Digital TV for Residential Care Homes
 

"Tony" wrote in message ...
Bill Wright wrote:


Ah. Well, if you can obtain 20dBmV of DTT signal from the aerial I'd be
with you all the way. You'd need to be very close to the tx though!

I'm not familiar with these units and cannot find a converter, they don't
seem to be dBm(W) or dBuV.


20dBmV = 80dBuV

Digital TV for Residential Care Homes
 

"Tony" wrote in message ...
Bill Wright wrote:
I don't think we dissagree on that, I just thought Aerial gain was the way
to eliminate amplifiers, and still think that it can.
In some cases it is the right way to go. We look after a large number of
blocks of flats which all have LOS to Emley Moor, which is about 14 miles
away I'd guess. There are 6 flats in each block, and each flat a has a
seperate downlead to the amplifier location. When we took these over (in the
1980s) we found that they all had antique amplifiers which had 26dB gain and
a maximum output across four channels of only around 32dBmV. In front of the
amps were attenuators, usually a fixed 12dB and a variable 0-18dB. Of course
the attenuation almost equalled the amplification! We had a policy of
replacing the aerial and feeder where necessary, replacing the attenuators
and amp with a splitter, and then checking the results. These were always OK
except the wallplate or something was faulty. The improvement in picture
quality was striking. The figures we
+27dBmV at the ae
+23dBmV at the splitter input
+13dBmV at the splitter output
+7 to +10 at the outlet.
Luckily the living rooms mostly faced away from Emley, so pre-echo wasn't a
problem. Those systems are still there, unaltered, and we get very few
problems even with DTT.

There's no doubt that a clean signal off the aerial will beat an amplified
one, if the levels are equal. This is of course because the noise floor is
lower. But the level MUST be much better than adequate at the receivers. If
in doubt turn up the wick!

Bill

Digital TV for Residential Care Homes
 

In message , Carpy
writes

"Tony" wrote in message ...
Bill Wright wrote:


Ah. Well, if you can obtain 20dBmV of DTT signal from the aerial I'd be
with you all the way. You'd need to be very close to the tx though!

I'm not familiar with these units and cannot find a converter, they don't
seem to be dBm(W) or dBuV.


20dBmV = 80dBuV

And, as there's been a couple of dBm used in this thread, 0dBm = 48dBmV
(appx).
--
Ian

Digital TV for Residential Care Homes
 

Ian Jackson wrote:

And, as there's been a couple of dBm used in this thread, 0dBm = 48dBmV
(appx).

pedant
49 is closer. 0 dBm = 48.751 dBmV = 108.751 dBuV in a 75 Ω system.
/pedant

--
Andy

Digital TV for Residential Care Homes
 

In message , Andy Wade
writes
Ian Jackson wrote:

And, as there's been a couple of dBm used in this thread, 0dBm =
48dBmV (appx).

pedant
49 is closer. 0 dBm = 48.751 dBmV = 108.751 dBuV in a 75 0 /pedant

Ah, but I always allowed for 0.751dB loss in the test lead! ;))
--
Ian

Digital TV for Residential Care Homes
 

On Fri, 25 Jul 2008 20:42:13 +0100, Ian Jackson
wrote:

And, as there's been a couple of dBm used in this thread, 0dBm = 48dBmV

As one is power and the other is voltage, it is invalid to compare the two
unless you also specify the impedance.

Digital TV for Residential Care Homes
 

On Thu, 24 Jul 2008 15:36:18 +0100, Tony wrote:

I'm not sure what figures you're working in but -32dBm will easily drive
loads of TVs. A DTV reciever needs about 28dBuV (-80dBm ish) of mux
to get a good picture, more for a stable picture/weather etc

It is pointless quoting signals levels without considering what the noise
level is as well.

Why not? It does, I have designed several setups for test in labs and
factories for companies I have worked for. All passive distrbution its
all about the level you start with.

And what happens when you can't get a high enough level to start with
compared to what you require at the end?

But then I also don't
understand why ethernet networks are built like stars aswell, when local
hubs would significantly reduce the amount of cabling required and do
not reduce bandwidth.

You are clearly a fool if you think local hubs don't reduce bandwidth
available (per branch) on the root.

Digital TV for Residential Care Homes
 

In message , Paul Ratcliffe
writes
On Fri, 25 Jul 2008 20:42:13 +0100, Ian Jackson
wrote:

And, as there's been a couple of dBm used in this thread, 0dBm = 48dBmV

As one is power and the other is voltage, it is invalid to compare the two
unless you also specify the impedance.

TV? TV Aerials? Cable TV? Satellite? Now what impedance would YOU think
we were talking about?
[Clue - it's not 4, 8, 30, 50, 300, 450 or 600 ohms.]
--
Ian

Ethernet (was Digital TV for Residential Care Homes)
 

Tony wrote:

I would describe my system as a tree, but how you split it doesn't
matter. I would have thought placing the 2ndary splitters around the
building will allow less coax than a star. But then I also don't
understand why ethernet networks are built like stars aswell, when local
hubs would significantly reduce the amount of cabling required and do
not reduce bandwidth. Is there some inherant benefit in reliability by
using a star?


I'm assuming you're talking about twisted pair Ethernet, not the
obsolete coax.

It's not about reliability, it's about cost and convenience.

The first thing is that there are fundamentally two kinds of box that
act as the hub of the star. A plain hub takes the signal from each of
the connections, and sends it out on all the other connections. I think
they are getting rare now. The other type, the switch, sends the
messages from each connection to whichever single other port is
required. It might even be sent at a different speed, and the ways in
which the other port is decided varies. Some messages get sent to all
other ports, but not many. A good switch can send messages between
several different pairs of ports simultaneously.

Most networks are designed around a few server computers, which are one
end of most conversations, and a lot of clients which are the other end
of each conversation. You put the servers on high speed links near the
middle of your stars (maybe even several links, which they can use to
spread traffic) and the workstations out near the edges.

If you have a dozen workstations in one room, they can share a hub and a
single long wire back towards the servers. Much less copper.

It's a fundamentally different problem to RF distribution, and one I
wouldn't want to cover in too much detail for fear of running out of
knowledge and making myself look like an idiot!

Andy

Digital TV for Residential Care Homes
 

On Sat, 2 Aug 2008 12:04:56 +0100, Ian Jackson
wrote:

And, as there's been a couple of dBm used in this thread, 0dBm = 48dBmV

As one is power and the other is voltage, it is invalid to compare the two
unless you also specify the impedance.

TV? TV Aerials? Cable TV? Satellite? Now what impedance would YOU think
we were talking about?

33 and a third. Or maybe it's 45.

[Clue - it's not 4, 8, 30, 50, 300, 450 or 600 ohms.]

Take your patronising attitude somewhere else. Oh and you missed out 110.

Things get taken out of context all the time, so it is important to specify
it to prevent a cock-up later.