GB2420671A

GB2420671A - Composite video signal decoding

Info

Publication number: GB2420671A
Application number: GB0603154A
Authority: GB
Inventors: Martin Weston
Original assignee: Snell and Wilcox Ltd
Current assignee: Snell Advanced Media Ltd
Priority date: 2002-09-12
Filing date: 2002-09-12
Publication date: 2006-05-31
Anticipated expiration: 2022-09-12
Also published as: GB0221162D0; GB2393061A; GB2420671B; GB2393061B; GB0603154D0

Abstract

A method of decoding a composite colour television signal comprises the steps of recovering high frequency luminance information from the output of a chrominance demodulator, adjusting the phase of the recovered luminance information by an offset to correct colour-under processing errors and adding the phase adjusted recovered luminance information to the luminance output. The phase may be adjusted to give the sharpest luminance (i.e. to minimise the rise time of a luminance edge or maximise the peak height of a luminance pulse). The chrominance demodulator may use a subcarrier derived using later and earlier phase measurements, the DC component may depend on earlier and later level measurements and the high frequency luminance may be separated using earlier and later phase measurements.

Description

COMPOSITE DECODING 2420671 This invention concerns the decoding of

composite colour television signals to luminance and chrominance, or red green and blue components.

In many digital processes the choice of the sampling grid has a profound effect on the complexity of the process. Composite decoders have often used a grid related to the chrominance subcarrier. International patent application WO 97/39589 describes a digitally-implemented decoder in which the composite input is sampled at four times colour subcarrier frequency, and the output components are delivered on an orthogonal (i.e. line-locked) sampling grid. The arcane relationship between the input and output sampling structures leads to a complex structure, and it is necessary for the input sampling structure to "track" any phase perturbations in the incoming subcarrier if the best results are to be obtained. This can be difficult if a distorted analogue input has to be processed.

The generation of an orthogonal sampling grid from an analogue signal usually requires a sampling-clock generator which tracks the line phase of the input signal; and this is also difficult to achieve if a distorted input is provided.

The Applicants' copending UK patent application number GB 0212430.3 describes a novel solution to this problem in which the sampling clock is derived from the long-term average of the input line frequency. The specification which follows extends this concept to the art of highquality composite decoding.

The invention consists in one aspect of a method of decoding a composite colour television signal compvising the steps of recovering high frequency luminance information from the output of a chrominance demodulator; adjusting the phase of the said recovered luminance information by an offset to correct colour-under processing errors; and adding the phase adjusted recovered luminance information to the luminance output.

In a second aspect, apparatus for decoding a composite colour television signal, comprises filter means for recovering high frequency luminance information from the output of a chrominance demodulator, phase adjustment means for adjusting the phase of the said recovered luminance information by an offset to correct colour- under processing errors; and means for adding the phase adjusted recovered luminance information to the luminance output.

The invention will now be described by way of example with reference to the Figure, which shows a composite decoder in accordance with an embodiment of the invention.

A digital composite colour input (1) has been sampled with a sampling clock which need not have any defined relationship to the input signal, but preferably is an integral multiple of the long-term average input line frequency. There also need not be any defined relationship between the black level of the input signal and the digital quantisation levels. Preferably a slow-acting clamp or DC- restorer precedes the analogue to digital converter so as to prevent the signal exceeding the converter's coding range without unduly distorting the low frequency content of the signal.

A sync identification stage (2) identifies the approximate position of the line- sync reference in the input signal (1). This information is passed to three blocks which make measurements on the input (1): * A black-level measurement block (3), which measures the level of the black level reference point (back-porch or average of colour burst) on each input line.

* A sync-tip-level measurement block (4), which measures the level of the tip of each input sync pulse.

* A burst phase measurement block (5), which measures the phase of the reference colour burst on each line relative to the sampling structure.

In order to represent this value with high precision it may be helpful to express the phase as the difference between the measured phase and the phase that would be expected from an ideal input signal sampled at the nominal frequency of the sampling clock.

A sync-edge phase measurement block (6) uses the measured values of black level and sync-tip level to identify the mid level of the reference sync edge and to measure its phase relative to the sampling grid to sub-sample precision.

The output of the black-level measurement block (3) is smoothed and interpolated in a filter (8). This may be an FIR filter which temporally interpolates the once-per-line measurements to give a black level value at every sample time, and also removes high-frequency noise from the measured values.

The output of the sync-phase measurement block (6) is smoothed and interpolated in a similar way by the block (9) to give a value every sample for the position of that sample relative to an ideal, line-locked, orthogonal sampling grid. This output grid may be defined by an externally derived timing reference (not shown).

The output of the burst phase measurement block (5) is also smoothed and interpolated in the block (10) to give a value of the input signal's reference subcarrier phase at every sample.

The input (1) is delayed by a compensating delay (7) which allows for the time taken by the above smoothing and interpolation processes. This delay allows the interpolation filters to make use of information from both "past" and "future" measurement values in arriving at the parameters of the current sample.

A black-level compensation block (11) subtracts the smoothed and interpolated black level values from the input samples at the output of the delay (7) to give a "clamped" video signal which is input to the subtractor (13).

The compensating delay (7) also feeds a chrominance demodulator (12). This uses the smoothed and interpolated reference subcarrier phase values from the block (10) to demodulate the chrominance into two phaseorthogonal baseband components (e.g. U and V, or I and Q). The twicesubcarrier component from the demodulation is rejected, and the baseband frequency response is shaped by, the low pass filter block (14). (This may be two separate filters, possibly with different characteristics in the case of NTSC, or one filter multiplexed between the two demodulated components.) The resulting baseband chrominance signals (which will be contaminated by cross-colour) are remodulated (27), using the same subcarrier phase as the demodulation, to give a modulated chrominance signal which is subtracted (13) from the black-level corrected composite signal, and input to a timebase corrector (TBC) block (16). The bandwidth of the filter (14) is chosen to be wide enough to include all the chrominance sidebands so that the output of the subtractor (13) is only luminance and includes no cross-luminance. The baseband chrominance is also input to second TBC block (17).

The two TBCs (16), (17) use the smoothed and interpolated sample phase signal from the block (9) to shift the chrominance and luminance samples onto an orthogonal output sampling grid, for example, in the manner described in GB0212430.3. A third TBC (18) shifts the reference subcarrier phase signal from the block (10) onto the same output sampling grid.

These luminance and baseband chrominance components are further processed to remove cross-colour and ensure that the high frequency luminance which falls into the chrominance channel is recovered. This processing makes use of the subcarrier phase information from the TBC (18) and is shown in the lower half of the Figure.

A multi-tap delay (20) provide a plurality of differently-timed baseband chrominance inputs taps for comb-filters (22), (23). The chrominance comb (23) removes cross-colour, and the comb (22) selects cross-colour. These filters use appropriate, adaptive or non-adaptive, spatial, temporal or spatio-temporal transversal filters. A further multi-tap delay (19) provides a plurality of differently-timed luminance signals which may be used to control the adaptation of the comb-filters (22) and (23).

The cross-colour from the comb (22) is input to a phase estimating block (24), and a remodulating block (25), which converts the cross-colour to true luminance; the remodulating subcarrier phase can usually be controlled by the timebase corrected (18) and correctly-timed (21), reference phase signal.

However, if the signal has undergone "colour-under" processing, or some other process in which high frequency luminance has been treated as chrominance, the remodulation may result in incorrectly-phased luminance. In this case the correct phase can be established, in the phase estimating block (24), as that which gives the "sharpest" luminance (i.e. the shortest rise-time of edges, or the greatest peak height of pulses) when added to the combed luminance.

Provided the correct subcarrier to line phase relationships have been maintained, the correct phase will be related to the subcarrier reference phase signal by an offset which will remain constant for long periods of time.

The output from the decoder consists of the sum of the combed luminance and the remodulated cross-colour from the adder (26), and the combed chrominance from the block (23). These components are locked to the orthogonal output sampling grid and may be combined (matrixed) to give red, green and blue components if required.

It will be appreciated by those skilled in the art that the invention has been described by way of example only, and that a wide variety of alternative approaches may be adopted without departing from the scope of the claims.

Claims

1. A method of decoding a composite colour television signal comprising the steps of: recovering high frequency luminance information from the output of a chrominance demodulator; adjusting the phase of the said recovered luminance information by an offset to correct colour-under processing errors; and adding the phase adjusted recovered luminance information to the luminance output.

2. A method as claimed in claim 1 in which the phase is adjusted to minimise the rise-time of a luminance edge.

3. A method according to claim I in which the phase is adjusted to maximise the peak height of a luminance pulse.

4. Apparatus for decoding a composite colour television signal, comprising: filter means for recovering high frequency luminance information from the output of a chrominance demodulator; phase adjustment means for adjusting the phase of the said recovered luminance information by an offset to correct colour-under processing errors; and means for adding the phase adjusted recovered luminance information to the luminance output.

5. Apparatus for decoding a composite colour television signal as claimed in claim 4, wherein the phase adjustment means comprises a phase estimating means for establishing the correct phase for the recovered luminance information; and re- modulating means for re-modulating the recovered luminance information using a reference subcarrier, where the phase of the reference subcarrier is determined by the phase adjustment means.

6. Apparatus as claimed in claim 4 or 5 wherein the phase adjustment means adjusts the phase to minimise the rise-time of a luminance edge.

7. Apparatus as claimed in claim 4 or 5 wherein the phase adjustment meansadjusts the phase to maximise the peak height of a luminance pulse.