[IRCA] AM vs MW-IBOC range differences

["Dan Strassberg" <dan.strassberg@xxxxxxx>]

Scitt Fybush wrote:

We didn't have a field-strength meter with us (or at least, Kent didn't dig
it out of the back of the truck if we did), but WBBM's calculated to
deliver about 7 mV/m of signal on its analog carrier where we were
listening. The digital carriers would have been down at least 3 dB from
that.

-----

Unless I'm missing something, if the IBOC carriers are 1500W, their power is
1/33 that of the 50-kW main carrier. 1/33 is very close to 1/32 or 1/2^5 or
(1/2)^5. Since 1/2 power is -3 dB, (1/2)^5 is -15 dB. Now, even in the
midwest with its excellent ground conductivity, the range of a MW signal 15
dB greater than another signal coming from the same tower on almost the same
frequency is nothling like 32 times the range of the lower-power signal, but
it IS a reasonable amount greater. If the IBOC receiver has sensitivity
equal to that of the AM receiver, WBBM's IBOC range should be less than its
AM range, but nowhere near as much less as most people would guess from the
33:1 AM:IBOC power ratio. And if the AM were to go away and the IBOC
carriers were moved back next to the center of the channel (where the AM
sidebands used to be), the range would be about the same--also assuming that
the AM and IBOC radios were equally sensitive and the interference had
approximately equal effect on both. In fact, the much more expensive IBOC
radio may be more sensitive than the AM radio and it just may be
significantly less susceptible to interference. So, if IBOC ever reaches the
pure-digital stage, the range of MW stations may actually increase.

The problem, however, is the ill-conceived iBiquity system. Although I've
heard that the radios are supposed to work with no AM on the main carrier
and with the digital sidebands moved back into the area of the spectrum
previously occupied by the analog modulation, I'm not sure that this
statement is really true. And although the radios COULD be designed so that
their characteristics could be modified by software patches
transmitted over the air in place of the normal digital modulation, the
standard does not--as far as I know--allow for such modifications--or indeed
for ANY software modifications. By not allowing for software modifications,
the designers of the system have eliminated one of the most important and
powerful features of any digital radio system and are guaranteeing that the
expensive digital radios will not be able to keep up with the inevitable
advances in the technology. This flaw is every bit as serious as will be the
adjacent-channel interference in the early stages of deployment. It can be
argued that the adjacent-channel interference is an unavoidable consequence
of making the system work on the existing band with the existing
allocations, There is, however, no similar excuse for making the radios
incapable of downloading software modifications.

--
Dan Strassberg, dan.strassberg@xxxxxxx
eFax 707-215-6367




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