[opendtv] Re: Freeview business model
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 31 Jan 2007 16:25:11 -0800
Bert wrote:
> However, the higher you go up the UHF band, the more practical this
> might be. Right?
Yes, the higher a transmitter's frequency the wider its possible bandwidth
(in MHz, not in percentage of deviation). A problem with wideband tuning, as
you have previously stated, is that power output capability is inversely
proportional to bandwidth.
As a practical matter, the higher a TV channels frequency, the more
transmission power is requires to achieve equivalent coverage. The FCC uses
a "dipole factor" when assigning UHF DTV channels. The channels RF power is
assigned to cause replication of a station's analog coverage - where
possible, and the higher channels are assigned more power to counteract the
increased signal attenuation.
IMO, such a FDM transmitter could not be constructed, regardless of cost,
that provides the needed output power* or the required intermod rejection
and bandpass filtering.
Some facts to consider:
DTV and analog transmitters have very critical FCC transmitter filtering
requirements to prevent interference with adjacent channels. Analog must
filter it's lower vestigial sideband plus it's color and aural images, as
well as any other deviations outside of it's assigned channel. DTV
transmitters filtering requirements are even more strenuous, mandating the
use of a very fine Mask (bandpass) filter at it's output.
The frequency division multiplexing of such signals would make such
filtering and IM control impossible.
See the following URL for some signal filtering and precorrection
requirements.
"TVTechnology - RF Technology Therefore, emissions at the output of a DTV
transmitter must be attenuated at least 75.9 dB ... and an external bandpass
filter to meet these requirements...."
www.tvtechnology.com/features/On-RF/f_rf_technology-02.04.04.shtml - 22k -
* - typical DTV station ERPs are from 50KW to 1000KW and the majority (those
that were analog VHF and full power UHFs)are at the higher end. 1000 KW DTV
stations using tall towers can require transmitter power outputs near 100KW
range, depending upon towers height and antenna gain/directionality. I would
estimate that the average full power DTV transmitter output is in the 20KW
to 50KW range.
A quick scan of the U.S. DTV assignment table shows 320 1000KW ERP
assignments with a huge number of stations at 500KW and up.
Dale
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