[opendtv] Re: Freeview business model
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 30 Jan 2007 12:23:10 -0800
Bert wrote:
> If there is a Freeview sort of service involved, there's no reason to
> assume you need multiple transmitters feeding a single antenna. In
> principle, markets could be assigned a contiguous set of frequencies,
> and a single broadband transmitter could be used to modulate X different
> 6 MHz channels.
In principle such a wideband system makes sense but as a practical matter,
it's impractical :~)- for several reasons:
1. Wideband transmitters have significant power output limitations and I'm
not aware of any technology* that can operate many channels wide with any
amount of power.
2. TV transmitters that multiplex signals must generally operate near the
Class A mode (very linear) to minimize intermodulation. That is also the
least efficient mode of operation.
3. Such a design creates a major "single point of failure" scenario where
any single problem impacts all channels and, of course, there will always be
problems.
Operating the system with several smaller (relatively) combined transmitters
will generate the required high RF power levels and with significantly more
efficiency (lower power bills). Those signals will be of higher quality and
there will be no single point of failure - other than the passive
combiner/antenna system, which are inherently very reliable.
* = Thompson introduced a Diacrode amplifier about 15 years ago that could
generate about 40 - 60KW peak power tuned 12 MHz wide. Accrodyne designed a
transmitter around that device which multiplexed an analog and digital
signal and I believe they installed it in Las Vegas. I did see the
transmitter operate in the factory and it did work, though I believe that
wide band tuning instability was an issue, which would make it undesirable
for long term field operations.
> -----Original Message-----
> From: opendtv-bounce@xxxxxxxxxxxxx
> [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Manfredi, Albert E
> Sent: Monday, January 29, 2007 3:46 PM
> To: opendtv@xxxxxxxxxxxxx
> Subject: [opendtv] Re: Freeview business model
>
>
> Tom Barry wrote:
>
> > Is there any power savings when modulating 2 similar channels
> > on a single antenna as opposed to two antennas?
>
> If there is a Freeview sort of service involved, there's no reason to
> assume you need multiple transmitters feeding a single antenna. In
> principle, markets could be assigned a contiguous set of frequencies,
> and a single broadband transmitter could be used to modulate X different
> 6 MHz channels.
>
> The total power would be the product of X times the effective power
> dedicated to each of the 6 MHz channels.
>
> If you had non-contiguous channels assignments, the single transmitter
> would have to be even broader band, so I would expect it to cost that
> much more. And then you'd want to transmitter to basically filter out
> any power it might emit on the unused frequency bands.
>
> Bert
>
>
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