DE29615897U1 - Frequency diversity arrangement - Google Patents

Description

Frequency diversity arrangement

The invention relates to a frequency diversity arrangement for the mobile reception of RF signals, in particular for mobile television reception, with the generic features specified in the preamble of the main claim.

Frequency diversity specifically for television reception has not yet become established. There are therefore no receivers that can automatically choose between, for example, two frequencies broadcast within a service area with the same television program and switch reception to the channel with the better signal quality, nor is there the option of choosing between, for example, more than two different frequencies with the same television program across regions, as has been introduced in radio broadcasting in the form of the Radio Data System (RDS). In radio broadcasting, RDS is - alongside spatial diversity - a basis for largely interference-free and consistently good reception on long journeys in motor vehicles. There is still no comparable system for television broadcasting.

In television technology, it is common to store a certain number of stations, each with a frequency, in the receiver, which can be called up manually by setting a reception channel. This principle fails when receiving television in moving vehicles, at least at the boundaries of the service area - with weakening of the currently received signal - and for other known reasons, such as fading, multipath reception, interference, etc.

In addition, it is known that the function of diversity circuits can in turn be associated with impairments in reproduction, such as noise when switching between antennas or frequencies and, above all, due to frequent switching back and forth between the signal paths when the signal quality on the available channels is approximately the same. This is particularly true under the operating conditions of the

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Television. In addition to all correction processes to ensure the most continuous possible provision of a signal with a sufficiently high level and signal-to-noise ratio, stable image synchronization and the synchronization of image and sound must always be guaranteed. If, therefore, the diversity principle is to be used to improve reception under the conditions of reception in a moving vehicle, it must be ensured that the advantages gained with the increased effort are not reduced by deficiencies in the diversity function.

The invention is based on the object of creating the basis for mobile and at the same time stable and as undisturbed as possible reception of television programs that are broadcast simultaneously in several service areas and on different frequencies by applying the frequency diversity principle, whereby the effort associated with the introduction of, for example, additional program identifiers is to be avoided and whereby interference and side effects of the diversity function are to be largely prevented.

This object is achieved according to the invention with the features specified in the characterizing part of the main claim. The subclaims contain preferred design details and variants.

It can be seen that the objective of the task of a largely universal principle for the automatic recognition of alternative frequencies to a specific television program that has been set once and thus a basis for the use of frequency diversity when receiving television broadcasts in a moving vehicle has been achieved.

For the comparison, the invention covers the simplest case, i.e. use of a program identifier, e.g. the VPS signal (if available), as well as the use of certain selected areas of the audio and video signal and can thus be used to detect certain programs independently of the presence of definitive information with an address component within the transmitted frequency band and without additional effort on the part of the transmitter or additions or modifications to the signal.

The invention is explained in more detail below using exemplary embodiments. The accompanying drawing shows Figure 1. Block diagram of a frequency diversity system for

mobile television reception according to the invention Figure 2. Diversity system as in Figure 1, for sound broadcast reception

In Figure 1, the receiver for sound and picture reproduction, as found in every television set, is designated 2. Receiver 5 has also been provided for the frequency diversity function. Both receivers are connected to the receiving antenna 1 via a fork. When the device is switched on, receiver 5 of the diversity circuit searches the transmission range for other stations with the current program, independently of the signal processing by the main receiver 2. If another station is found, the search stops and the received signal is fed to the VPS decoder 6. If this is a signal with a VPS identifier, the identifier is compared with the VPS identifier of the station running via the main receiver 2. At the same time, the evaluation and control circuit 7 checks and compares the reception quality of the two stations. If the identifiers match and if the reception quality of the comparison station is better, the main receiver 2 is set to the new frequency. Immediately before the switching process, the monitor is connected to the receiver 5 by means of the signal source switch 3, and the receiver 5 is switched as the main receiver during the transient response of the receiver 2.

Then switch 3 is switched back and the receiver of the diversity circuit continues its search.

The evaluation and control circuit 7 has another function: It stores all the stations found with their VPS identifiers and the current signal quality in a non-volatile memory. The stored information is included in the ongoing test and comparison process. This means that no separate VPS decoder is required for the main receiver 2.

In parallel to the search over the entire frequency range - which requires a certain amount of time - the frequencies whose identifier matches the signal transmitted by receiver 2 are checked sequentially at regular intervals. By selecting and comparing the preferred stations,

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the system can react more quickly to changing reception conditions.

The circuit shown in Figure 2 of a frequency diversity system according to the invention for audio-only radio reception works in principle in the same way as the circuit in Figure 1. The difference is that here the output signals of the main receiver 2 and the receiver 5 of the diversity circuit are used. The signals from both receivers are fed into the comparator 9, which evaluates them and then generates the corresponding pulse for the control circuit 7.

Claims (6)

j. Protection claims 1. Frequency diversity arrangement for the mobile reception of HF signals in the meter and decimeter wave range, in particular for mobile television reception, essentially consisting of an additional receiver and an evaluation circuit which, while driving, continuously searches for all incoming frequencies of a program that has been set once and switches the main receiver to the frequency with the best signal quality,
characterized by the following features: - The criterion for finding the alternative frequencies of a program is an existing station identifier that has been added to the wanted signal for other purposes, or the wanted signals of the alternative frequencies are directly compared and evaluated with the wanted signal of the set program. - In addition to the information about alternative reception frequencies of the selected program, local location coordinates are processed so that the evaluation is limited from the outset to the alternative frequencies expected in the area and the entire control process is accelerated. - The control pulse for switching the main receiver (2) to another frequency is simultaneously used to switch the useful signal path between the main receiver and the receiver (5) of the diversity circuit, such that the useful signal is passed through the receiver (5) of the diversity circuit during the gap given by the transient phase of the main receiver (2). - Threshold values of the selected signal quality criteria are specified for the response of the evaluation and control circuit (7) and for generating the switching control pulse, so that if the signal quality is approximately the same on the available frequencies of the set program, switching back and forth between the stations is avoided. •i &kgr;.·: I 2. Frequency diversity arrangement according to claim 1, characterized in that the VPS identifier is used as the station identifier for searching for alternative frequencies of a television program. 3. Frequency diversity arrangement according to claim 1, characterized in that when comparing the useful signals on the different frequencies, signal samples are taken and compared at identical positions in relation to the synchronization pulses, whereby in particular the videotext, luminance, chrominance and audio signals as well as recorded subtitles can be used and whereby the selected signal components are preparatory preferably subjected to low-pass filtering to suppress higher-frequency interference. 4. Frequency diversity arrangement according to claim 1 and 3, characterized in - that instead of the point-by-point comparison of the components taken from the useful signals, the spectral composition or an envelope of the signals, preferably the envelope of the audio signals, are compared, whereby in the latter case 50 measurements per s are preferably carried out for the comparison, and - that the comparison is made by forming the difference between the signals, whereby signals are considered to belong to the same program if, on average, a predetermined absolute maximum value of the difference is not exceeded over a certain period of time. 5. Frequency diversity arrangement according to claim 1 and 3, characterized in that the signals, made mean-free, are compared by cross-correlation, the measurements being carried out at moments in which the signals to be compared are largely free of interference and in which there is no modulation pause, and identity is assumed when the value of the cross-correlation reaches approximately half of the autocorrelations. 6. Frequency diversity arrangement according to claims 1 to 5, characterized in that when there are several receivers (2) for separate processing of different signal components, preferably a receiver (2) for picture and a receiver (2) for sound in television reception, a separate frequency diversity arrangement operates on each reception branch and separately switches on the best picture frequency and the best sound frequency for the selected program.