Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N9/00—Details of colour television systems
  • H04N9/64—Circuits for processing colour signals
  • H04N9/646—Circuits for processing colour signals for image enhancement, e.g. vertical detail restoration, cross-colour elimination, contour correction, chrominance trapping filters
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N17/00—Diagnosis, testing or measuring for television systems or their details
  • H04N17/02—Diagnosis, testing or measuring for television systems or their details for colour television signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N5/00—Details of television systems
  • H04N5/14—Picture signal circuitry for video frequency region
  • H04N5/20—Circuitry for controlling amplitude response
  • H04N5/205—Circuitry for controlling amplitude response for correcting amplitude versus frequency characteristic
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N5/00—Details of television systems
  • H04N5/14—Picture signal circuitry for video frequency region
  • H04N5/21—Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
  • H04N5/211—Ghost signal cancellation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N9/00—Details of colour television systems
  • H04N9/64—Circuits for processing colour signals

Definitions

• 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.
• a method for reducing the effect of differential distortion on demodulated composite television signals comprising the steps of: 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 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.
• a correction to be applied to subsequently received broadcast signals for each such distortion at each of the measured average video signal levels.
• 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.
• the corrections are applied to subsequently received broadcast signals at closely spaces signal levels to give a smoothly varying correction which adds no visible distortions to the broadcast signal.
• 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; storage means connected to receive the derived required correction or corrections; and 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 the test signal, and at least one receiver of the type described above.
• a broadcast system including a transmitter configured to modulate and transmit a composite television signal including the test signal, and at least one receiver of the type described above.
• differential distortion of the broadcast signals may be corrected more accurately than by the average correction methods currently known and used.
• Figure 1 is a general schematic diagram of the correction 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.
• Figure 3 is a schematic diagram showing the receiver stages of Figure 1 implemented in digital circuitry.
• test signals 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 10A shown in Figure 2A is in the form of a television line ramp and the second 10B, in Figure 2B, is in the form of a television line staircase. Both signals shown are augmented by a subcarrier.
• 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 be 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 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.
• 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 34 generates a write-enable signal 36 which allows derived corrections to be written into the memory 28 for the duration of the test signal.
• 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 equaliser 38.
• the interpolator 40 derives further required corrections to be applied at average video signal levels other than those for 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 adds no visible additional distortions to the video signal.
• FIG. 3 A more detailed implementation of the receiver circuitry is shown in Figure 3.
• 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 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 42 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 sura 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.
• 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.
• the equaliser 38 is an adjustable transversal equaliser having a signal input 50 for the demodulated broadcast video signal and separate symmetric 52 and 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 coefficients symmetrically about the centre of the filter.
• the phase correction signal is applied to the anti-symmetric control input 54: phase correction is achieved by varying the size of the signal coefficients anti-symmetrically about the centre of the filter.
• averaging circuits 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.
• the method of the present invention may be implemented in software.
• the schematic diagrams of figures 1 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.

Landscapes

• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Health & Medical Sciences (AREA)
• Biomedical Technology (AREA)
• General Health & Medical Sciences (AREA)
• Testing, Inspecting, Measuring Of Stereoscopic Televisions And Televisions (AREA)
• Television Systems (AREA)
• Color Television Systems (AREA)

Priority Applications (8)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10700317

Family Applications (1)

Country Status (14)

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Citations (5)

Family Cites Families (2)

• 1991
  • 1991-08-21 GB GB9118057A patent/GB2258969A/en not_active Withdrawn
• 1992
  • 1992-08-20 CN CN92110458.8A patent/CN1074073A/zh active Pending
  • 1992-08-20 SK SK34293A patent/SK34293A3/sk unknown
  • 1992-08-20 CA CA002094381A patent/CA2094381A1/en not_active Abandoned
  • 1992-08-20 BR BR9205328A patent/BR9205328A/pt not_active Application Discontinuation
  • 1992-08-20 HU HU9300945A patent/HUT64668A/hu unknown
  • 1992-08-20 AU AU24402/92A patent/AU2440292A/en not_active Abandoned
  • 1992-08-20 JP JP5504200A patent/JPH06501832A/ja active Pending
  • 1992-08-20 EP EP92917600A patent/EP0558704A1/en not_active Withdrawn
  • 1992-08-20 WO PCT/GB1992/001532 patent/WO1993004563A1/en not_active Application Discontinuation
• 1993
  • 1993-04-09 CZ CZ93616A patent/CZ61693A3/cs unknown
  • 1993-04-20 NO NO93931440A patent/NO931440L/no unknown
  • 1993-04-20 BG BG97648A patent/BG97648A/xx unknown
  • 1993-04-21 FI FI931798A patent/FI931798A0/fi not_active Application Discontinuation

Patent Citations (5)

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