Apco technical questions

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bradlington
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Apco technical questions

Post by bradlington »

May I ask a simple question on APCO repeaters.

1-Are commercial APCO repeaters made to pass the digital stream in a transparent manner , or does it digitally regenerate it before sending it out again.(I have seen the maxtrac version back to back for APCO 25 ham use. )

2-Where can I get hold of an online version of the CAI specification document to read up on how this digital stream is made up.

Thnaks

Brad
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bradlington
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Post by bradlington »

Firstly thanks for the standards link.

.(I have seen the maxtrac version back to back for APCO 25 ham use. )""

I take it that this type of passing of the callers digital stream (in the Ham back to back radios) is not going to do justice to quality and the BER %.I assume it would reduce latency?

Thanks for the tips.

Regards

Brad
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Post by MattSR »

To add to what r0f has said - here is the P25 trianing manual. Very informative and goes into quite a bit of depth.

http://www.p25.com/resources/P25TrainingGuide.pdf

Enjoy!
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Post by mikenet »

Since we're on the subject of standards...there's a couple of tidbits missing form that training manual that I've been wondering about(otherwise that training manual, especially "CHAPTER 4: ANATOMY OF THE
COMMON AIR INTERFACE", is great).

Do the standard receive filters support 6.25KHz spacing? That is, could I throw up some adjacent CQPSK transmitters and have current equipment be happy?

Does anyone know what the shaping filter used for C4FM is?

In the voice code word, how and why is the PN sequence generated? I never quite understood why the PN sequence was based on part of the vocoded stream as opposed to a fixed sequence.

I should probably just break down and buy the standard, but I'm lazy, and it's just my idle curiosity(I'm not actually building anything at this point). I had planned to try to demodulate and decode P25 data(but not recover any audio due to the proprietary vocoder), and possibly remodulate it(transmitters are always the easy part). But, due to some &$*%, my 3-month summer vacation didn't actually happen, so it doesn't look like I can even start on the project. (The project may have justified actually buying the CAI standard)
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Post by mr.syntrx »

The vocoder isn't a problem, you can buy a DSP preloaded with IMBE from DVSI for about $20 a pop. Drop it on a board with a serial codec and you're laughing.
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Post by MattSR »

Where from? Ive sent emails to DVSI and they never reply...
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Post by mr.syntrx »

DVSI themselves.

I emailed them from my work address though, which might have affected things.
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Post by Wowbagger »

mikenet wrote: Do the standard receive filters support 6.25KHz spacing?
No, most radios have the 12.5kHz IF, as if you don't, you distort the hell out of a C4FM waveform.
mikenet wrote: Does anyone know what the shaping filter used for C4FM is?
It's a raised cosine filter (NOT a root-raised as they *should* have used for minimum ISI). The RX filter is a one symbol time integrate-and-dump filter.
mikenet wrote:
In the voice code word, how and why is the PN sequence generated? I never quite understood why the PN sequence was based on part of the vocoded stream as opposed to a fixed sequence.
I don't know the PN sequence is generated (I've not had to do the voice frame recovery, I work more at the trunking protocol level), but I can tell you my guess as to the reason: it's to prevent a run of identical symbols in the data stream - that really screws up clock recovery and can cause issues with non-DC coupled systems. They are probably using VC0 as the seed rather than a fixed code to help increase the entropy of the signal, again to help clock recovery.
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Post by ka8ypy »

mr.syntrx wrote:The vocoder isn't a problem, you can buy a DSP preloaded with IMBE from DVSI for about $20 a pop. Drop it on a board with a serial codec and you're laughing.
Negative ghost rider...Any plain jane hobbiest cannot get their hands on the IMBE codec info. Per DVSI it is the firmware CODE for the codec you sign the license for.

The AMBE vocoder is a different animal, and can be had for $20 with a minimum order of 5 units.

When I get home I will copy and paste the e-mail I got as a ham looking to play with both. Like you said, the reply you got might have had something to do with your e-mail addy....
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Post by mr.syntrx »

Sorry, yes, you're correct. I originally only asked about AMBE, but I thought I asked about both. It was a fair while ago, so I had to check my emails to be sure.

They do have this board though... http://www.dvsinc.com/products/VC-55.htm ... though I imagine it's at least a few hundred bucks.
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Post by MattSR »

ka8ypy wrote:
mr.syntrx wrote:The vocoder isn't a problem, you can buy a DSP preloaded with IMBE from DVSI for about $20 a pop. Drop it on a board with a serial codec and you're laughing.
Negative ghost rider...Any plain jane hobbiest cannot get their hands on the IMBE codec info. Per DVSI it is the firmware CODE for the codec you sign the license for.

The AMBE vocoder is a different animal, and can be had for $20 with a minimum order of 5 units.

When I get home I will copy and paste the e-mail I got as a ham looking to play with both. Like you said, the reply you got might have had something to do with your e-mail addy....
Would the AMBE $20 vocoder be of any use? I know AMBE is backwards compatible with IMBE - does this mean you can just feed an AMBE vocoder with IMBE frames and it will all be sweet?

Im interested to read the email you got from then - I havent gotten a response from DVSI :S

Cheers,
Matt
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Post by escomm »

MattSR wrote:I know AMBE is backwards compatible with IMBE
Got a cite on this? I thought they were incompatible
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Post by 10-95 »

escomm wrote:
MattSR wrote:I know AMBE is backwards compatible with IMBE
Got a cite on this? I thought they were incompatible
That would be super cool if it is possible!
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Post by mr.syntrx »

There's nothing in the AMBE2020 manual to that effect.
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Post by MattSR »

From DVSI's website - http://www.dvsinc.com/products/VC-55.htm
provides a number of other vocoder advancements while remaining fully interoperable with the existing APCO Project 25 vocoder standard (TIA-102BABA)
I have read other references on the net but nothing from an offical source.

Given the P25 vocoder spec mandates IMBE, I believe the above paragraph confirms that the codecs are backwards compatible.
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Post by MattSR »

Also from the glossary inside the P25 training manual posted above:-
AMBE+2™
Abbreviation for “Advanced Multi Band
Excitation”. An enhanced Vocoder that
is backwards compatible with IMBE™.
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Post by Victor Xray »

Surely Wowbagger will weigh in on this.



Although it's probably not ever likely, it would be quite :ham: sexy to be able to monitor P25 public safety with a D-STAR radio (which uses AMBE).
There's nothing more permanent than a temporary solution.
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Post by Wowbagger »

escomm wrote:
MattSR wrote:I know AMBE is backwards compatible with IMBE
Got a cite on this? I thought they were incompatible
No, they are quite compatible in the sense that an AMBE codec can encode and decode IMBE data.

Now an IMBE codec cannot handle AMBE data.

How do I know? I (or rather my employer) *AM* an AMBE licensee, with full access to the code.
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Post by ka8ypy »

Wowbagger wrote:
escomm wrote:
MattSR wrote:I know AMBE is backwards compatible with IMBE
Got a cite on this? I thought they were incompatible
No, they are quite compatible in the sense that an AMBE codec can encode and decode IMBE data.

Now an IMBE codec cannot handle AMBE data.

How do I know? I (or rather my employer) *AM* an AMBE licensee, with full access to the code.
So would this mean that only one shipset would be needed if one wanted to make a P25/D-Star/Conventional radio? Instead of a chipset for each codec?
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Post by MattSR »

One chipset.

The smarts for decoding the different protocols (P25, DSTAR etc) are all in the host microcontroller.

Matt
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