Digital TV page
The subject of digital TV is of personal interest. This page describes some of my findings while researching the subject and attempting to receive all programs that are broadcast from the Midhurst transmitter.
The Freeview website (ww.ukfree.tv) shows the Midhurst transmitter as radiating six multiplexes at 1kW , which are quite low power compared with the transmitter at Crystal Palace which has a 20kW output per multiplex. Only four of the multiplexes were available. This page shows my attempts to receive all six.
The plot below shows the recorded trace for Midhurst, using a 17 dBi group C/D Yagi and 10 dB masthead preamp:
Midhurst transmission
The analogue vision carriers can be seen inter-spaced with the DVB multiplexes, which appear as "blocks" with an extremely well defined edge. Mux C and D are very weak and these were not detected by my fairly old Pace STB.
Examining a smaller span reveals the analogue vision carrier with its colour subcarrier at Fc + 4.43 MHz, the NICAM spectrum (digital sound on analogue service) and the DVB mux. Averaging has been used to improve the trace:
Detail of mux and analogue transmission
Detail of mux, with averaging
This DVB mux is very close to analogue signals. To try to receive all six multiplexes, I fitted an Antiference mast head amplifier with 10 dB gain and 3.5 dB noise figure. The feeder is CT100 coax with a loss of approximately 2 dB. It is a relatively straight forward task to show that the actual signal to noise ratio at the receiver is somewhat worse with the preamp in, as its noise figure is worse than the cable loss. However, the signal strength at the receiver input is higher. It is difficult to see how an aerial amplifier can improve reception, unless the cable is very long or a power divider is used to drive several sets.
Freq response of Antiference amplifier, 200 - 1000MHz, -50 dBm drive, 10 dB/div
When the mast head amplifier was fitted, it was no longer possible to receive channels that I could obtain previously. When I fitted a 6dB attenuator after the preamp, the channels were available again. Interestingly, this shows that the Pace STB may have been overloaded by the strong analogue channels.
The Pace box has now been replaced with a Sony RDR HXD710 HDD/DVB tuner . The Sony receiver now allows all six multiplexes to be received with almost zero uncorrected bit error rate even on mux C and D. The correction removes all the errors, with the result there is no blocking whatsover. Possibly the Sony has a better front end in that it can decode the digital transmission in the presence of the strong analogue signals.
In summary then, fitting the mast head preamp actually made things worse and did not allow reception of the two additional multiplexes. Replacement of the DVB decoder solved the problem, but to be fair to Pace, this was quite an old unit. Perhaps when the analogue switch-off occurs, reception of digital TV will be improved by removal of the analogue carriers and an increase in power to the digital mux's.
A note on the Sony HXD710: Works very well in all respects, with simple A-B scene removal possible on HDD before "dubbing" to DVD. Seems to be able to improve synchronisation on old VHS-C camcorder tapes, i.e, the DVD copy plays better than the original tape. As mentioned above, the DVB receiver is excellent but it crashed once and went into "recovery" mode. Timer and program listings work well.
A warning: While copying old family videos to DVD, some old Hi-8 tapes (particularly Fuji) seem to have a problem where the edge of the tape bonds to the spool. On trying to play these tapes for the first time in 8 years, the tape rips lengthways. However, in my case I managed to salvage most, by spooling by hand - very carefully to avoid ripping, onto another reel . Hint: Don't leave video tape backup too long, your tapes may not be playable in the future! How long does a "home recorded DVD last?
Contact details:
JWD Ltd
Telephone/Fax: +44 (0)1403 261251, Mob: 07973 173808
email:
julian @ jwdltd.demon.co.uk