DVB-T aerial cable
ian field
cable for DVB-T as opposed to regular 75 Ohm TV cable as used for analogue
UHF (channels 21 - 68). Its not satellite, and uses the usual cheap & nasty
UK type UHF connectors not the screw together types used elsewhere.
Albert Manfredi
Don't know about any connectors different from the standard F type,
either screw-in or push-on, that might be used in the UK. The general
answer is that the cable and connector for a digital TV antenna
(aerial) should be identical to what is used for analog TV. I use both
RG-59 and RG-6 cables. The RG-59 were popular back when, the RG-6 seem
to be about all you can find today. Both types are 75 ohm coax, and
both are terminated with F type connectors.
Bert
ian field
"Albert Manfredi" <bert...@hotmail.com> wrote in message
news:9bda96ca-1214-4737...@k10g2000prm.googlegroups.com...
Bert
Thanks.
It seems like an exercise in cutting through the hype, plenty of shops sell
expensive pre-packed ready made leads emblazoned with the word "Digital",
none of which are long enough and it seems wasteful to cut the moulded
connector off the end just to pass it through the hole in the floor. So far
I only found one shop that sells any length co-ax off the reel, unlike all
the expensive pre-packed cables, the label doesn't say "Digital" so I
thought I'd better check.
Albert Manfredi
> It seems like an exercise in cutting through the hype, plenty of shops sell
> expensive pre-packed ready made leads emblazoned with the word "Digital",
> none of which are long enough and it seems wasteful to cut the moulded
> connector off the end just to pass it through the hole in the floor. So far
> I only found one shop that sells any length co-ax off the reel, unlike all
> the expensive pre-packed cables, the label doesn't say "Digital" so I
> thought I'd better check.
Oh indeed, just as they did when color TV came out.
No, I literally disconnected my antenna lead from the analog receiver
and reconnected it to the digital receiver, did a channel scan, and
off we were to the wonderful new world of DTV. Did this for the TV and
for the PVR.
Bert
ian field
Bert
We had a broken antenna wedged between the beams in the loft, with the first
DVB-T box there was no reception so an installer had to be called - we made
it clear to the installer that the new rooftop antenna was to be used with a
DVB-T box, so it was all new supplied by the installer.
Don Stauffer in Minnesota
> "Albert Manfredi" <bert22...@hotmail.com> wrote in message
While I am sure that since the frequencies are the same, regular cable
would be okay. However, it does bring up the question in my mind- how
is the DTV signal modulated?
Albert Manfredi
wrote:
> While I am sure that since the frequencies are the same, regular cable
> would be okay. However, it does bring up the question in my mind- how
> is the DTV signal modulated?
It depends on the country. However the short answer is, all digital
transmissions over RF channels are ultimately transmissions of analog
waveforms. In fact, that shouldn't be limited to just RF. Same applies
to digital over fiber or twisted pair. At the bottom of it all, you
need to decode an analog waveform correctly, to be able to decide
whether a 0 or a 1 had been intended.
Ian is from the UK. They use a scheme called Coded Orthogonal
Frequency Division Multiplexing (COFDM). You take the (in their case)
8 MHz TV frequency channel, subdivide it into thousands of
subcarriers, and transmit the signal in parallel, over these
subcarriers. Each subcarrier is modulated with a QPSK, 16-QAM, or 64-
QAM signal. So, each subcarrier is modulated with a mix of AM and PM
(phase modulation). Which means, 2 possible amplitude levels and two
possible phase variations, or 4 amplitudes and 4 phases, or 8
amplitudes and 8 phase variations are represented by each symbol, on
each subcarrier. The more amplitude and phase levels each symbol is
expected to represent, the less robust the signal becomes. Because it
becomes easier for ambiguity to result, if the transmission becomes
distorted in any way.
That system is used by most DTV standards around the world. It is the
"physical layer" of the DVB-T standard used in Europe, Australia, New
Zealand, and elsewhere, and also the physical layer of the ISDB
standard used in Japan and planned for use in Brazil.
In the US, Canada, Korea, and a few other places, we use a scheme
called 8-level trellis-coded vestigial sideband (8T-VSB, usually
simply called 8-VSB). In this scheme, you start with an 8-level AM
signal, but you do not use the phase of the symbol to carry
information. So, it's just AM initially. Then you use the fact that
the spectrum of a pure AM signal is symmetrical. Instead of
transmitting this symmetrical spectrum, you chop off the lower half
entirely, leave a suppressed carrier as pilot signal, and transmit
only this suppressed carrier and the upper half of the spectrum. The
receiver then regenerates the missing half of the spectrum and
demodulates the symbols to retrieve the intended bit pattern. While
phase variations are not used to carry data, they can be used to help
adjust the receiver's equalizer. That's because you know ahead of time
that any phase variation is caused by signal distortion. So in
principle, if the receiver can equalize the incoming signal to remove
the phase variance, it has restored the intended shape of the symbols.
As you can see, in terms of information-carrying capacity, 8-VSB and
64-QAM are about the same. In 8-VSB, you use 8 amplitude levels only,
but you can save bandwidth by removing the redundant half of the
spectrum before transmission. In 64-QAM, you use 8 amplitude levels
and 8 phase variations, but the resulting asymmetric spectrum prevents
you discarding half of signal's spectrum before transmission.
Bert
Albert Manfredi
wrote:
> However, it does bring up the question in my mind- how
> is the DTV signal modulated?
Damn. It looks like the news group is filtering out answers to this
question, perhaps to avoid another "holy war." I had a fairly detailed
reply, but it seems to have been exorcised.
The short answer is, the scheme used depends on the country. One
scheme uses multiple subcarriers and a combination of amplitude and
phase modulation on each subcarrier (quadrature amplitude modulation).
That's the one used in Europe. The standard in the US is instead
single carrier, with eight-level amplitude modulated symbols and
vestigial sideband. Actually, it's more similar to single sideband,
with only a suppressed carrier used as pilot.
So whatever standard is used, ultimately the receiver has to be able
to demodulate what ends up being an analog waveform. In essence, this
is true of any digital interface, though, no matter whether it's RF
modulation or baseband.
Let's give this one the old college try.
Bert
ian field
Bert
It arrived here OK, thanks for the detailed description.
Paul Ratcliffe
wrote:
> Ian is from the UK. They use a scheme called Coded Orthogonal
> Frequency Division Multiplexing (COFDM). You take the (in their case)
> 8 MHz TV frequency channel, subdivide it into thousands of
> subcarriers, and transmit the signal in parallel, over these
> subcarriers. Each subcarrier is modulated with a QPSK, 16-QAM, or 64-
> QAM signal.
We don't use QPSK on our terrestrial system.
Albert Manfredi
wrote:
> We don't use QPSK on our terrestrial system.
True, but you could if you wanted to. Meaning, if a broadcaster
decides to use QPSK mode for any reason, all receivers out there
should accommodate that without a hitch. It's one of the modes
included in the DVB-T standard.
Bert
Sal M. Onella
"ian field" <dai...@ntlworld.com> wrote in message
news:J8aLj.1472$B83...@newsfe3-gui.ntli.net...
I also saw it.