually not integrated on the chip comprising the video signal decoder, but it is in the form of a separate component.
SUMMARY OF THE INVENTION
The object of this invention is to provide a demodulator for frequency modulated signals, and particularly for SECAM chrominance signals, without the disadvantages mentioned above.
One object is to provide such a demodulator with a low capacitance memory capacitor, particularly suitable for production in the form of an integrated circuit.
Another object is to provide such a demodulator with an oscillator with a particularly stable frequency, and which is not very sensitive to leakage currents from the memory capacitor.
To achieve these objects, the demodulating device for a frequency modulated signal comprises an oscillator with controlled frequency, forming a demodulation loop with a phase comparator, and an adjuster for adjusting the oscillator frequency as a function of a charge voltage of a memory capacitor connected to an oscillator adjustment input.
The adjuster according to the invention comprises a fine adjustment channel to output a first adjustment value at the oscillator adjustment input, that depends on the voltage of the memory capacitor, and a coarse adjustment channel. The coarse adjustment channel outputs a second adjustment value added to the first adjustment value at the oscillator input. The second adjustment value is increased when the capacitor voltage exceeds a high limitation voltage or first threshold and is reduced when the capacitor voltage is below a low limitation voltage or second threshold.
The charging voltage of the capacitor may be fixed by appropriate means during an oscillator matching phase, for example as described previously with reference to FIG.
1
.
The coarse adjustment channel may be designed to output an adjustment value that preferably corresponds to a majority fraction of the total adjustment value applied to the oscillator input. This adjustment value is not affected by any parasitic variation and is maintained independent of the variation of the state of charge of the capacitor between two oscillator matching phases.
The fine adjustment channel still contributes to variations in the charge of the memory capacitor. However,
Bret Gerard
Salle Didier
Allen, Dyer, Doppelt, Milbrath & Gilchrist, P. A.
Miller John W.
STMicroelectronics
LandOfFree
Demodulator, particularly for a SECAM chrominance signal,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Demodulator, particularly for a SECAM chrominance signal,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Demodulator, particularly for a SECAM chrominance signal,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2964320