Cheers to the Maryland State Police

[bear105]

Out "en force" on this Memorial Day...

Waterloo Post all over I-95 between the beltways this morning...
Howard Co MSP1 TG45776 (Simulcast 39.42Mhz)
Also hearing MSP2 TG47344 alot this whole weekend...

 

[ka3jjz]

I wouldn't doubt that at all; I'd bet every barrack's got some extra staff on today. I'd also check the various 150 mhz freqs that have been reported to see if they're being used, too (155.73, 159.405, ect.)

73s Mike

 

[w9ofa]

The spectrogram on my radio shows them active at 39.41925.

Can anyone replicate this result? I wonder if they are using 250 Hz splits instead of the nice rounded numbers in the database.

 

[TinEar]

w9ofa, I don't doubt for a minute that they are not precisely at 39.4200. I think you may find that to be the case for a lot of freqs. Back many years ago when I had the capability to measure frequencies with some precision, I found bunches of them slightly off the advertised frequency. However, having said that, there is probably not one in a thousand of us that can either measure the frequency to the minus 75 hz observation, in 25 hz steps, and even fewer that can tune their scanner or radio to it. And if we could, I doubt we'd hear any difference in signal strength/quality or voice clarity.

Just from a nerdy point of view and to be anal retentive, I'd be interested to know which others you find that are not precise even though I can't do a thing about it after I know.

 

[w9ofa]

You are probably right TinEar. I'm sad that my radio only goes up to 60 Mhz, or else I might be able to solve some more riddles

You are right, it is highly unlikely that most consumer grade scanners would show any difference. Most of them probably don't have oscillators that are accurate to within 750 Hz, if they are built for higher VHF reception. As you point out, it is highly unlikely to make any difference for narrowband FM reception. Mainly, it is a nerdy curiousity.

I'm moving from my basement Columbia apartment to a fine townhouse in Piney Orchard in Odenton next week. When I set up a few antennas in the attic I'll do some more measurements, I expect to be able to hear the state police for a few counties around.

My friend is borrowing my ICOM PCR 1000, I can compare with that once I get it back

 

[ka3jjz]

One interesting point - there really isn't any narrowband being used by MSP right now. There's only one community in all of Maryland that is using true narrowband (Kent County fire), and DNR/NRP is beginning to start implementing it (151.2425 has been testing for a long time now). But it's also true that any consumer grade scanner could not tell such a small error; but when true narrowband comes around, that could all change.

73s Mike

 

[w9ofa]

What do you mean when you say "true narrowband"? The spectrum appears to be relatively narrow already, around 4 kHz for the main spectral mass and the rest of the products are spread out over 10 kHz or so. Are they going to use a narrower filter? Are they going to eliminate the preemphasis? I'll have to grab my communications theory text to calculate the Beta value of the current signal.

 

[Admin0140434]

...my radio only goes up to 60 Mhz...

damn, how old is your radio?

...my friend is borrowing my ICOM PCR 1000...

you let someone BORROW your radio? what kind of scanner nut are you?

 

[ka3jjz]

What do you mean when you say "true narrowband"? The spectrum appears to be relatively narrow already, around 4 kHz for the main spectral mass and the rest of the products are spread out over 10 kHz or so. Are they going to use a narrower filter? Are they going to eliminate the preemphasis? I'll have to grab my communications theory text to calculate the Beta value of the current signal.

There's an excellant article about this very subject in the June 2005 MT. A little basic, but rather informative. It's written by David Stark NF2G, who runs the popular NF2G NYS page.

Anyway, there's another factor to consider when thinking about narrowband. Since we have one narrowband system here in Maryland, let's talk a little about that. Standard spacing for a VHF system is 15khz, with deviation to match. When you go narrowband (as Kent county fire has), that spacing gets chopped to 7.5 khz, with deviation reductions to match. As far as I have heard (I live too far away on indoor antennas to actually hear it), this is what Kent has done.

So listening to a true narrowband signal on a standard scanner would be difficult; it's likely to sound very low in audio and muffled. And many older scanners simply can't tune in 7.5 khz steps, so that's going to be a problem as well. Tuning off freq might cause even more noise and distortion. As far as I know, there are a few newer scanners that can tune that fine, but do they compensate for the lower deviation? That I don't know.

Basically, I think (from the article) that what the consumer thinks of narrow FM and what is thought of as narrow FM by techs, the FCC, ect. are not really the same thing. Channel spacing is only 1 piece of the puzzle; just because it looks like a narrowband allocation, it might not actually be.

73s Mike

 

[w9ofa]

I don't subscribe to MT, so I can't follow that. They only have the May version at Borders

I agree with your sentiment, though. The scanner of the future ought to just output a digital IF stream, so you can do the final filtering and demodulation in the computer.

Sadly, no major manufacturer has signed up to this idea yet. The time will come, I'm convinced. The first "black box" scanner with an ethernet jack will be an instant hit.

 

[ka3jjz]

They've got the June edition at the Books A Million store at Arundel Mills Mall; I got my copy there.

73s and stay cool...Mike