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G8MNY > DIGATV 18.09.21 08:42z 99 Lines 4361 Bytes #999 (0) @ WW
BID : 47017_GB7CIP
Read: GUEST
Subj: Re: Digital ATV experience
Path: HB9ON<IW2OHX<IR1UAW<I0OJJ<GB7CIP
Sent: 210918/0838Z @:GB7CIP.#32.GBR.EURO #:47017 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : DIGATV@WW
(updated Sep 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Hi Readers,
Tony VK3FBD asked a long time back if anyone is doing it. Well yes, I first saw
a digital ATV demo at a BATC rally about 25 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.
GB3HV REPEATER DIGITAL Tx
Since then several hams in the old GB3HV 23cms ATV repeater's service area were
into digital & have built up digital systems. One on them was incorporated into
the repeater about 20 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
working)....
ÚÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄ¿\³/ \³/
Ä´VISION³ ³ TIMEBASE³ ÚÄ´ 1W 1308MHz FM TX ÃÄÄÄ´MODE RELAYÃÄ´2x 50W FETÃÄÙ ³
Ä´SWITCHÃ>´CORRECTORÃ>´ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÂÙ ³ PA SYSTEMÃÄÄÄÄÄÙ
ÀÂÄÂÄÂÄÙ ÀÄÄÄÄÄÄÄÄÄÙ ³ÚÄÄÄÄÄÄ¿qÚÄÄÄÄÄÄÄÄÄÄ¿ÚÄÄÄÄÄÄÄÄÄÄÄ¿³ ÀÄÄÄÄÄÄÄÄÄÄÙ
Video À´ MPEG ÃÄ´70MHz QPSKô70-1308MHz ÃÙ
Sources ³ENCODEÃÄ´MODULATOR ³³TRANSVERTER³
ÀÄÄÄÄÄÄÙpÀÄÄÄÄÄÄÄÄÄÄÙÀÄÄÄÄÄÄÄÄÄÄÄÙ
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!
REPEATER RX DIGITAL
This repeater also had automatic changeover to digital Rx if a valid data
signal Rx, this is not easy to implement on the old GB3HV as the Rx aerial
strategy searches over 4 switched aerials on Rx. (to get 6dB Rx advantage over
an omni system).
N E S W
\³/ \³/ \³/ \³/ÚÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿
³ ³ ³ ÀÄ´FOUR³ ÚÄÄÄÄÄ¿ Ú´FM RXÃÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ´ VISION ÃÄÂÄ>Tx
³ ³ ÀÄÄÄÄÄ´MAST³ ³SHACKÃÄ´ÀÄÄÄÄÄÙ ÚÄÄÄÄÄÄ)ÄÄÄÄÄÄ´SWITCHER³ ÀÄ>VCR
³ ÀÄÄÄÄÄÄÄÄÄ´PRE-ÃÄ´AMP ³ ³ÚÄÄÄÄÄÄÄ¿ ³ ³ ÀÄÂÄÄÂÄÄÂÙ
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄ´AMPS³ ÀÄÄÄÄÄÙ ³³DIGITALÃÄÄÙ ÚÄÁÄÄÄÄÄ¿
ÀÄÂÄÄÙ À´ RX ÃÄÄÄ>ÄÄÄ´P GRADEÃÄ>SYNCS, P Grade &
³ ÀÄÄÄÄÄÄÄÙquality³ & ANT ÃÄ>Ant, to computer
ÀÄÄÄÄÄÄ<ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´CHOICE ÃÄ>
ÀÄÄÄÄÄÄÄÙ
Only 4 stations have the capability for digital Tx, as it is very complex to do
it properly (linear PA etc).
QRM
Another problem with 23cm in the UK was the primary user Air Traffic Radar,
that used to run Giga Watts ERP. On FM this gives annoying white spots or
streaks, but in digital it causes servier corruption, from error blocks on the
screen, to freezing the picture & in extreme cases stops the data lock
compleatly every sweep. In my case fron Croydon 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
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.
WEBSITE
For further info & live video streaming (10 seconds delay) etc. have a look at
www.gb3hv.com or www.batc.org.uk
UPDATE
GB3HV Closed down @ High Wycombe 2008 due to increasing site costs, but was
reinvented a few years later (Rx only on 23cms) with differnet kit near
Reading.
_____________...,
!\! ! !
!/! C O L O U R ! /----/'\
! !! o= ( )
! A T V ! \----\./ 73 de JOHN
!_____________!,.!
//!!\\ Home Counties ATV.
// !! \\
// !! \\ G8MNY @ GB7CIP
// !! \\
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