[opendtv] Re: DVB-T HDTV demo using 19.7 Mbps in a 6 Mhz channel in costa rica
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 7 Mar 2006 10:40:11 -0500
Doug McDonald wrote:
> The 3dB or 0.6 DB preamp is to amplify the signal
> without adding noise. TV sets and STBs have crappy
> 6 dB or 8 dB preamps. In the far field you need a
> better one. Cable length does not matter much,
> since you need the good preamp even if the cable
> length is five feet.
I think we are talking about different things.
When you say 0.6 dB or 3 dB, you are talking about a noise figure.
Instead, the previous mention of 3 dB was an attempt to compensate for
the 3 dB deficit in C/N margin of COFDM, compared with 8-VSB, in
gaussian channels. So it was 3dB of gain. At least, that's how I
understood it.
But the gain provided by an antenna amp amplifies everything arriving at
the antenna, signal and noise. In terms addressing the C/N threshold
problem specifically, the antenna amp's contribution is to make all
noise sources and losses downstream of the antenna as close to
insignificant as possible. But it can't do a thing to improve the SNR
received by the antenna itself.
This effect is really obvious in my own setup. In the past, for various
reasons, I had to split my antenna signal more ways to feed all the
separate boxes I used to have. The antenna amp improved the signal
significantly. As I cut down on the splits, e.g. no more VHF/UHF
splitters at each appliance, no separate hifi TV audio tuner, the
beneficial effect of the antenna amp has diminished.
Bert
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