GB2173365A

GB2173365A - Tuning system for a receiver

Info

Publication number: GB2173365A
Application number: GB08607025A
Authority: GB
Inventors: Raymond James Dominey; Terence Michael Cassell
Original assignee: I G G ELECTRONICS Ltd
Current assignee: I G G ELECTRONICS Ltd
Priority date: 1985-03-23
Filing date: 1986-03-21
Publication date: 1986-10-08
Also published as: GB8507594D0; GB8607025D0

Abstract

A tuning system arranged to receive an incoming signal, e.g. a signal modulated with teletext data in the case of a television signal, comprises a tuner (10), a decoder (14) and a microprocessor (16). Digital data on the incoming signal is used by the microprocessor to modify the tuner (10). The microprocessor (16) is used in a closed loop system to carry out fine tuning of the receiver and/or selection of a particular transmission. Fine tuning can be accomplished by monitoring errors in the data in the incoming signal. Automatic selection of a particular transmission is achieved by inspecting the data in the incoming signal for a content that is unique to a particular signal source. <IMAGE>

Description

SPECIFICATION Tuning systems SPECIFICA T1ON This invention relates to tuning systems and to methods of and apparatus for tuning a receiver, in particular-for tuning a receiver to a signal modulated with teletext data, such as, in the United Kingdom, the current BBC and IBA television broadcasts.

The user of a domestic television receiver will tune the receiver into a transmission and then verify that the set is tuned to the desired channel by looking at the content of the programme. This method works well for television receivers where the user has instant visual and audio feedback via the screen and sound.

In situations where a receiver is required to operate from a broadcast transmission and that receiver is not fitted with visual display or sound facilities, the problem arises of how to tune that receiver into a particular transmission.

It is an object of the present invention to provide a system which provides for automatic tuning of such a receiver.

It is a further object of the present invention to carry out a tuning procedure using digital data present on the transmitted signal instead of using the video signal directly.

The tuning of such a receiver can be considered in two parts. The first involves the fine tuning of the receiver to an incoming signal, and the second involves selection of the desired transmission from a number of incoming transmissions with different programme contents.

It is yet a further object of the present invention to provide a system for carrying out one or both of these tuning procedures.

In accordance with the present invention there is provided a tuning system for a receiver arranged to receive an incoming signal, the system comprising a tuning circuit, means for decoding said incoming signal, and control means coupled to said decoding means and arranged to modify the tuning of the tuning circuit in response to information in the decoded signal.

Preferably, the control means comprises a microprocessor.

Preferably, the control means responds to digital data contained in the incoming signal.

The control means is preferably used in a closed loop control system to carry out fine tuning of the receiver and/or selection of a particular transmission.

The fine tuning of the receiver is preferably accomplished by monitoring errors within portions of the data, e.g. teletext data, in the incoming transmission.

In a preferred embodiment, the system is adapted for the reception of teletext signals, the control means having stored therein a portion of teletext data which is characteristic of the transmission to be received and being arranged to detect errors in the corresponding portion of the decoded signal and cause the tuning frequency to be adjusted to a point at which the number of errors is at a minimum, whereby the tuning circuit is finely tuned to the transmission frequency.

In another embodiment, the system is adapted for selection of a particular transmission, the control means being arranged to search the decoded signal for data which is unique to the transmission it is desired to select and to vary the tuning frequency until that data is found.

The tuning system of the present invention is extremely cost effective.

One presently preferred embodiment of the system in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram illustrating one method of tuning in accordance with the invention; and Figure 2 is a block schematic diagram illustrating a second method of tuning in accordance with the present invention.

Considering first the aspect of the fine tuning of the receiver, this is achieved by monitoring errors within protected portions of the teletext data where one has a transmission signal which is modulated with teletext data.

Errors within teletext data may be detected, for example by checking the parity of portions of the teletext data protected with a parity bit, or by checking the CRC (cycle redundancy check) of a portion of teletext data protected by a CRC, or by the checking of hamming coded bytes. An example of the method using hamming coded bytes is further explained hereinafter. In the teletext system important data is sent in a hamming coded byte. A hamming coded byte contains four message bits and four protection bits. This enables a single bit error to be corrected on reception and one or more errors to be detected.By monitoring the number of errors in the hamming coded bytes received, while simultaneously controlling the frequency at which the receiver operates by using a microprocessor, the frequency of the receiver can be adjusted to the optimum point where the minimum number of errors is detected. Figs. 1 and 2 show two alternative ways in which the microprocessor can control the precise frequency at which the receiver operates.

As shown in Fig. 1, a tuner or tuning circuit 10 receives incoming signal transmissions modulated with teletext data and incorporates a local oscillator. This is a voltage-controlled oscillator. The tuner 10 is connected to an IF stage 12 which has its output connected to a teletext decoder 14. The output from the de decoder 14 is fed to a microprocessor 16, the digital output of which is fed to a digital-toanalogue converter 18. This converter 18 provides a tuning voltage to the local oscillator within the tuner 10. The frequency of oscillation of the local oscillator is controlled by the DC tuning voltage applied to it, in this em embodiment a voltage derived from the output of the digital-to-analogue converter.

In contrast, in the arrangement shown in Fig. 2, where the first local oscillator in the tuner 10 is also a voltage-controlled oscillator, here the voltage applied to that oscillator is supplied by a digital frequency synthesiser 20.

The microprocessor 16 again supplies the fre -quency data to the synthesiser 20 and thus again has control of the receiver frequency.

In each system the receiver frequency is finely controlled by the microprocessor 16 and the frequency is adjusted by the microprocessor whilst the hamming codes decoded by the decoder 14 are inspected for errors. The receiver frequency is then automatically adjusted to the point where the minimum number of errors occurs. In other words, in contrast to the conventional television set where the user performs the adjustment, with the system of the present invention the microprocessor 16 is exerting the control function.

The second aspect of the invention, as referred to above, involves selecting a particular transmission from a number of transmissions which have different programme contents.

Automatic selection of a particular transmission is achieved by adjusting the receiver frequency whilst inspecting part of the teletext data for a content which is unique to a particular signal source. For example, with the cur rent UK system, if one requires the receiver to operate on BBC1, then the receiver frequency is adjusted by the microprocessor 16 until a signal is received. The microprocessor 16 then inspects the content of line 0 on magazine number 1 for the text "CEEFAX". If this is not found then the receiver frequency is adjusted until another signal is received. This next signal is then checked by the microprocessor for data unique to that signal source.

This continues until the page header of the teletext data is found to contain the text "CEEFAX''. This is- then used to verify that BBC1 has been selected. This system can be implemented using digital frequency synthesis for tuning control where the frequency to be synthesised is controlled by the microprocessor as in Fig.2, or by a means where the tuning voltage is more directly controlled from the microprocessor as in Fig. 1.

Claims

1. A tuning system for a receiver arranged to receive an incoming signal, the system comprising a tuning circuit, means for decoding said incoming signal, and control means coupled to said decoding means and arranged ;to modify the tuning of the tuning circuit in response to information in the decoded signal.

2. A tuning system as claimed in claim 1, wherein said control means comprises a mi croprocessor.

3. A tuning system as claimed in claim 1 or 2, in which the control means responds to digital data contained in the incoming signal.

4. A tuning system as claimed in any pre ceding claim, in which the control means is part of a closed loop control circuit.

5. A tuning system as claimed in claim 1 or 2, wherein the tuning circuit includes an oscillator and wherein the output of the con trol means is supplied to a digital-to-analogue converter which supplies a tuning signal to the oscillator.

6. A tuning system as claimed in claim 1 or 2, wherein the tuning circuit includes an oscillator and wherein the output of the con trol means is supplied to a digital frequency synthesiser which supplies a tuning signal to the oscillator.

7. A tuning system as claimed in any pre ceding claim adapted for the reception of tele text signals, the control means having stored therein a portion of teletext data which is characteristic of the transmission to be re ceived and being arranged to detect errors in the corresponding portion of the decoded sig nal and cause the tuning frequency to be adjusted to a point at which the number of er rors is at a minimum, whereby the tuning cir cuit is finely tuned to the transmission fre quency.

8. A tuning system as claimed in claim 7, wherein said portion of teletext data is the hamming code.

9. A tuning system as claimed in any of claims 1 to 6, adapted for selection of a parti cular transmission, the control means being ar ranged to search the decoded signal for data which is unique to the transmission it is de sired to select and to vary the tuning fre quency until that data is found.

10. A tuning system substantially as here inbefore described with reference to Fig. 1 of the accompanying drawings.

11. A tuning system substantially as here inbefore described with reference to Fig. 2 of the accompanying drawings.