Note: Descriptions are shown in the official language in which they were submitted.
. ~vo 93/04563 2 0 9 ~ 3 81 PCT/CB92/01532
DECODING OF COMPOSITE TELEYISION SIGNALS
Technical Field:
The present invention relates to the field of composite
television broadcasting systems and in particular to a method and
apparatus for reducing the effect of differential distortions on
demodulated broadcast video signals.
Back~round Art:
It is a known problem of PAL, NTSC and other similar
composite tele~ision broadcasting systems that the broadcast
signal is often subject to amplitude and phase distortions.
These distortions are usually introduced to the signal as a
result of modulation of the signal onto a high frequency c~rrier
signal prior to transmission and demodulation of the transmitted
signal at the receiver. Several factors contributing to the
distortion of the video signal are influenced by the mean level
of the modulated signal carrier with the result that, when the
video signal is demodulated, the distortions become differential
distortions which vary with the mean level of the signal.
While these distortions are allowed for in the design of
known composite television signal receivers, by the use of
delay-line decoders and a saturation control in PAL system
receivers and by the use of hue and saturation controls in ~TSC
receivers, these measures allow only an average correction to be
applied. In order to improve the quality of signal that can be
delivered by~e~isting-terrestrial television systems and by new
television systeims which use the established.composite coded
signal methods, improved receivers employing more accurate
distortion correction methods are required.
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Disclosure of Invention~
; '' In accordance with the present-invention, there is
provided a method~for reducing the effect of differential
; distortion on demodulated composite television signals,
comprising the steps of:
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broadcasting a test signal at predetermined intervals, the
test signal including a component at at least one average video
signal level;
receiving the test signal at a receiver, the receiver
being adapted to carry out the steps of:
measuring the distortion of the video signal at the said
at least one average video signal level;
deriving the required correction at that signal level; and
applying the said derived required correction to subsequently
received broadcast video signals at that signal level.
The present invention also provides a test signal for use
in the above method, comprising one line of a composite
television signal having an identifying portion to identify it as
a test signal and having high frequency components at at least
three average video signal levels. ,
The test signal may be transmitted once per frame or over r-
a longer period with successive test signals being spaced by
several seconds. , ,
The method of the present invention may include the step
of measuring the amplitude and phase distortion for a range of
average video signal levels of the test signal and deriving a
correction to be applied to subsequently received broadcast
signals for each such distortion at each of the measured average
video signal levels. Thus, if the levels of demodulated '
chrominance are significantly reduced at high average signal
levels (i.e. close to peak white) by distortion of the signal
during modulation, transmission and demodulation, the effect of
these distortions mayibe corrected by increasing the chrominance
gain when the signal level is high. These corrections are ;
preferably applied.to the signal,in ,its com,posite form to allow ,
the level of high-frequency luminance (picture detail) to be
corrected also. ,,The correction may be derived and applied at a
number OI different frequencies to improve the accuracy of the
luminance correction. ,~
. ,., ,The derived,corrections may be interpolated or
extrapolated to derive the correction to,be applied at average
,,,~,video signal levels other than the signal levels of the test
signal components. , , -
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Preferably, the corrections are applied to subsequentlyreceived broadcast signals at closely spaces signal levels to
give a smoothly varying correction which adds no visible
distortions to the broadcast signal.
Also in accordance with the present invention there is '
provided a receiver adapted to receive and demodulate a composite
television signal, including: -'
a test signal detector operable to detect a test signal
broadcast at predetermined intervals;
measurement apparatus adapted to determine the level of
differential amplitude and/or phase distortion of the received ;
test signal at at least one average video signal level and to
derive the correction required to reduce the distortion at that
signal level or levels;
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storage means connected to receive the derived required -
correction or corrections; and ,L~
- signal processing apparatus adapted to apply the required
correction or corrections to subsequently received television
signals at the average video signal level or levels for which the
correction or corrections was derived.
The receiver operates to derive and apply corrections in
the way described above. The apparatus may be implemented in
analogue or digital'circuitry or, where the receiver includes a
high speed computer, certain of the apparatus operations may be
' implemented in software.
' ' '' ~ The present invention provides a broadcast'system,
including a transmitter configured to modulate and transmit a
' composite'television'signal'including t~e test signal, and at
least one receiver'of the type described above. In such a :
' ' broadcast system differential distortion of the broadcast signals '
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' may be corrected more accurately than by the average correction '
methods currently known and used. '
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' ~rief DescriPtion of Drawinqs: '
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One particular preferred embodiment of the present
' invention will now be described, by way of e~ample only, with
reference to the accompanying drawings in which:
Figure 1 is a general schematic diagram of the correction
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derivation and application stages of the receiver of the present
invention;
Figure 2 shows two suitable forms of the test signal of
the present invention; and
Figure 3 is a schematic diagram showing the receiver
stages of Figure 1 implemented in digital circuitry.
Modes for CarrYinq Out the Invention:
Referring initially to Figure 2, two suitable forms of
test signals according to the preser.t invention are shown. Both
signals, employed to correct chrominance errors, are in the form
of a broadcast line signal including high frequency components
; which may be measured at at least three different average video
signal levels. The first signal lOA shown in Figure 2A, is in
the form of a television line ramp and the second lOB, in Figure
2B, is in the form of a television line staircase. Both signals
shown are ausmented by a subcarrier.
i; Signals of this type are easy to detect, being identified
by a preceding chrominance burst 12 of at least 20 microseconds
duration.
The circuitry measuring the test signal distortion and
deriving the corrections required to minimise its effect includes ~ -
storage for the corrections to allow them to be applied to
successive received television broadcast signals.~ It is
therefore the case that the test signal, which sets up the stored
corrections, need ~e transmitted only sufficiently often to
ensure that there is no appreciable delay in obtaining a good
picture after the viewer has changed channel or after a change in
received signal quality as a result of changes to the signal path
for distribution, transmission or reception.~ Accordingly,
whilst the test signal may be transmitted once per frame, lower
repetition frequencies such as once per second or once per ten
seconds may be used.
The receiver circuitry which derives and applies the '
required correction of amplitude and phase distortion affecting
one signal frequency (colour subcarrier) at different average
video signal levels is shown in Figure 1.
The television broadcast signal, including at periodic
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~VO 93/0~563 2 0 9 ~ 3 81 PCT/G892/01532
intervals the test signal, is received and demodulated by the
receivers vision demodulator 20. The demodulated signal 22 is
applied to measurement apparatus 24 which measures the difference
between the chrominance level of the received (distorted) signal
and an expected chrominance level for the transmitted test signal
and derives a correction which will minimise the effect of the
distortion. The derived corrections 26 are then stored in a
memory 28 at address locations 30 determined by the average
luminance level of the demodulated signal 22. The average
luminance level, and hence the memory address locations 30, are
derived by passing the demodulated signal 22 through a suitable
low-pass filter arrangement 32. -
To ensure that only corrections derived from a receivedtest signal are stored in the memory 28, a test signal detector
34 is provided. ~ When a test signal is received, identified as- - -
such by the preceding 20 us chrominance burst 12 (Figure 2), the
detector ~4 generates a writ~F~-enable signal 36 which allows
derived corrections to be written into the memory 28 for the
duration of the test signal.
The stored corrections are applied to subse~uently
received television broadcast signals by a controllable equaliser
38. Control of the equaliser 38 may be achieved by a number of
established methods.
The stored corrections are passed through an interpolator
40 prior to application by the ëqualiser 38. The interpolator ' -
40 derives further required corrections to be applied at average
video signal levels other than those fo,r which the measurement
apparatus 24 measured the distortion'of the test signal. Use of ¦
the interpolator 40 allows the corrections to be applied at
closely spaced average video signal levels to provide a smoothly
varying correction which àdds no'visible additional distortions
to the video signal. -
- ~''`" A more'detaiied-implementation of'the receiver circuitry
is shown in Figure 3. In the circuitry shown, the distortion is
measured and the corrections derived from the test signal of
Figure 2A (the television line ramp). The output from the
receivers vision demodulator 20 is digitised by an analogue to
digital converter 42: this allows the subsequent measurement and
93/04563 20~38~ PCl`tGB92/01532.
correction stages to be defined in terms of known digital
operations although it will be appreciated that some of these
stages could be implemented in analogue circuitry.
The digitised output from the analogue to digital
converter g2 is applied to a chrominance demodulator 44. The
chrominance demodulator 44, which could be the normal receiver
demodulator, is shown in Figure 3 as a separate demodulator for
the sake of clarity.
The demodulated chrominance components U,V are applied to
the measurement apparatus 24. The amplitude of the demodulated
signal is calculated, in a PROM (programmable read-only memory)
24A, as the root of the sum of the squares of the two demodulated
colour components U,V. The phase of the demodulated signal is
also calculated in a PROM 24P as the inverse tan ratio of the two
demodulated colour components U,V. These two measurements 26A,
26P which define the corrections to be applied to a subsequently
received broadcast signal are then stored in amplitude and phase
correction memories 28A, 28P respectively.
As described previously, the corrections are stored at
memory address locations 30 derived from the average luminance
level of the demodulated signal by a low pass filter 32. As also
described, these corrections may only be stored when a
write-enable signal 36 from a test signal detector 34 indicates
the presence of a test signal.
Following interpolation by respective amplitude and phase
interpolators 40A, 40P the corrections are applied to a
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controllable egualiser 38. The equaliser 38 is an adjustable
transversal equaliser having a signal input 50 for the
demodulated broadcasL video signal and separate symmetric 52 and
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anti-symmetric 54 control inputs. The amplitude correction
signal is applied to the symmetric control input 52: amplitude
correction is achieved by varying the size of the signal I -
coefficients symmetrically about the centre of the filter. The
phase correction signal i5 applied to the anti-sy~metric control
input 54: phase correction is achieved by varying the size of the
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~: signal coefficients anti-symmetrically about the centre of the ¦
filter.
In order to reduce the sensitivity of the circuitry to
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disturbances which may affect the received test signal, averaging
circuits (not shown) may be provided around the correction
memories 28A, 28P. Such averaging circuits may average
successive test signals prior to measurement of distortion or may
average successively derived corrections for a given average
video signal level.
It is envisaged that new designs of broadcast receiver,
which include means for improving the quality of the received
television signal, will include a wide range of sophisticated
circuits. The present invention provides such means for
improving the quality without requiring a significant increase in
receiver circuitry.
It will be appreciated that, where the receiver apparatus
includes high-speed computing capability, the method of the
present invention may be impiemented in software. In such a
case, the schematic diagrams of figures l and 3 may be regarded
as flow charts representing the steps of a software decoding and
correction process.
The decoding and correction circuitry of the present
invention may also include means for correcting static losses
within the receiver to compensate for multipath effects, receiver
; and/or transmitter responses'and the''low'frequency effects of
mistuning. ' ' '
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