In message , Jerry Stuckle
writes
On 3/17/2014 3:15 PM, Rob wrote:
Jerry Stuckle wrote:
Most likely the company reduced the transmitted power by a factor
of 10 at the time of the switchover, to put the added link margin
in their own pockets.
(transmitting a megawatt of ERP as was regular in the analog days
puts a serious dent in your electricity bill, even when you have
a lot of antenna gain)


Not at all. If anything, they raised their power.

Here they went from 1 megawatt to about 50 kilowatt (ERP).
And then there are several programmes on one transponder, instead
of one analog programme. This gives significant savings in power.


OK, you mean absolute power. Yes, they can lower the ERP - but that
does not necessarily lower the power for the signal. Remember at 1MW
the power was spread over 4.25 Mhz (assuming video only, of course).
Digital requires much less bandwidth, so they don't need as much power
to get the same effective signal. However, digital still requires a
stronger signal than analog, in the bandwidth provided. You need quite
a bit of noise before it becomes visible in analog. Digital, a single
noise pulse can cause the loss of several bits of information. Because
of the compression involved, this is more than one or two pixels.

I think not much of that is correct.
The systems differ a bit between US and elsewhere, but over here the
channel spacing of digital and analog is the same, and the bandwidth
is similar (a bit more for digital if anything).

Also there is no discission of "spreading", we are just discussing
peak envelope ERP.
You could argue that a single digital stream sending 5 programmes
means that 1 programme is transmitted at 1/5 the power, but that is
not what I mean. The total ERP for 1 transmitter has been lowered,
and it transmits multiple programmes to boot.

Digital requires less power because it requires less signal-to-noise
ratio at the receiver.


There are major differences between Europe's PAL and the U.S.'s NTSC.

Where do you get these strange ideas? Yes, there are differences (the
major one being the subcarrier phase alternation of PAL), but otherwise
the systems are very similar - and none of the differences really affect
the basics of RF system measurements.

But the digital signal has much LESS bandwidth than the old analog one.

No it doesn't. It's the same. In the UK the 8MHz wide analogue channels
have been replaced with an 8MHz wide digital QAM signal, and this
occupies the same channel frequency. Unless I'm very much mistaken, the
US has done the same (except that the channels are 6MHz wide).

That was the major impetus over here to switch to digital - to free
up major bandspace in the VHF and UHF spectrums.

The 'freeing-up' is because each QAM multiplexed transport stream
carries typically up to 6 TV channels and 6 high-quality stereo radio
channels.

We now have as many (or, in some areas, more) stations in a much
smaller band than before.

Digital requires less power because the bandwidth is much lower.

Only insofar as the digital decoder can successfully extract accurate
data from the 6 or 8MHz wide multiplexed transport stream at
signal-to-noise ratios which, for analogue, would produce barely
watchable pictures.
--
Ian