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Duty cycle and transmitters



 
 
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  #21  
Old October 1st 17, 07:49 AM posted to uk.tech.digital-tv
Brian Gaff
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Posts: 6,807
Default Duty cycle and transmitters

I have used Klystrons for Radar, lethal things to me but I suppose they are
a good way to get high power. Surprised they did not use TWT devices, but
maybe the frequencies were a bit low for making those work.
Brian

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"Woody" wrote in message
news

"Woody" wrote in message
news
Top posted for Brian.

Right, where do we start.

Taking Emley Moor ITV as an example, in the olden days pre D S O we had
two transmitters of 25KW each running in parallel. Each transmitter
comprised two Klystrons one for video putting out 25KW and one for sound
putting out something lower although I cannot remember the exact level.
Although peak black could be higher the peak power was measured on the
sync pulse and the average video level was typically 10dB below peak.
With an antenna gain of 13dB and a feeder loss of 1dB this gave an e.r.p.
of 870KW peak or an average of about 87KW. The Klystrons ran the cathode
at -21KV which meant that the anode was very close to or at earth
potential making feed much simpler.

DVB being a COFDM has a pretty constant level when measured by a typical
power meter. The transmitters are IMSMC 10KW and run main/standby. Again
with 13dB antenna gain and 1dB feeder loss this gives an e.r.p. of about
174KW - which is a 3dB increase on the original average power.

Yes the transmitters are running constantly at that power but as they are
modern 'valves' which are being underrun and water cooled under very
tight processor control they are in fact much more reliable.

Lower powered sites such as Sheffield use Rhode and Schwartz or NEC solid
state units, one for each mux, duplicated on some sites but not all.

Relays use a single unit tray non-duplicated of 20W or 50W capability
which, should they fail, are a simple swapout which takes about 20 mins
as the replacement try can be programmed before leaving base.

There is no longer any need to go to site for periodic maintenance
checks. As every transmitter sits on the Arqiva engineering network an
engineer can be detailed to do a PM on a site from his office. All
parameters are visible including power up and down the aerial which shows
if the outside bits are healthy.

Oh what fun it is.


--
Woody

harrogate3 at ntlworld dot com


I should have said that a Klystron was a ceramic unit about 4ft tall and
maybe 4-5 inches diameter: the 'valves' now in use are an inverted cone in
shape about 18 or so inches deep and maybe 15 inches diameter at the
widest with contact rings that supply power etc etc. They run off -35KV
and on stations like Emley there are UPS units (a whole room full of 'em)
that will keep all four of the main transmitters (PSB1-3 and Com4) going
for up to 10 minutes whilst the backup gen starts.


--
Woody

harrogate3 at ntlworld dot com



  #22  
Old October 1st 17, 07:55 AM posted to uk.tech.digital-tv
Brian Gaff
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Default Duty cycle and transmitters

Yes I agree that is what it actually is, its just like the old dial up
modems only very much wider in bandwidth and hence has more subcarriers.


Brian

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"Bill Wright" wrote in message
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On 30/09/2017 21:35, Woody wrote:

DVB is a COFDM of 1705 carriers which to a power meter looks like a
block of continuous level.


Yes, well, it depends on the measurement bandwidth.

Bill



  #23  
Old October 1st 17, 08:09 AM posted to uk.tech.digital-tv
Ian Jackson[_7_]
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Posts: 66
Default Duty cycle and transmitters

In message , Bill Wright
writes
On 30/09/2017 20:00, NY wrote:

As other people have mentioned, 625-line TV was negatively modulated,
so the highest power is for the sync pulses, then for black, with the
white parts of the signal having least modulation. This was done to
reduce the visible effect of spark interference (eg from
badly-suppressed car ignitions), because black dots are less
noticeable than white ones.


Funny how we still had white dots


Black dots were indeed predicted as being an advantage of the
forthcoming 625-line TV, but they seem just as white as with 405. Could
it be that, with both positive and negative modulation, we have both
kinds of dots - but the whites are more noticeable?

If ignition interference is less noticeable on 625, it's far more likely
to be because the interference is less at UHF (less radiated, and the TV
aerials are far more directional).

450-line TV used the same type of VSB AM but with positive modulation
so spark interference showed as white speckle on the picture. I'm not
sure whether 405-line sound was AM or FM.


It was AM.

Negative vision modulation and FM sound do have several advantages over
positive mod and AM sound - particularly when processing the RF and IF
signals.


--
Ian
  #24  
Old October 1st 17, 10:37 AM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,759
Default Duty cycle and transmitters


"Mark Carver" wrote in message
...
On 30/09/2017 21:35, Woody wrote:

DVB is a COFDM of 1705 carriers which to a power meter looks like a
block of continuous level.


Was 1705 (aka 2k) That mode ceased at DSO. All UK DVB-T1 is now
6,817 (aka 8k)

UK DVB-T2 is at 32k I think ?



Well yes and no.

I too had an idea that it changed from 1705 (which I have a feeling
has some relationship to DAB now) but a number over 7000 comes to mind
as now used.

I'll call a former colleague tomorrow and confirm.


--
Woody

harrogate3 at ntlworld dot com


  #25  
Old October 1st 17, 11:03 AM posted to uk.tech.digital-tv
David Woolley[_2_]
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Posts: 526
Default Duty cycle and transmitters

On 01/10/17 08:55, Brian Gaff wrote:
Yes I agree that is what it actually is, its just like the old dial up
modems only very much wider in bandwidth and hence has more subcarriers.


It's like the current xDSL modems. The old dialup modems generally only
used one sub-carrier, or one per direction. (56k is funny, but is
downlink only.)
  #26  
Old October 1st 17, 11:10 AM posted to uk.tech.digital-tv
David Woolley[_2_]
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Posts: 526
Default Duty cycle and transmitters

On 30/09/17 20:00, NY wrote:
The sound was FM on a carrier spaced a few MHz from the video carrier (I
forget the spacing) - presumably on the sideband which has had the
higher frequencies filtered out.


The sound sub-carrier (whether or not it was treated as such by the
transmitter, it was treated as a sub-carrier by the receiver) was on the
the non-vestigial side, at, I think 6MHz from the main carrier. (The
receiver would extract the sound sub-carrier from the the video IF,
rather than directly from the RF stages.)

Incidentally, one of the advantages of negative video modulation is that
it allows an AGC reference on each line, whilst keeping blacker than
black sync pulses. (The band 1/2 system was not good for night scenes.)
  #27  
Old October 1st 17, 02:07 PM posted to uk.tech.digital-tv
Ian Jackson[_7_]
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Posts: 66
Default Duty cycle and transmitters

In message , David Woolley
writes
On 30/09/17 20:00, NY wrote:
The sound was FM on a carrier spaced a few MHz from the video carrier
(I forget the spacing) - presumably on the sideband which has had the
higher frequencies filtered out.


The sound sub-carrier (whether or not it was treated as such by the
transmitter, it was treated as a sub-carrier by the receiver) was on
the the non-vestigial side, at, I think 6MHz from the main carrier.
(The receiver would extract the sound sub-carrier from the the video
IF, rather than directly from the RF stages.)

Incidentally, one of the advantages of negative video modulation is
that it allows an AGC reference on each line, whilst keeping blacker
than black sync pulses. (The band 1/2 system was not good for night
scenes.)


In the earlier 405-line days, most sets (all B&W in those days, of
course) did use a sample of the black level as an AGC reference, had a
video black-level clamp, and DC coupled the video to the CRT. As a
result, the picture contrast, brightness and black level were displayed
correctly.

Later on, for simplicity and cheapness, almost all sets started using
mean-level AGC, and AC coupling between the video amplifier and the CRT.
As a result, the contrast, brightness and black levels wandered all over
the place, depending on the picture content.

When 625-line started, very few sets took any advantage of the constant
amplitude positive syncs (although I guess the AGC line was fortuitously
less dependent on picture content) - but the real killer was that they
still stuck with AC video coupling to the CRT. [I have seen circuits of
American B&W sets - and they also normally used AC coupling.] It was
only with the advent of colour that sets had to be designed to 'do
things properly' again.

I recall a Wireless World article (around 1968) about a simple add-on
black-level stabiliser for B&W sets (which I built). This sampled the
video back porch, and adjusted the video brightness to help keep black
black. There were no other alterations to the circuit (ie the set still
had mean level AGC and AC video coupling). Although it did not address
the contrast expansion on dark scenes, at least the black stayed much
more constant (both on 405 and 625).

I think the only B&W sets I've seen that had a proper black, brightness
and contrast were a small Russian set (about 5" - can't recall the
model) and a larger 10" or 12" set (called 'Gypsy', I believe - and
possibly intended for use in a caravan). Both had metal cases, and
worked off mains or 12VDC. They worked really well with a ZX81 - and
when eventually they packed up and I had to use a 'normal' set, I really
missed the constant black-level.
--
Ian
  #28  
Old October 1st 17, 07:19 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,909
Default Duty cycle and transmitters

On 01/10/2017 09:09, Ian Jackson wrote:

If ignition interference is less noticeable on 625, it's far more likely
to be because the interference is less at UHF (less radiated, and the TV
aerials are far more directional).


Yes. Aerial directivity was important. We had terrible problems with
ignition interference on a estate where the aerials pointed over a
scramble bike track a good mile away. The actual signals were weak as
well, so masthead amps were in use.

Speaking generally, attempts to reduce ignition interference were often
doomed to failure. The reason was that the pulses of RF, although being
of very short duration, were often much stronger than the actual signal.
The result was that you could improve the ratio between the two by, say,
20dB, only to find that the interference looked just as bad on the
screen, or almost so. The spikes of interference were still above the
peaks of the video signal.

405 sets sometimes used white spot limiters.

Bill
  #29  
Old October 1st 17, 07:30 PM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,909
Default Duty cycle and transmitters

On 01/10/2017 15:07, Ian Jackson wrote:

I think the only B&W sets I've seen that had a proper black, brightness
and contrast were a small Russian set (about 5" - can't recall the
model)


Rigonda and Vega

Bill

and a larger 10" or 12" set (called 'Gypsy', I believe - and
possibly intended for use in a caravan). Both had metal cases, and
worked off mains or 12VDC. They worked really well with a ZX81 - and
when eventually they packed up and I had to use a 'normal' set, I really
missed the constant black-level.


  #30  
Old October 1st 17, 09:24 PM posted to uk.tech.digital-tv
Indy Jess John
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Posts: 1,202
Default Duty cycle and transmitters

On 01/10/2017 20:19, Bill Wright wrote:

Yes. Aerial directivity was important. We had terrible problems with
ignition interference on a estate where the aerials pointed over a
scramble bike track a good mile away. The actual signals were weak as
well, so masthead amps were in use.


Early HT leads to spark plugs had a wire conductor, and these radiated
the spark. If you wanted to fit a radio in a car thus equipped, you had
to buy suppressors to fit between the wire end and the spark plug. This
requirement led to the development of carbon cored HT leads which
carried the voltage for the spark but didn't radiate it. It probably
weakened the spark a bit (when I changed the leads on my car I had to
worry about the size of the spark gap for the first time) but it did
mean that I could tune into weaker stations and hear them clearly.

At a guess, a scramble bike would have preferred the heftier spark from
a copper cored HT lead.

Jim
 




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