[opendtv] Re: Digital radios outstrip analogue

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
  • Date: Thu, 3 Mar 2005 13:12:04 -0500

Craig Birkmaier wrote:

> Not only this, but it is NOT correct to lump
> all video services together.

Which is why I didn't lump them together. I said
specifically high quality TV, not the portable
kind.

With an analog design, it's perfectly possible for
a TV station to use its 6 MHz channel for radio,
perhaps during the day. It can create 30 200-KHz
FM radio channels, or perhaps 15 to allow for
poor receiver selectivity, and deliver an
equivalent of 33+ KW output for each radio
program (for 1 MW TV transmitter). Then revert
back to TV at night.

Same transmitter, same transmitting antenna,
different exciters. The effect is more robust
radio channels during the day, and less robust
TV channel (AM-VSB) at night, for the same
transmitter. But oh wow, now my radios might
have to work over the UHF band too.

You rob Peter to pay Paul.

With a digital scheme, you can do something
similar *or* you can use hierarchical modulation or
DVB-H to transmit radio and TV simultaneously on
the same 6 MHz band. So what? You're still robbing
Peter to pay Paul. There's no free lunch. If
you transmit radio, you have to reduce the TV
channel. Either in bit rate or in robustness or
both.

And at the same time, your radio channels are also
compromised, compared to what they could be if you
did not have TV running simultaneously. Check the
ETSI EN 300 744 spec to see how degraded QPSK is
when it's run in HM rather than by itself.

Instead of detracting from the TV channel and from
the radio channels, the small aggregate bandwidth
used for radio can be collected in a small sliver
of spectrum.

Aside from providing more optimized services for
both applications, other advantages of doing this
are (1) radio receivers can be made cheaper,
because they can be optimized to operate over a
considerably smaller set of frequencies, and (2)
receiving antennas can also be optimized, because
they can be designed for higher gain in with a
simpler structure (because you'll only have a
narrow range of wavelengths of interest to the
radio receiver).

So once again, there are tradeoffs. And the
difference between analog and digital, once again,
is not so huge after all. Just because something
was not done in the past does *not* mean that it
couldn't be done. And just because something *can*
be done now does not mean it should be done.

> The only sharing of infrastructure between Radio
> and TV is when we put both TV and Radio antennas
> on a common tower.

Sharing of infrastructure for transmission was
discussed above. As far as reception, I use the
same receiving antenna for TV and FM, and the lead
serves as a decent MW and SW antenna as well. Same
infrastructure. So this is not by any means a new
concept, Craig, and all the same considerations
apply now as they always have.

Bert
 
 
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