Re: [IRCA] NAB wants it both ways

[Barry McLarnon <bdm@xxxxxxxxxx>]

On Friday 12 October 2007 21:40, Craig Healy wrote:
> Right now digital power is 1% of analog.  Coverage increases as the square
> root of the power increase.  Double the coverage, four times the power.
> With the coverage radius at half the analog, the area covered is 1/4.  So,
> to equal the coverage of the analog, it would take 16 times the power - or
> 16% of analog.  That's really optimistic considering the interference
> potential of the digital signal.  So, 20% maybe for an all-digital signal
> to equal analog?  10kw from a 50kw station?  Does anyone know if that has
> ever been tried?  I did ask a local engineer if his station was capable of
> an all-digital transmission, and he didn't think so.  Someone said a
> Broadcast Electronics transmitter could, but a Harris rig could not.

The digital power is not 1% of analog, though everyone seems to think it is.
You're thinking of the FM system, which is a different breed of cat.  In the 
AM system, the digital power is either 5.8% or 6.8% of the analog power, 
depending on a power setting that is part of the NRSC-5 standard.  So, if 
your theory about coverage was correct, you would need roughly equal digital 
power to get the same coverage as analog.  However, you can't really compare 
the hybrid digital and all-digital systems in that way, since they are quite 
different in their characteristics (bandwidth, power spectrum, etc.).

All that said, it would still be interesting to try some all-digital tests at 
some power level, and see how it shakes out.  In theory, all HD receivers and 
exciters should support the all-digital mode, though reality may differ. :-)

Of course, if an all-digital mode is actually used, it should be DRM, which is 
greatly superior to the iBiquity all-digital system.

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON

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