[BC] Nighttime AM HD Status

[Barry McLarnon]

On Monday 15 October 2007 10:33, Dana Puopolo wrote:
> When I lived in Waltham, MA,  WBZ wouldn't stay locked on my Accurian HD
> radio either. This was at the edge of their measured 25 mV/m contour. I was
> unable to pick up any other AM stations' IBOC at all, even though their
> analog signals could be received perfectly.
>
> Here in LA, I can't receive ANY HD stations on AM whatsoever, even though
> their analog signals come in perfectly. I also can't receive about 1/4 of
> the FM HD signals of Los Angeles licensed FM stations, even though I live
> within the city limits, and I'm on the second floor using a dipole antenna
> aimed at the stations. All the analog FM stations come in perfectly in
> stereo.
>
> This technology is hopelessly flawed. How long can this charade continue?

I'm still waiting for the reports of the stunning coverage being achieved by 
AM-HD at night, thereby justifying the havoc being caused to other stations.
Here's something on the subject that I posted elsewhere:

<snip>

I'd like to see some evidence presented to back up these assertions about 
AM-HD coverage at night.  I haven't seen any broadcasters bragging about 
their wonderful digital nighttime coverage, and with good reason - it's 
almost certainly going to be embarrassingly small. This is an inevitable 
consequence of usurping your adjacent channels in order to run digital.

Let's try a few back of the envelope calculations. In order to get any digital 
audio, you've got to decode at least one of the primary digital sidebands 
(preferably both, if you want some robustness against fading), which have 
64QAM modulation. Now, what SNR do we need for decoding? According to the 
textbook I just grabbed off the shelf, 64QAM with coherent detection and a 
fairly powerful BCH error-correcting code will deliver a low (10E-6) bit 
error rate when the SNR is 19.4 dB. But let's be generous, and assume that 
system can work with an SNR of 16 dB.

Now we come to the tricky part - what's the "N" in the SNR? It's basically the 
RSS sum of the skywave signals on the 1st adjacent channels. Since our AM-HD 
station has little or no protection on these channels, these signal levels 
tend to be quite high - the received field strength (10% skywave) can easily 
reach 2-4 mV/m, but let's be conservative and say that it averages 1 mV/m. If 
our primary digital sidebands need a 16 dB margin over these signals in order 
to be decoded, then they need to have a field strength of at least 6 mV/m or 
so. But the power in each digital sideband is 16 dB below the analog carrier 
power, so the analog field strength at this point is thus about 36 mV/m.

So, my back-of-the-envelope estimate says that nighttime digital coverage 
won't extend much beyond the AM-HD station's 36 mV/m contour. Even if I was 
out by a factor of two, that's still pretty shabby, and hardly worth doing, 
considering the havoc that those digital sidebands are wreaking on other 
analog stations. A system that provides service only within a small radius, 
and creates interference over a radius that is many orders of magnitude 
larger, is a bad system, period.

This stuff has been on for nearly a month now, so where's all the measurements 
to show how it's performing at night?  I recall seeing someone on this board 
from the Chicago area posting that at night, he loses digital service from 
all of the local HD stations except one, WLS. He gave his approximate 
location, so I used the V-Soft zip code lookup service to guesstimate the 
field strengths of the local HD blowtorches at his location. WLS came in at 
48 mV/m, while the others ranged from 11 to 16 mV/m. Kinda fits with the 
above calculations, doesn't it?  It should be easy for other folks who have 
HD receivers and nearby AM-HD stations to make the same kind of observations, 
so let's hear about 'em!

</snip>

I got no response when I originally posted the above, and I don't expect much 
here, but so far, the limited anecdotal results I've seen seem to support my 
bleak predictions.

Barry

-- 
Barry McLarnon  VE3JF  Ottawa, ON