Abstract: If an N.T.S.C. signal is to be demodulated using a line-locked sampling rate such as 800 times the line frequency, the ratio between the sampling period and the subcarrier period has the awkward value of 455/1600. Line-locked sampling is nevertheless desirable, e.g. in digital standards conversion. The subcarrier digital signals for digital demodulation are derived with the correct frequency from the sampling rate clock pulses CP by an adder and accumulator into which the number 582 is added modulo-2048 (the register is an 11-bit register) to generate an 11-bit number which represents the subcarrier phase angle at each sampling pulse and which addresses a ROM providing sin and cos values representing subcarrier samples. 582/2048 is not exactly equal to 455/1600 but 2048 is a desirable denominator as it is a power of 2 and implies a ROM of suitable size. ##EQU1## is exactly equal to 455/1600.

Abstract: A signal processing circuit (10) for color television signals, e.g. PAL signals, has first and second one-line delays (14, 16) connected in series to the input (12), an averager (18) connected across the two delays and a subtractor (20) connected to subtract the averager output from the output of the first delay (14). In this way the circuit provides (i) a first signal at the output of the first delay (14) which is the input signal delayed by one line period and (ii) a second signal at the output of the subtractor (20) which is the resultant of the average of the undelayed input signal and the input signal delayed by two line periods subtracted from the input signal. A band-pass filter (22) covering the chrominance band is connected to the output of the subtractor (20). A second subtractor (24) subtracts the filtered signal from the one-line delayed signal.

Abstract: Each broadcast page contains a row such as row 29 which is outside the set of displayed rows. In addition to the standard initial bytes CR (clock run in), FC (framing code) and MRAG (magazine number of 3 bits and row number of 5 bits), there are 4 bytes allocated to a 16-bit cyclic rundancy check code CRC for the page and 6 groups, of 6 bytes each, NEXT 0 to NEXT 5. Bytes are Hamming coded and thus contain only 4 message bits. Each 6-byte group is a pointer to another page address made up of page tens and page units (4 bits each), hour code tens and units (2 bits plus 4 bits) and minute code tens and units (3 bits plus 4 bits). The three spare bits from the tens bytes for hours and minutes contain a number which is normally 0 but, if not 0, is added modulo-8 in the decoder to the current magazine number in MRAG to derive the magazine number for the `next` page.

Abstract: Demodulated luminance (Y) and chrominance (U, V) signals are stored in field stores 1 to m each with line outputs 1 to n. Output lines are formed by summing the producs of the line outputs with weighting coefficients which are specified for each line in the output standards. Separate sets of coefficients for luminance and chrominance are stored in respective memories. The chrominance sets are arranged to reduce the vertical and temporal resolution of the chrominance information. This has the beneficial effect of reducing cross-color interference. In the implementation shown the signals are stored as time multiplexed digital samples YYYUV (i.e. with one pair of chrominance samples to three luminance samples) and the luminance and chrominance coefficient sets for the current output line are stored as a.sub.11 to a.sub.mn and b.sub.11 to b.sub.mn respectively in buffer registers. A switching waveform controls electronic switches to apply a.sub.11 to a.sub.

Abstract: An electronic control system for a television receiver (12) incorporates a light pen (10) which can be applied to selected portions (16) of the screen (14). These portions (16) are modulated (FIG. 2) at the field scan rate by data inputs such that in successive television fields they are black or white in accordance with the values 1 or 0 of the bits of the data. The data can be used to pre-program the receiver, or alternatively simply stored or printed. The light pen can also be used to read bar codes (36) in a broadcasting periodical (38), and to distinguish whether the light pen is reading the CRT or a bar code a differentiator (42) and asymmetry detector (44) sense whether the light pen output pulses have sharp or decaying trailing edges. In an alternative arrangement the light pen can be used with a light emitting diode such as on a radio receiver or a telephone.

Abstract: Digital sampling of the luminance component of an N.T.S.C. color television signal is achieved by sampling with a mean frequency of twice the color subcarrier frequency to produce a sub-Nyquist sampled luminance signal. The sampling phase is changed between each pair of lines by an amount equal to half the interval between samples. The digital sampling can be applied to separate luminance and chrominance signals, to encoded N.T.S.C., and to the conversion of N.T.S.C. signals into signals of other form including PAL. Further, phase-perturbed sampling techniques permit the construction of an advantageous N.T.S.C. coder and de-coder system. The coder operates to cause spectrum folding of a luminance input signal by multiplying by a phase perturbed signal of twice the color subcarrier, and adding, the resultant being comb filtered to select frequencies which are multiples of the line frequency. The chrominance signals I, Q are combined into I.+-.

Abstract: A television test signal generator generates a test signal comprised of varying sinusoidal components each of which has a phase defined by the equation:.phi.=k.sub..phi. +k.sub.t .multidot.t+k.sub.t 2.multidot.(t.sup.2 /2) +k.sub.y .multidot.y+k.sub.yt .multidot.y.multidot.t+k.sub.y 2.multidot.(y.sup.2 /2) +k.sub.x .multidot.x+k.sub.xt .multidot.x.multidot.t+k.sub.xy .multidot.x.multidot.y+k.sub.x 2.multidot.(x.sup.2 /2)The use of more complex terms enables more sophisticated qualities of the television equipment to be gauged. The signal can be generated with the use of appropriate accumulators.

Abstract: To provide improved resolution of a television, e.g. CRT, display with reduced large-area flicker, twittering of horizontal edges, and line structure of the picture, the field rate is doubled and the number of lines per field is doubled from that of a received input television signal. Preferably a 21/2 field store arranged in half-field units is used, together with a four-line store arranged in one line units to provide line interpolation, which can be adaptively dependent upon the presence or absence of motion.

Abstract: Encoded digital television signals having a defined bandwidth are transmitted at reduced bandwidth by regularly omitting (18) one in every n of the signal samples, where n is greater than two and is preferably three or four. At a receiver the omitted samples are regenerated by estimating (26) their values from the samples of the reduced-rate transmitted signal by making use of frequency components outside the normal defined bandwidth of the original signal. The estimation can be achieved by a symmetrical digital transversal filter of which one in every n coefficients is zero, the amplitude/frequency characteristics of the filter being unity throughout the defined bandwidth of the TV signal and antisymmetric about half the mean lower sample rate.

Abstract: A method of and apparatus for decoding a shortened cyclic block code in which the received codeword is serially shifted forwardly through a bidirectional syndrome register with the generator word being so subtracted from the register content as to effect division of the received codeword leaving the remainder in the register, and, if the remainder is larger than the maximum correctable error, the register contents are so reverse shifted as to reverse the division operation until the register contents are not larger than the maximum correctable error, the register contents then being combined with the received codeword to produce a corrected word. The codeword length does not have to be artificially increased to that of the maximum length code, and thus a reduction of the processing time can be obtained.

Abstract: To portray movement in a television display, a field of the signal is stored in an image store (110), and subsequent input fields are compared in a motion detector (104) with the stored field to detect differences between them due to movement. A flag is retained in an auxiliary store (114) to indicate the positions in the field of the detected differences. For those positions for which a new difference is detected, i.e. for which no difference has been detected previously, the corresponding portions of the then current input scan are stored in the image store (110), by a switch (106) being connected to the input (102). For portions which are flagged, the output of the auxiliary store (114) causes the switch (106) to take the output of the image store (110), thus insetting past positions of movement into the current field. The background of the field in the image store is nevertheless updated by the background of the input field.

Abstract: Digital processing of a 525 line N.T.S.C. color television signal is achieved by digitizing (40) with a sub-Nyquist sampling frequency f.sub.s which has a mean value 2 f.sub.sc .+-.1/2 f.sub.F, or more generally m f.sub.L .+-.(p+1/2) f.sub.F (where f.sub.sc, f.sub.L and f.sub.F are the subcarrier, line scan, and field scan frequencies, and m and p are integers). Conveniently the sampling frequency is exactly 2 f.sub.sc during each field and suffers a 180.degree. phase shift between fields. After processing, the signal is converted back to analogue form (50) and, to remove alias components, is comb filtered (30) over the frequency range f.sub.s -f.sub.v to f.sub.v (where f.sub.v is the maximum video frequency in the original signal) by averaging across a 262 line delay (24'), or more generally a delay of a field minus half a line.

Abstract: A single field of a PAL color television signal is stored in a store (50) and provides a signal at an input (52). The signal is delayed by one-line delays (56,58) in series to provide one and two-line delayed signals. Separate switching units (60,62) select desired ones or combinations of the input and delayed signals to constitute respectively the low-frequency and chrominance part of the signal at the output (82). The switch positions are changed between fields, and for at least some fields of the PAL sequence each component of the output signal is formed by combining signals from more than one line, i.e. a halving adder (64;66) averages successive lines for the low frequency part of the signal and a halving subtractor (68) forms the difference between lines two lines apart for the chrominance part. The selector unit outputs are combined in an adder (78) which receives the output of the first selector (60), and also the difference between the output of the two selectors after band pass filtering (76).

Abstract: A cyclic redundancy check word is customarily placed at the end of a data block, occupying one or more of the last bytes of the block. This can impose an undesirable restraint upon the person composing the data block, e.g. the editor of a teletext page. The invention allows the check word effectively to be embedded anywhere convenient in the block. A location in the block is selected (one or more bytes--preset address 16) and all preceding bytes are cyclically encoded (encoder 18) to yield a first check word. The succeeding bytes are subjected in reverse order to complementary encoding (encoder 28) to yield a second check word. A third check word is formed from the first and second check words (encoding logic 32) and inserted in the selected location.

Abstract: Data receiving apparatus for receiving repetitively transmitted blocks of data stores a block of data (register 10), and compares (comparator 12) each received byte in DI with the corresponding stored byte in D(n). If the bytes match (M) a data selector 15 selects either byte for storage and sets a flag bit for that byte in a register 14 to 1. If the bytes do not match, the data selector selects the new byte when the existing flag bit is 0 and selects the old, stored byte when the existing flag bit is 1, in either event setting the flag bit to 0. In this way the probability is that each stored byte is an error free byte. A status circuit 16 signals when all flag bits are not 1 to indicate that suspicion nevertheless attaches to the block as stored.

Abstract: A method of and apparatus for simultaneously enabling different parts of an input television signal to be made available is used as a television standards converter. Successive lines of an input television signal are cyclically written into n (eg 4) sucessive random access store sections, using in each cycle the same addresses for the different sections. The write addresses are incremented for sucessive cycles. The sections are read from using for each section addresses which are related to each other and to the write addresses so as to access a desired set of up to n sucessive lines. These lines are combined by weighted addition to provide an interpolated output line. The read addresses are derived by counting the desired output lines during each field and multiplying the count by the line conversion ratio. The integral part of the resultant controls the read addresses and the non-integral part controls the weighted addition. A total storage of two fields is used, each divided into sections.

Abstract: Reduction of noise in a television signal is achieved by comparing signals from successive scans to produce a difference signal. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier which receives as multiplier the output of a non-linear transfer characteristic element. The non-linear element receives the differences after rectification and after passing through a variable-gain element. The non-linear element is such that above a predetermined value a constant predetermined minimum attenuation factor applies. A noise measurement circuit measures the noise present in the difference signal and controls the gain of the variable-gain element accordingly. Several noise-measurement circuits may be provided, a selected one of which is used in dependence upon the magnitude of the input signal to the apparatus.

Abstract: Reduction of noise in a television signal is achieved by comparing signals from successive scans to produce a difference signal. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier which receives as multiplier the output of a non-linear transfer characteristic element. The non-linear element receives the differences after rectification and after passing through a variable-gain element. The non-linear element is such that above a predetermined value a constant predetermined minimum attenuation factor applies. A noise measurement circuit measures the noise present in the difference signal and controls the gain of the variable-gain element accordingly. Several noise-measurement circuits may be provided, a selected one of which is used in dependence upon the magnitude of the input signal to the apparatus.

Abstract: In a method of and apparatus for processing 625-line PAL colour television signals, in which in conjunction with each line of the signal there is provided an additional signal for use therewith, the additional signal is formed essentially of the luminance component of a signal equivalent to the signal 625 lines previous to the current line, and the chrominance component of a signal substantially equivalent to the signal 624 lines previous to the current line. Alternatively, the difference of the chrominance components 624 and 626 lines previous may be used. For 525-line N.T.S.C. the luminance component is derived from the signal 525 lines previous, and the chrominance component is the chrominance component of the signal 524 or 526 lines previous to the current line, or an average of these. The form of the chrominance component may be selected automatically in dependence upon vertical chrominance detail.

Abstract: Reduction of noise in a television signal is achieved by comparing signals from successive scans and noting the differences. Low amplitude differences are assumed to represent noise and are attenuated. Differences above a predetermined level are assumed to represent movement and are not so attenuated. The attenuation is achieved by a multiplier which receives as multiplier the output of a non-linear transfer characteristic element. The non-linear element receives the differences after rectification and after passing through a variable-gain element. The non-linear element is such that above a predetermined value a constant predetermined minimum attenuation factor applies.