From To Received Bytes Subject
G8MNY DIGATV @ WW 2008-08-03 23:24 4125 Re: Digital ATV experience


Hi Readers,                                        (updated Feb 07)

Tony VK3FBD asked some time back if anyone is doing it. Well yes, I first saw a
digital ATV demo at a BATC rally about 16 years ago where a ham working in the
field, put together an encoder & decoder units both the size of a large PC &
but has a commercial value of something like 50000 UKP for just the encoder at
the time.

Since then several hams in the GB3HV 23cms ATV repeater's service area were
into digital & have built up digital systems. One on them was incorporated into
the repeater 2 years ago.

If DTMF commanded the repeater will Tx in digital mode for up to 15 mins at a
time. The digital mode uses satellite QPSK type but @ 4Msym/s & needs all the
sources need to be of broadcast standard for digital. So we start off with a
PAL timebase corrector in a 1U rack unit, a P&Q channel MPEG encoder 3U rack
unit, a 70MHz phase modulator 3U rack unit, a very linear 70MHz to 1308MHz 1W
transverter 4U rack unit. This then feed the PAs at reduced power (for linear

 ┌──────┐ ┌─────────┐   ┌──────────────────┐   ┌──────────┐ ┌──────────┐\│/ \│/
─┤VISION│ │ TIMEBASE│ ┌─┤ 1W 1308MHz FM TX ├───┤MODE RELAY├─┤2x 50W FET├─┘   │
─┤SWITCH├>┤CORRECTOR├>┤ └──────────────────┘   └─────────┬┘ │ PA SYSTEM├─────┘
 └┬─┬─┬─┘ └─────────┘ │┌──────┐q┌──────────┐┌───────────┐│  └──────────┘
 Video                └┤ MPEG ├─┤70MHz QPSK├┤70-1308MHz ├┘
 Sources               │ENCODE├─┤MODULATOR ││TRANSVERTER│

At this low data rate the signal is about 4MHz wide & much narrower than the
19MHz theoretical maximum of normal FM ATV. However most compatible digital Sat
Rx are 30MHz wide & so a strong signal is needed to overcome the Rx bandwidth
noise (8dB worse than 4MHz) if using one of these Rx. Compared to my well
tweeked down standard FM Rx I need a good P3.5 in FM to get an early type
digital Rx to see anything at all!

This repeater also as automatic changeover to digital Rx if valid data is
present, this is not easy to implement on GB3HV as the Rx aerial strategy
searches over 4 switched aerials on Rx. (to get 6dB Rx advantage over an omni).

 N   E   S   W
\│/ \│/ \│/ \│/┌────┐          ┌─────┐                  ┌────────┐
 │   │   │   └─┤FOUR│ ┌─────┐ ┌┤FM RX├───────────┬──────┤ VISION ├─┬─>Tx
 │   │   └─────┤MAST│ │SHACK├─┤└─────┘    ┌──────)──────┤SWITCHER│ └─>VCR
 │   └─────────┤PRE-├─┤AMP  │ │┌───────┐  │      │      └─┬──┬──┬┘
 └─────────────┤AMPS│ └─────┘ ││DIGITAL├──┘    ┌─┴─────┐
               └─┬──┘         └┤  RX   ├───>───┤P GRADE├─>SYNCS, P Grade &
                 │             └───────┘quality│ & ANT ├─>Ant, to computer
                 └──────<──────────────────────┤CHOICE ├─>

Only 4 stations have the capability for digital Tx, as it is very complex to do
it properly (linear PA etc).

Another problem with 23cm in the UK is the primary user's Air Traffic Radar,
this runs Giga Watts ERP. On FM this gives annoying white spots or streaks, but
in digital it causes corruption, from error blocks on the screen, to freezing
the picture & in extreme cases stops the data lock compleatly every sweep.
In my case with only a weak signal the 510MW ERP L/HR radar is every 4 seconds
& this is not enough time for the Rx to relock!

Sound time delay over the digital mode is also a annoying, as it is in the
order of a second or so! So having a normal talkback full duplex conversation
is not easy.

For further info & live video streaming (10 seconds delay) etc. have a look at or

(PS I am not on the internet, my hobby is Amateur Radio)

!\!             !  !
!/! C O L O U R ! /----/'\
  !             !! o= (   )
  !    A T V    ! \----\./  73 de JOHN
       //!!\\          Home Counties ATV.
     //  !!  \\
   //    !!    \\        G8MNY @ GB7CIP
 //      !!      \\

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