Hetrodynes... does it happen with...
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- KC9UZB
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Hetrodynes... does it happen with...
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.
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.
Re: Hetrodynes... does it happen with...
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.
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.
- motorola_otaku
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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.
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.
And the sign says you got to have a membership card to get inside.
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Thanks Josh and "Wowbagger".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.
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
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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
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
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.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??)
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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Re: Hetrodynes... does it happen with...
You're not the only Amateurs in the UK using p25 ...
theres at least 3 of us 'round here, and we're growing :)
Mark
theres at least 3 of us 'round here, and we're growing :)
Mark
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Re: Hetrodynes... does it happen with...
After 6 years I'd certainly hope so ![THE BABY! :baby:](./images/smilies/smileybaby.gif)
![THE BABY! :baby:](./images/smilies/smileybaby.gif)
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...
Yup, there's nothing like a good necro-post :)
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Re: Hetrodynes... does it happen with...
Damn Mark. Where abouts are you? I'm about 4000 miles to the west.
KC9UZB - ex M3ECM - expat dirty socialist Brit