Hetrodynes... does it happen with...

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KC9UZB
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Hetrodynes... does it happen with...

Postby KC9UZB » Tue Sep 04, 2007 1:43 pm

IMBE?

Say for instance I'm talking to my buddy on FM, and some idiot tries to "key me out", all my mate will hear is a hetrodyne.

What happens if:
someone keys over me talking on IMBE, with FM? (assuming my mate is listening non-mixed mode)
someone keys over me with another IMBE transmission?

Any thoughts? Since there's only two of us in the UK using IMBE in the ham world its difficult to test.

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Re: Hetrodynes... does it happen with...

Postby Wowbagger » Tue Sep 04, 2007 2:56 pm

First of all: VOCODERS ARE NOT PROTOCOLS! You are using some protocol which uses IMBE as a protocol (e.g. you are using APCO-25 CAI). Saying you are using "IMBE" is like saying you are using "big".

If anything - analog or digital - interferes with a digital protocol, then the result will be that the error rate will increase.

If the error rate does not exceed the forward error correction coding's ability to correct it, you hear no change in your signal. That's what is great about using digital protocols with FEC.

If the error rate does exceed the forward error correction coding's ability to correct it, you will either hear garbage or silence - depending upon the protocol's ability to detect the fact that it is having uncorrectable errors and how the protocol handles such cases.

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Postby motorola_otaku » Tue Sep 04, 2007 3:59 pm

Okay, now for the real-world answer: I have personally seen an Astro Spectra attempt to decode two simultaneous P25 conventional transmissions with identical NACs. The end result was.. bizarre. You could tell two people were talking, and pick out parts of both transmissions, but the overall effect was like 5 or 10 people trying to transmit in AM at once with lots of digital artifacts thrown in.

Now if someone with a different NAC or in another mode tries to double with a P25 transmission, yeah, you get packet loss (choppy audio) or complete loss of audio.
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Postby KC9UZB » Tue Sep 04, 2007 4:58 pm

motorola_otaku wrote:Okay, now for the real-world answer: I have personally seen an Astro Spectra attempt to decode two simultaneous P25 conventional transmissions with identical NACs. The end result was.. bizarre. You could tell two people were talking, and pick out parts of both transmissions, but the overall effect was like 5 or 10 people trying to transmit in AM at once with lots of digital artifacts thrown in.

Now if someone with a different NAC or in another mode tries to double with a P25 transmission, yeah, you get packet loss (choppy audio) or complete loss of audio.


Thanks Josh and "Wowbagger".

In order to not embarrass myself further, what IS the correct term to use? I generally use "IMBE" or "P25" to describe wtf it is I'm speaking in and most people understand me, but obviously I screwed up in the OP :baby:

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Postby Wowbagger » Tue Sep 04, 2007 5:50 pm

M3ECM wrote:In order to not embarrass myself further, what IS the correct term to use?


P25 is fine. APCO-25 is more complete, but really it's just being verbose.

If you are referring to D-Star, say D-Star.

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Postby KC9UZB » Wed Sep 05, 2007 4:12 am

Since I try to be a snob, I'd never refer to D-star except to pour scorn on it ;) *tongue in cheek*.

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Postby KB9SXK » Wed Sep 05, 2007 8:42 am

D Star?
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Postby KC9UZB » Thu Sep 06, 2007 12:28 pm

Back on topic.

Does transmit deviation matter for P25? I just thought, if I'm using 2.5kHz deviation and my mate is using 5kHz, does it matter? (apart from bandwidth use??)

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Postby Josh » Thu Sep 06, 2007 1:51 pm

Yes, of course it matters.

You'll find that range will suffer if you try wide-band receiving a P25 channel meant for 12.5Khz use.

I had issues like that with the "local" P25 ham repeater 30 miles away, but mostly with the really local analog ham repeater located about 1/2 mile from here. desense-city on an astro spectra. fixing the bandwidth to match the system cut it all out.

-Josh

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Postby KC9UZB » Thu Sep 06, 2007 2:29 pm

I thought it did but wanted to be sure.

Thanks, I'll get him to set all his to match mine. Ta.

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Postby Wowbagger » Fri Sep 07, 2007 7:25 am

M3ECM wrote:Back on topic.

Does transmit deviation matter for P25? I just thought, if I'm using 2.5kHz deviation and my mate is using 5kHz, does it matter? (apart from bandwidth use??)


P25 is not like analog FM - there is a specified deviation for the symbols, and if you aren't at that deviation you aren't making a correct P25 waveform. The symbols SHALL BE at +/-600 Hz or +/-1800 Hz from carrier. There is no wiggle room.

Due to the shaping filters, there is no easy way to use a conventional deviation meter to measure if you are hitting those numbers at symbol time - you have to use an instrument that can determine when symbol time is and measure deviation at that time. Think of it like a timing light on a car engine - you cannot just eyeball the timing mark, you have to see where the mark is just as #1 fires.

If you don't have the equipment to measure deviation at symbol time, you have to put the radio into a test mode where it is only sending the _/-1800 Hz symbols and make your measurement.

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Postby mr.syntrx » Fri Sep 07, 2007 7:30 am

FWIW, CPS will let you set the channel width, even if a channel is P25 only, but in practice the radio will ignore that setting when dealing with P25 signals.

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Postby KC9UZB » Fri Sep 07, 2007 10:16 am

Ok I guess I mean channel width. I'll just leave it as is then for now, it works. :p

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Postby n3jfw » Fri Sep 07, 2007 4:17 pm

oh it also happens when a cetain user logs into batboard
N3JFW 4:29 pm
(4:29:48 PM): so he's gay huh
(nameless ham) 4:30 pm
(4:30:02 PM): haha no, but he is a music major
N3JFW 4:30 pm
(4:30:15 PM): so he's still in the closet

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What radios do you own?: Motorla MX 1000s, Motorola GP300s, Motorola CP040, Motorola MTS2000, Motorola XTS 2500 model 3, Motorola Astro Spectra, Philips Simoco FM1100 UHF / Low band, Simoco TETRA Handheld, also various other Amateur SSB & HF transceivers

Re: Hetrodynes... does it happen with...

Postby Mark_UK » Sun Mar 31, 2013 5:54 pm

You're not the only Amateurs in the UK using p25 ...
theres at least 3 of us 'round here, and we're growing :)

Mark

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Re: Hetrodynes... does it happen with...

Postby escomm » Tue Apr 02, 2013 4:13 am

After 6 years I'd certainly hope so :baby:
I'd hit it so hard you'd have to be to King of England to pull me out

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Re: Hetrodynes... does it happen with...

Postby Mark_UK » Fri Apr 12, 2013 6:46 am

Yup, there's nothing like a good necro-post :)

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Re: Hetrodynes... does it happen with...

Postby KC9UZB » Mon Jul 15, 2013 9:29 am

Damn Mark. Where abouts are you? I'm about 4000 miles to the west.
KC9UZB - ex M3ECM - expat dirty socialist Brit


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