Digital signal processing system

Abstract: A processor for use in synchronising the sampling frequency of an analogue to digital converter (5) to that of an analogue colour television signal to be digitised, comprises a signal processor (12) which is arranged to process signal samples received from the converter (5) during a colour burst of the television signal in order to calculate any phase error, to correct any phase error above a given magnitude by controlling the phase shift provided by a phase shifter (7) connected between the analogue to digital converter and a clock oscillator (6) defining the sampling frequency, and to correct any phase error below a given magnitude by adjusting the frequency of the clock oscillator (6).

Description: This invention concerns a digital signal processing system of the kind wherein an incoming analogue signal is sampled at a rate determined by a frequency component of the analogue signal and converted to a series of digital words. Such a processing system is used, for example, in converting a composite colour television signal into the corresponding digital signal required for use in digital television systems. In the operation of such a digital signal processing system, it is necessary that the sampling frequency have a fixed phase relationship to the incoming signal in order that information represented by the phase of a component of the signal can be correctly decoded. It is also important that any phase error that should occur between the incoming signal and the sampling frequency, for example upon change of a signal source, should be corrected as rapidly as possible. From US-A-4 797 730 and US-A-4 404 583, there are known signal processing systems according to the preamble of Claims 1 and 2. US-A-4 404 583 further describes a signal processing system according to the preamble of Claim 1. Both described systems enable a colour television signal to be sampled so that a clock oscillator can be locked to the phase of a colour burst of the television signal, but there is still a requirement for a system enabling more rapid adjustment of the phase of the clock oscillator, upon change of a signal source, as referred to above. One object of the invention is therefore to provide means enabling rapid measurement of a phase error to be corrected and a further object is to provide means for providing a correction of the measured phase error that is able to deal more rapidly with relatively large magnitudes of error. the frequency of a signal component of the incoming signal, and a processor unit programmed to receive said output signals, to store and compare one or more sets of said amplitude signals corresponding to one or more cycles of the said signal component, and to derive from said comparison a value representing a phase error between the sampling frequency of the analogue to digital converter and the said signal component. Preferably said processor unit is further programmed to determine, by comparison of a set of sampled amplitude signals corresponding to one cycle of said signal component, that sample closest to a given zero crossing point of said cycle of said signal component, and to compute said phase error value by comparison of the amplitude of said sample with a notional value of a sample coinciding with said crossing point. A second aspect of the invention provides a digital signal processing system comprising an analogue to digital converter arranged to receive an incoming analogue signal that is to be digitised, a variable frequency clock oscillator for controlling a sampling frequency at which said analogue to digital converter is to be operated, a phase shifter coupled between said clock oscillator and said analogue to digital converter and a processor unit arranged to receive sampled digital signal values provided by said analogue to digital converter and to provide output control signals to said variable frequency oscillator and to said phase shifter, said processor unit being programmed to compute from sampled signal values provided thereto a phase error between the said sampling frequency and a signal component of the incoming analogue signal, to provide a corresponding control signal to said phase shifter when the phase error is greater than a predetermined magnitude and to provide a corresponding control signal to said variable frequency oscillator when the phase error is below said predetermined magnitude. These objects are achieved by the features of the characterising clauses of Claims 1 and 2 respectively. Further preferred features and advantages of the invention will become apparent from the following description and accompanying drawings, in which Figure 1 is a block circuit diagram of one embodiment of a digital signal processing system in accordance with the invention for use in the analogue to digital conversion of colour television signals, Figure 2 is a more detailed circuit diagram of the phase shifter shown in Fig. 1, Figure 3 is a flow chart illustrating the programming of the processor of Fig. 1, and Figures 4 and 5 are diagrams illustrating a process embodied in the flow chart of Fig. 3. Referring to Fig. 1 of the drawings, a signal processing system in accordance with one embodiment of the invention is adapted for the analogue to digital conversion of a composite colour video signal received at a signal input 1. An analogue conditioning circuit indicated generally at 2 is arranged to set the signal voltage limits to defined levels and to extract timing information from the conventional video synchronising pulses of the composite video signal. This timing information is provided from an output indicated at 3, whereas the conditioned composite video signal is applied from an output 4 to an input of an analogue to digital converter 5. The analogue to digital converter 5 is clocked to sample the composite video signal at a repetition frequency corresponding to four times the frequency of the colour subcarrier of the composite video waveform, by means of clock pulses derived from a voltage controlled oscillator 6 and applied to the analogue to digital converter via a phase shifter 7, a flywheel circuit 8 and a divider circuit 9. At each clock pulse applied to the analogue to digital converter 5 the latter provides at an output 10 a corresponding digital word corresponding to the voltage amplitude of the incoming analogue signal. Because the incoming analogue signal is sampled at four times the frequency of the colour subcarrier, when the phase of the clock pulses supplied to the analogue to digital converter 5 are correctly locked to the phase of the colour subcarrier of the incoming signal the voltage amplitudes of the four samples obtained during each cycle of the subcarrier represent four different relationships of sample sign or phase from which the colour information carried by the original can be decoded. Figure 5 illustrates the appropriate phase relationship between the colour subcarrier and the four samples required to be obtained during each cycle, and the locking of the sampling axes to the phase of the colour subcarrier is achieved by adjusting the frequency of the clock oscillator 6 and the phase shift provided by the phase shifter 7 with the aid of a programmed microprocessor control system as indicated within the box 11 shown in broken lines in Fig. 1. The operation of the microprocessor system 11 will be described in more detail below, but meanwhile it will be noted that the system comprises a main processor unit 12 and a tag word counter 13, both of which receive the same clock pulses as the analogue to digital converter 5 and thus operate at a system frequency corresponding to the sampling frequency of the analogue to digital converter. The processor 12 is associated with read only memory 14A containing the control program and random access memory 14B for storing the results of processing operations. The processor 12 incorporates a data selector indicated diagrammatically at 15, which, when enabled by an appropriate signal from an output 16 of the processor 12, causes the processor 12 to read selectively digital words from the output 10 of the analogue to digital converter 5 and to read associated tag words from the tag word counter 13. The processor 12 further comprises an input 17 for receiving video synchronising signals from the output 3 of the conditioner 2, a data output 18 to control ports 19 of the processing system and control outputs 20 and 21 for the phase shifter 7 and variable frequency oscillator 6 respectively. As will be described below, the processing system 11 is programmed to compute, from the digital signals received at 10, the synchronising signals received at 17 and the clock pulse signals received from the divider 9 a phase error between the clock pulse signals and the colour subcarrier of the incoming analogue composite video signal and to provide a corresponding error signal on one of the two outputs 20 and 21 in accordance with whether the magnitude of the error is greater or less than a predetermined value. Fast locking of the phase of the clock pulse signals to that of the composite video signal can thus be obtained in a manner that will now be described. The variable frequency oscillator 6 comprises a stable voltage controlled oscillator which runs at eight times the nominal subcarrier frequency and generates the system clocks for feeding into the phase shifter 7. The oscillator 6 can be steered in frequency under control of the processor 12 via a digital to analogue converter 22. However changes in phase will take place over large numbers of cycles because of the limited pulling range of the oscillator and hence the primary fast control of phase lock is achieved with the phase shifter 7, which is adapted to provide fast switching of increments of phase shift via a selector 23 controlled from the output 20 of processor 12. The oscillator 6 is slowly pulled onto frequency with a second order loop filter implemented in the processor instructions as described below. Referring to Fig. 2 the phase shifter 7 comprises a tapped delay line 71 and a programmable selector 72. Each tap off the delay line 71 is equivalent to the required resolution of phase step at subcarrier frequency, and is applied to individual inputs of the selector 72. Since the input frequency into the delay line 71 is eight times subcarrier, the delay taps move by a total of one eighth of a cycle of subcarrier frequency for a complete cycle of 360 degrees, in steps of approximately one nano-second. To reduce the total delay taps required, a programmable inverter 73 can be placed after the selected output and the total delay reduced to 180 degrees or half a cycle of the oscillator frequency. When the end tap is reached the selector 72 switches back to zero delay, and the inverter 73 is enabled, resulting in the required 180 degrees of phase shift. In operation the processor 12 applies a continuous series of steps to the programmable selector in response to a measured colour burst phase error, which has the effect of sliding the selector output along the delay line and consequently shifting the output phase. Each 360 degree movement of the oscillator phase is equivalent to 180 degrees at the system clock frequency. The system will provide the necessary rotation of a full 360 degrees at four times subcarrier by sliding the selector along the delay line up to twice per line time. An error can therefore be corrected extremely quickly using the phase shifter 7. The clock flywheel circuit 8 removes any spikes or glitches produced on the clock by the action of the processor in moving the phase shifter. The clean clocks are then divided by two in the divider 9 to produce a clean, even mark/space ratio system clock at four times nominal subcarrier frequency. The manner in which the processing system 11 of Fig. 1 computes the phase error from incoming signals will now be described in more detail with reference to Figs. 3 to 5. As already mentioned above, Fig. 5 is a vector diagram showing the correct, in-phase relationship between the sampling axes as defined by the system clock pulses, and the axes of the conventional chrominance signal components of the colour subcarrier of the composite video signal. The upper part of Fig. 4 shows the corresponding in-phase relationship between the waveform of the conventional colour burst signal of the composite waveform indicated at 100, and the digital values, indicated at 102, obtained by sampling the colour burst of the composite video signal, when the system clock pulses are correctly phase locked with no phase error. The lower part of Fig. 4 shows, for comparison, the corresponding relationship when a phase error of less than 45° occurs between the colour burst waveform and the clock pulses. Provided that the amplitude of the colour burst waveform is known, it will be seen that the phase error can be determined from the corresponding amplitude error that will occur in the digital value of that sample which is closest to a zero crossing point of the colour burst waveform. The phase error will thus be the arc-sine of the amplitude error represented by the difference between the sampled digital value and the digital value that would correspond to a zero crossing point of the waveform. Fig. 3 is a flow chart illustrating the programming of the processor system 11 required to affect the above-mentioned computation and provide a corresponding control signal to the oscillator 6 or phase shifter 7 of Fig. 1. The individual steps of the flow chart will be understood by one skilled in the art without further detailed explanation, and therefore the relationship between the flow chart and the foregoing description will be summarised only briefly. The program commences with the wait for 'burst window', the presence of a burst window within which the colour burst of the composite video signal will occur being determined from the conventional synchronising signals received at input 17 of the processor 12. When the presence of a burst window is established, the sampled digital values from the analogue to digital converter 5 are received via a digital subcarrier band-pass filter, not shown in Fig. 1, and the data selector 15 and stored in the random access memory 14 to provide, for each of the ten cycles of the colour burst, four sampled digital values corresponding to the samples 102 of Fig. 4. The processor 12 then also receives from the tag word counter 13 corresponding tag words that are associated with each of the sampled and stored digital values, so that corresponding samples from each of the cycles of the colour burst signal are each identified by the same tag word to identify the relative positions of the samples within the cycle. This relationship between samples and tag words is illustrated in Fig. 4. Each set of samples identified by the same tag word is then accumulated in an individual register to provide an integrated value for the entire period of the colour burst signal. This averaging will provide immunity to burst envelope distortion and noise. The four accumulated samples are then processed to determine the phase error from the amplitude error of that sample identified as being closest to the positive going zero crossing point of the colour burst waveform. Selection of the appropriate sample type will ensure that the phase error is within plus or minus 45° and a corresponding adjustment of the phase shifter 7 or the oscillator 6 can then be initiated in accordance with the determined magnitude of the phase error. The above described measurement and correction loop enables the system to acquire the phase, and where appropriate the PAL ident, of the incoming colour burst within three line times and provides extremely quick recovery from non-synchronous changes in the incoming signal to bring the clocks back into a known phase. As can be seen by reference to the flow chart of Fig. 3 the use of digital techniques in the system also conveys other advantages to enhance performance over conventional analogue locking methods, by directly measuring the amount of noise present during the colour burst, and indicating the colour burst amplitude. These measurements may then be used to modify characteristics within the system such as alteration of loop filtering for noisy signals, and automatic corrections for low amplitude incoming subcarrier by simple condition testing in the processor program. All the functions provided by a conventional analogue phase locked oscillator, such as the loop filtering and PAL ident extraction are implemented in the program and are therefore capable of being dynamically altered by the processor to optimise locking conditions.

Claims: A digital signal processing system, comprising an analogue to digital converter (5) arranged to receive an incoming analogue signal to be digitised and to provide digital output signals representing the amplitude of the incoming analogue signal sampled at a frequency corresponding to a multiple of the frequency of a signal component of the incoming signal, means (12,14) for receiving and storing more than two of said output signals for each of a plurality of cycles of the said signal component, and means for comparing the values of said stored signals to derive a value representing a phase error between the sampling frequency of the analogue to digital converter and the said signal component, characterised in that said comparing means comprises a processor unit (12) programmed to select by comparison of sets of said stored signals corresponding to a plurality of cycles of said signal component, that set of signals that was sampled at a point closest to a given zero crossing point of each of said cycles of said signal component, and to compute said phase error value by comparison of the amplitude of said selected signal set with a notional value of a sample coinciding with said crossing point. A digital signal processing system comprising an analogue to digital converter (5) arranged to receive an incoming analogue signal that is to be digitised, a variable frequency clock oscillator (6) for controlling a sampling frequency at which said analogue to digital converter (5) is to be operated, a phase shifter (7) coupled between said clock oscillator and said analogue to digital converter (5), and means (12) arranged to receive sampled digital signal values provided by said analogue to digital converter (5), to compare said sampled values and to provide output control signals to said phase shifter for adjusting the phase relationship of said sampling frequency and the incoming analogue signal, characterised in that said comparing means comprises a processor unit (12) programmed to compute from sampled signal values provided thereto a phase error between the said sampling frequency and a signal component of the incoming analogue signal, to provide a control signal to said phase shifter (7) when the phase error is greater than a predetermined magnitude and to provide a control signal to said variable frequency oscillator (6) when the phase error is below said predetermined magnitude. A signal processing system according to claims 1 or 2 for use in digitising colour television signals, characterised in that the sampling frequency of the analogue to digital converter (5) is four times the frequency of the colour subcarrier of the composite video waveform, and that the signal component of the incoming analogue signal with reference to which the phase error is computed is the colour burst of the composite video waveform. A system as claimed in claim 2 or claim 3 as appended thereto, characterised in that the said phase shifter (7) comprises a tapped delay line (71) and that the control signal provided to the phase shifter (7) from the processor unit (12) controls the selection of that tap of the delay line from which the oscillator signal is supplied to the analogue to digital converter (5). A signal processing system as claimed in claim 3 or claim 4 as appended thereto, characterised in that the processor unit (12) is programmed to receive and store sampled signal values received during each cycle of a colour burst, in such a manner that the respective values corresponding to a single cycle of the colour burst are averaged over a plurality of cycles before the phase error is computed. A signal processing system according to claim 2 or any one of claims 3-5 as appended thereto, characterised in that said processor unit (12) is further programmed to determine, by comparison of a set of sampled amplitude signals corresponding to one cycle of said signal component, that sample closest to a given zero crossing point of said cycle of said signal component, and to compute said phase error value by comparison of the amplitude of said sample with a notional value of a sample coinciding with said crossing point. A signal processing system as claimed in claim 3, or any one of claims 4-6 as appended thereto, characterised in that the processor unit (12) and a tag word counter (13) are both arranged to be controlled at a system frequency corresponding to the sampling frequency of the analogue to digital converter (5), and that the processor unit (12) is arranged to receive from the tag word counter (13) tag words associated with each of the samples received from the analogue to digital converter (5) during a cycle of the colour burst signal, in order to identify the relative positions of the samples within said cycle. A signal processing system according to claim 7, as appended to claim 5, characterised in that averaging of said respective values is effected by said processor unit (12) accumulating consecutive samples identified by the same tag word in a corresponding register to provide an integrated value. A signal processing system according to claim 3 or any one of claims 4-8 as appended thereto, characterised in that it further comprises means (3) for extracting synchronising reference signals from the composite video waveform of the analogue signal and for providing said reference signals to said processor unit (12) to define a colour burst window during which samples may be accepted from the analogue to digital converter (5).

Inventors: TYSON PETER, TYSON, PETER
Applicants: QUESTECH LTD, QUESTECH LIMITED
Application year: 1991
Main classification: H04N9
IPC codes: H04N9