GB1573159A

GB1573159A - Wired broadcasting systems

Info

Publication number: GB1573159A
Application number: GB4516177A
Authority: GB
Original assignee: Communications Patents Ltd
Current assignee: Communications Patents Ltd
Priority date: 1977-10-29
Filing date: 1977-10-29
Publication date: 1980-08-13
Also published as: NL7808662A; DE2829667A1

Description

(54) IMPROVEMENTS IN AND RELATING TO WIRED BROADCASTING SYSTEMS (71) We, COMMUNICATIONS PATENTS LIMITED, a British Company of, Carlton House, Lower Regent Street, London, SW1Y 4LS, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to wired broadcasting systems and seeks to provide an improved form thereof.

In wired broadcasting systems operating in the high frequency range, that is in the frequency range 2-20 MHz, difficulties arise in the choice of an optimum carrier wave frequency particularly for colour television transmissions because of the inband beat effects which arise as a result of unavoidable nonlinearities in the active components of the system and of unavoidable crosstalk between adjacent signal paths. Frequencies in the range 4 - 6 MHz with upper vestigial sideband have often been successfully employed but difficulties are increasingly arising as a result of the proliferation of high-power short-wave transmitters operating in the broadcasting bands around 5 MHz and 6 MHz. These short-wave transmissions cause additional patterning problems to arise on television pictures derived from HF wired broadcasting systems. Although these problems can be minimised by increasing the signal level on the HF wired broadcasting network this solution is both commercially uneconomic and technically self-defeating as the higher signal levels demand the achievement of even higher degrees of linearity in the active elements of the network which are in existing wired broadcasting systems already at a very high level.

A solution to the problem of short-wave broadcasting interference is to move the carrier frequencies used on the HF wired broadcasting system. The choice of an alternative frequency is however limited by the design of the subscribers terminal units since to modify or exchange the very large numbers already in use would involve prohibitive expense and labour effort. The alternative carrier frequencies available lie in the range of about 8 - 10 Mhz with lower vestigial sideband but the use of a carrier in this range poses an additional problem in that as well as the beat effects previously mentioned any second-harmonic of the colour subcarrier generated in the system will beat with the carrier wave to give a coarse and obtrusive pattern on the received pictures.

In an attempt to provide a wired broadcasting system in which the adverse effects of in-band beats resulting from unavoidable non-linearities in the active components of the system, of unavoidable crosstalk between adjacent signal paths, of interference from shortwave transmitters operating in the region of 5 MHz and 6 MHz and of any second-harmonic component of the colour subcarrier may be simultaneously mitigated, it has been proposed to provide a wired broadcasting system in respect of television signals which comprise means for generating an amplitude modulated high frequency carrier wave in respect of picture signals, the frequency of which corresponds to an odd mutliple of one quarter of the line scanning frequency of the television signals concerned.

When the above proposals were implemented on practical networks, it was found that the quarter-line frequencies which were theoretically correct suffered from patterning in certain saturated colour areas.

It is an object of the present invention to overcome the practical problems referred to above.

According to the present invention there is provided a wired broadcasting system in respect of television signals which comprises means for generating an amplitude modulated high frequency carrier wave in respect of picture signals, characterised in that the frequency of the carrier wave is related to the line scanning frequency and the field frequency in accordance with the following equation: (2n + 1) (2m + 1) F f= L+ 4 2 where L is the line scanning frequency and F is the field frequency of the television signals concerned, and n and m are integers.

The advantages of the invention are realised over the whole range of frequencies at intervals equal to the field frequency and also at intervals equal to the line scanning frequency above and below any particular carrier selected from the above formula.

The explanation as to why the carrier frequency selection according to the invention works is that in the PAL system the relative amplitudes of the two colour sub carrier components at 4.43361875MHz present as a result of the phase alternation of one colour component vary under modulation depending upon the colour content of any particular scene being transmitted. Long term observations have established that there is an "average" colour content resulting in a corresponding average relative amplitude of the colour sub carrier components.

Thus it is possible to select a carrier frequency for the wired network which suits this average scene. As a result no patterning is evident other than occasionally in a highly saturated colour area of a cartoon transmission for example.

A wired broadcasting system in accordance with the present invention will tolerate the addition of a frequency modulated carrier wave in respect of accompanying sound signals at a frequency spacing, relative to the vision carrier, corresponding closely with that adapted for radiated broadcasting of signals to the same television standard. The distribution of the accompanying sound signals in this manner is becoming increasingly attractive with the advent of integrated circuits designed for processing the frequency modulated sound signals for television receivers designed for aerial reception and for use in VHF broadcast receivers.

Further, the presence on the wired broadcasting network of a frequency modulated sound signal having substantially the correct frequency spacing from the vision signal avoids complication in the operation of signal adaptors by means of which receivers designed for aerial reception can be fed with signals from the wired network.

A wired broadcasting system in accordance with the present invention may also be arranged to distribute sound only programmes utilising frequency modulated carrier waves located outside the band occupied by the television signals, for example, above the television signal band, and having a frequency spacing relative to the brightness carrier wave of the television transmission corresponding to an odd multiple of half the line scanning frequency of the television signals concerned. In addition it may be advantageous to arrange for the carrier wave in respect of each sound only programme to differ slightly from that of the others so as to enhance the crosstalk protection between them.

Further features and advantages of the invention will become apparent from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a wired broadcasting system in accordance with the present invention; and Figure 2 is a diagram showing the frequency relationship between the various signals in a wired broadcasting system as shown in Figure 1.

In Figure 1 there is shown a wired broadcasting system which is arranged to distribute signals over signal paths provided in a cable 1. These paths are formed by six pairs of twisted mutually isolated conductors 2 and a further six provided by the pairs of interstitial conductors 3 associated therewith. For simplicity transmitting equipment is shown in respect of only two signal paths. This transmitting equipment comprises a receiver 4, 5 associated with each path and each fed with signals from a corresponding aerial 6,7. These signals will normally be those of a regular radiated broadcasting transmitter. The receivers 4,5, are arranged to process the received off-air signals and to provide on their outputs 8,9, signals of a form suitable for direct application to the wired broadcasting system. These signals comprise an amplitude modulated carrier wave 10, see Figure 2, in respect of the picture signals 11 having a frequency which is approximately twice that of the suppressed carrier wave 12 in respect of the colour signals 13 and a frequency modulated carrier wave 14 in respect of the accompanying sound signals. The frequency of the picture carrier 10 is determined by automatic frequency control arrangements in the receivers 4, 5, which operate in association with a highly stable reference oscillator 15 which is arranged to provide a reference signal having a frequency satisfying the formula: (2n + 1) (2m + 1) F f = L+ ~~~~ 4 2 where L is the line scanning frequency and F is the field frequency of the television signals concerned, and n and m are integers.

Typically the reference oscillator 15 comprises an oven controlled crystal oscillator having a stability of + 2 Hz but a variation of about + 6 Hz is allowable without losing the advantages of the present invention. The frequency spacing between the sound and picture carrier waves is that employed by the radiated broadcasting transmitters. It will be observed that the amplitude of the sound carrier is less than that of the picture carrier. The reason for this is that the cable attenuation is much lower at the sound carrier frequency and that the amplitude of the sound signal must be kept as low as possible consistent with satisfactory operation of receivers connected to the wired network so as to minimise the generation of unwanted beats between it and the picture carrier.

Subscribers television receivers, of which one is shown at 16, are arranged to derive their signals from the signal paths 2 by means of a switch 17 which connects the input terminals of the receiver 16 to that signal path carrying the desired programme. Each subscribers switch is connected to the signal paths 2 through the intermediary of impedance networks 18 by means of which each subscribers installation is matched to the conductors of the signal paths and provided with a degree of isolation from other subscribers. Typically, the impedance networks 18, commonly referred to as 'inserts', may be of the inductively coupled type wherein no direct physical connection is made to the conductors of the signal path. Such inserts are described in our British Patent Specification No. 1 441 959.

If desired additional sound only programmes may be provided by means of frequency modulated carrier waves outside the band occupied by the television signals. One sound only programme is arranged to be carried on each signal path. To this end each receiver output 8, 9, has associated therewith a corresponding combining device 19, 20 by means of which signals from frequency modulated sound transmitters 21, 22 may be applied to the same signal paths. The sound transmitters 21, 22 include means which determine that their carrier waves have frequencies which are spaced from the brightness carrier waves by an odd multiple of half the line scanning frequency of the television signals concerned. Typically these means may comprise an automatic frequency control arrangement referred to an oven controlled crystal oscillator. Here a variation of + 100 Hz on the nominal frequency is allowable without losing the advantages of the present invention. However, in practice an oscillator of the type shown at 15 would be used for preference. The carrier wave 23 in Figure 2 represents a sound only programme. It may be advantageous for the sound only programmes on each of the signal paths in the cable 1 to have frequencies which differ slightly from each of the others thereby to enhance the crosstalk protection between them.

In a wired broadcasting system of the kind described above and operating in accordance with CCIR system I the frequency selected for use is that for which n = 1134, m = O, F = 50 Hz and L = 15625 Hz, giving f = (567.25L - 25) Hz = 8.86325625 MHz. The spacing between this vision carrier and the sound only carrier is chosen to be 168.5L giving a sound carrier frequency of 11.49606875 MHz. The offset between the various sound only programmes may be about 25 kHz.

It will be appreciated that the sound only programmes may alternatively be distributed over the interstitial conductors 3 rather than the main signal path conductors 2 although with the arrangement shown the interstitial conductors 3 are available, if required, for auxiliary purposes.

WHAT WE CLAIM IS: 1. A wired broadcasting system in respect of television signals which comprises means for generating an amplitude modulated high frequency carrier wave in respect of picture signals, characterised in that the frequency of the carrier wave is related to the line scanning frequency and the field frequency in accordance with the following equation: (2n + 1) (2m + 1) F f = L+ 4 2 where L is the line scanning frequency and F is the field frequency if the television signals concerned, and n and m are integers.

2. A wired broadcasting system in respect of colour television signals according to Claim 1, wherein the amplitude modulated high frequency carrier wave has a frequency approximately twice that of the colour subcarrier wave of the television signals.

3. A wired broadcasting system according to Claim 1 or 2, comprising means for generating a frequency modulated carrier wave in respect of sound signals accompanying the television signals.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

(2n + 1) (2m + 1) F f = L+ ~~~~

4 2 where L is the line scanning frequency and F is the field frequency of the television signals concerned, and n and m are integers.

Typically the reference oscillator 15 comprises an oven controlled crystal oscillator having a stability of + 2 Hz but a variation of about + 6 Hz is allowable without losing the advantages of the present invention. The frequency spacing between the sound and picture carrier waves is that employed by the radiated broadcasting transmitters. It will be observed that the amplitude of the sound carrier is less than that of the picture carrier. The reason for this is that the cable attenuation is much lower at the sound carrier frequency and that the amplitude of the sound signal must be kept as low as possible consistent with satisfactory operation of receivers connected to the wired network so as to minimise the generation of unwanted beats between it and the picture carrier.

Subscribers television receivers, of which one is shown at 16, are arranged to derive their signals from the signal paths 2 by means of a switch 17 which connects the input terminals of the receiver 16 to that signal path carrying the desired programme. Each subscribers switch is connected to the signal paths 2 through the intermediary of impedance networks 18 by means of which each subscribers installation is matched to the conductors of the signal paths and provided with a degree of isolation from other subscribers. Typically, the impedance networks 18, commonly referred to as 'inserts', may be of the inductively coupled type wherein no direct physical connection is made to the conductors of the signal path. Such inserts are described in our British Patent Specification No. 1 441 959.

If desired additional sound only programmes may be provided by means of frequency modulated carrier waves outside the band occupied by the television signals. One sound only programme is arranged to be carried on each signal path. To this end each receiver output 8, 9, has associated therewith a corresponding combining device 19, 20 by means of which signals from frequency modulated sound transmitters 21, 22 may be applied to the same signal paths. The sound transmitters 21, 22 include means which determine that their carrier waves have frequencies which are spaced from the brightness carrier waves by an odd multiple of half the line scanning frequency of the television signals concerned. Typically these means may comprise an automatic frequency control arrangement referred to an oven controlled crystal oscillator. Here a variation of + 100 Hz on the nominal frequency is allowable without losing the advantages of the present invention. However, in practice an oscillator of the type shown at 15 would be used for preference. The carrier wave 23 in Figure 2 represents a sound only programme. It may be advantageous for the sound only programmes on each of the signal paths in the cable 1 to have frequencies which differ slightly from each of the others thereby to enhance the crosstalk protection between them.

In a wired broadcasting system of the kind described above and operating in accordance with CCIR system I the frequency selected for use is that for which n = 1134, m = O, F = 50 Hz and L = 15625 Hz, giving f = (567.25L - 25) Hz = 8.86325625 MHz. The spacing between this vision carrier and the sound only carrier is chosen to be 168.5L giving a sound carrier frequency of 11.49606875 MHz. The offset between the various sound only programmes may be about 25 kHz.

It will be appreciated that the sound only programmes may alternatively be distributed over the interstitial conductors 3 rather than the main signal path conductors 2 although with the arrangement shown the interstitial conductors 3 are available, if required, for auxiliary purposes.

WHAT WE CLAIM IS: 1. A wired broadcasting system in respect of television signals which comprises means for generating an amplitude modulated high frequency carrier wave in respect of picture signals, characterised in that the frequency of the carrier wave is related to the line scanning frequency and the field frequency in accordance with the following equation: (2n + 1) (2m + 1) F f = L+

4 2 where L is the line scanning frequency and F is the field frequency if the television signals concerned, and n and m are integers.
2. A wired broadcasting system in respect of colour television signals according to Claim 1, wherein the amplitude modulated high frequency carrier wave has a frequency approximately twice that of the colour subcarrier wave of the television signals.
3. A wired broadcasting system according to Claim 1 or 2, comprising means for generating a frequency modulated carrier wave in respect of sound signals accompanying the television signals.
4. A wired broadcasting system according to any preceding claim, comprising automatic

frequency control arrangements associated with a highly stable reference oscillator providing a reference signal having a frequency equal to the frequency of the carrier wave.
5. A wired broadcasting system according to Claim 4, wherein the reference oscillator is an oven controlled crystal oscillator.
6. A wired broadcasting system according to any preceding claim, comprising a plurality of receivers each fed with signals from a corresponding aerial, and each providing television signals to a respective pair of mutually isolated twisted together conductors.
7. A wired broadcasting system according to Claim 6, as appendent to claim. 3, wherein the frequency spacing between the carrier waves in respect of the picture and accompanying sound signals is that employed in the received radiated broadcast.
8. A wired broadcasting system according to any preceding claim, comprising means for generating a frequency modulated carrier wave in respect of sound only programme signals which carrier wave is located outside the band occupied by the television signals and has a frequency spacing relative to the brightness carrier wave of the television signals corresponding to an odd multiple of half the line scanning frequency.
9. A wired broadcasting system according to Claim 8, wherein the carrier wave in respect of sound only programme signals is located above the said band.
10. A wired broadcasting system according to Claim 8 or 9, wherein means are provided for generating a plurality of carrier waves in respect of different sound only programme signals, the frequencies of the plurality of carrier waves differing so as to enhance crosstalk protection between them.
11. A wired broadcasting system according to Claim 10, wherein the frequencies of the plurality of carrier waves differer by approximately 25 kHz.
12. A wired broadcasting system according to Claim 8, 9, 10, or 11, wherein the means for generating a frequency modulated carrier wave in respect of sound only programme signals comprise an automatic frequency control arrangement associated with an oven controlled crystal oscillator.
13. A wired broadcasting system according to any preceding claim, operating in accordance with CCIR system I.
14. A wired broadcasting system according to Claim 13, wherein the carrier wave in respect of picture signals has a frequency of 8.86325625 MHz.
15. A wired broadcasting system according to Claim 14, wherein an associated sound only programme carrier wave has a frequency of 11.49606875 MHz.
16. A wired broadcasting system in respect of television signals substantially as hereinbefore described with reference to the accompanying drawing.