[opendtv] Re: DVB-T HDTV demo using 19.7 Mbps in a 6 Mhz channel in costa rica
- From: Doug McDonald <mcdonald@xxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 06 Mar 2006 10:51:43 -0600
John Shutt wrote:
>
> What is the point of your quote? It reinforces what was actually observed
> in the field by over a hundred different observers over the course of a
> summer in 1999. You think you can tell the difference in 2 dB of
> sensitivity in your living room with your receiver? Atmospherics alone will
> make the signal vary by more than 2 dB.
>
You BET I CAN tell a 2 dB difference, and it is large and
obvious.
Actually, of course, the difference is still more like 3 dB
between COFDM and ATSC, but let's say 2 dB because it's
still a big difference in reception.
I have a station I watch frequently that is some 68 miles
away. I get if off an indoor antenna, a 16 element Yagi
sticking out a window. This is attached to a preamp
with a 0.6 dB [sic] NF, which is is tuned for the
channel of interest, though the 3 dB NF bandwidth is +- 4
channels.
The signal is quite reliable with this setup. I get dropouts
for periods of a hour or so in the evening every 10 days or
so in early winter. The signal on a spectrum analyzer is
near perfectly flat when reception is OK. Typically the
S/N on the receiver is in the 16.5-18 dB range. The failures
start when it starts dropping below 16 dB into the middle
15s. The S/N shown on the receiver meter corresponds to the
power level shown on the spectrum analyzer as long as the
spectrum is flat. In some reception conditions, in very cold
winter conditions just after sunset, I start seeing a
non-flat spectrum that varies on a 1 second time scale.
These are very broad frequency drops, not sharp dips; as
they vary in frequency you can see them move around inside
this channel (which is 44) and also the same drops appear
just as expected by extrapolation in a channel 42 station
which is the same power, height, and on a tower about one
mile away from 44. If the average level drops below 16 dB,
I start seeing dropouts.
Now, I have frequently tried inserting a 2 or 3 dB
attenuator right at the input of the preamp. This has a
DRAMATIC influence on reliability in any season except
Aug-Oct. Instead of normally having completely reliable
reception, it becomes quite iffy. Some days it is fine, but
in early winter it is almost always zip, none, nothing all
evening. In late fall or spring/early summer it becomes
perhaps 75% reliable, which is near useless, compared to the
normal >99.9% reliability.
So I can say that IN THE ACTUAL REAL WORLD, 2 or 3 dB makes
a HUGE difference in actual real world useability of a station.
Now there is also another thing: that ch. 42 is, except in
late summer/early fall, virtually NEVER receivable, even
though the spectrum analyzer shows it to be exactly the same
signal strength as 44 (and the NF of the receiver is still
probably 1 or 1.3 dB at Ch. 42). This is because there is a
channel 41 digital signal that is typically 40 dB stronger.
When the propagation of 42 is good enough that the signal
levels are less than 35 dB different, I get 42 OK.
So in fact 2 dB DOES certainly make a REAL difference, at
least in the far field.
Doug McDonald
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