Field Trials with Videotext Systems in Switzerland

Summary

Videotext field trials have been carried out since slightly more than one year by the Research and Development Department of the Generale Directorate of the Swiss PIT at Berne, partly in collaboration with other European research centres. The aim of these trials was to check the reliability of transmission of such additional data information in the vertical blanking interval of a television signal. The quality of the Videotext “image” was compared with the quality of the colour television picture, in order to judge the relative service areas to be expected for Videotext signals.

The tests were carried out by using a van and comprised NRZ- and biphase coded data signals of various bit rates (original TELETEXT, biphase/RAI, DIDON/ CCETT). The test regions were in and around Berne, in the south-eastern mountainous areas of Berne (Pre-Alps) and in the Jura Mountains (Fig. 1). Direct reception and reception via transposers and transposer chains were accomplished in all the television Bands I to V. The receiving sites were chosen in such a way, that the colour picture quality was normally fairly good or good. A large and a fairly large community antenna system was included in one of the trials.

Four field trial periods have been carried out so far. The results can be summarized as follows. The quality of Videotext transmission depends on a number of parameters. The most important of these are:

• system parameters (type of coding used, bit rate etc.)

• conditions for transmission and reception (topography)

• quality of the receiving station: aerial, transmission line and adaptation, receiver(RF/IF-characteristic, demodulation, decoding).

Among the results gained so far are to be mentioned:

• There is only very little correlation between the quality of the colour picture and the quality of the Videotext “image”

• Random noise is not a problem in Videotext transmission

• Impulsive noise may be critical if it falls into the vertical blanking interval

• Echoes with short time delay are much more critical for Videotext signals than for colour television signals. Such critical short-term echoes may be created in mountainous terrain by superposition of a direct and a ground-reflected wave, if the transmitter or transposer is situated on a relatively high site fairly close to the service area. Difficulties may also arise in transposer chains by uncontrolled ground reflections. Data-signals with lower bit rates (biphase or MRZ) are less affected in such transmission conditions.

• Community antenna systems seem not to be subject of influencing largely the Videotext signals as long as they are of good design and well maintained. It is, however, important that the received signal be of very good quality at the head end.

• Teletext decoders of recent design are significantly better than older ones.