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2 x A/Spectras as a P25 repeater - Help needed

This forum is dedicated to discussions pertaining specifically to the Motorola ASTRO line of radios (those that use VSELP/IMBE/AMBE), including using digital modulation, digital programming, FlashPort upgrades, etc. If you have general questions please use the General or Programming forums.

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Johnno
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What radios do you own?: XTL2.5K,XTS2.5k, P25 stuff....

2 x A/Spectras as a P25 repeater - Help needed

Postby Johnno » Thu Jan 16, 2014 4:43 pm

Hi,

I'm trying to build a P25 repeater out of 2 Astro Spectras.

Has anyone tried this?

I would be grateful for any assistance.

My original idea was to establish a link via the 15 pin accy conn.

The end result I am after is to be able to pass DES-XL.

Is there a way of simply making a patch cable?

Thanks in advance,

Johnno

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Doug
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Re: 2 x A/Spectras as a P25 repeater - Help needed

Postby Doug » Thu Jan 16, 2014 5:13 pm

It's going to a bit more difficult than just a simple patch cable. The biggest milestone I see is getting a low COR to key your transmitter. IIRC Your going to have to do some mods to the radio to even get a cor signal out. This held true with normal spectras maybe astro spectra are different. That cor voltage is going to go high so you'll have to do some creative stuff with transistors to get a low cor to key the ptt. Just so your aware your going to be double vocoding the audio and you may not be happy with your results.
As an experiment I'd say go for it. P25 is pretty cool to mess with
Doug
May the Schwarz be with you.

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Johnno
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What radios do you own?: XTL2.5K,XTS2.5k, P25 stuff....

Re: 2 x A/Spectras as a P25 repeater - Help needed

Postby Johnno » Fri Jan 17, 2014 4:11 pm

Ouch! I kinda thought it wouldn't be easy.
Everyone would be doing it!

Thanks.

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Astro Spectra
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Re: 2 x A/Spectras as a P25 repeater - Help needed

Postby Astro Spectra » Sat Jan 18, 2014 1:13 am


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Johnno
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Posts: 86
Joined: Fri Oct 18, 2002 7:09 pm
What radios do you own?: XTL2.5K,XTS2.5k, P25 stuff....

Re: 2 x A/Spectras as a P25 repeater - Help needed

Postby Johnno » Sat Jan 18, 2014 5:19 pm

Great info!

Thanks Astro Spectra. I appreciate the link updating too.

Did your setup pass DES-XL?

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Astro Spectra
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Re: 2 x A/Spectras as a P25 repeater - Help needed

Postby Astro Spectra » Sat Jan 18, 2014 7:42 pm

Given that set-up was interconnected analog to analog, then no it can't pass through raw digital data like RIDs, conventional talk group info, etc. However, when both radios are keyloaded with the same key then yes encrypted speech will pass thru but of course it will be decrypted to analog at the receiver then re-encrypted again in the transmitter. That is a security risk if you don't have physical control of the repeater site.

This is the downside of using back to back P25 radios, they have no facility to output digital. You'll see various references to double vocoding because there are two passes through the IMBE vocoder, once in the receiver and again in the transmitter. Professional P25 repeaters demodulate the C4FM waveform, do error correction, re-timing, etc, and then re-transmit the digital stream. The do not do IMBE voice coding instead they preserve the original data.

If you want a bit transparent link then using Astro Spectra is not the right platform, you would be better off using back to back anlalog Maxtrac radios modified to pass the P25 C4FM like this:

viewtopic.php?f=4&t=45645

or this using Tait radios:

https://www.p25.ca/threads/1297-Tait-TM ... ossbanding

These solutions rely on receiving a good clean signal as the repeater has no ability to do error correction or re-timing.


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