ASTRO: CAI vs. IMBE, what's the difference??

[themedic66]

Ok, that was a beautiful explanation..the only thing is that I didn't understand half of the terms used....In any case, THere is an AS that's 512K and that has CAI, but the owner can't produce the flashcode. I want to buy this radio to monitor LAPD frequencies. Since from my knowledge their freqs are IMBE, I need IMBE to RX them right?? I also heard something about their freqs having "NACS"? Also the owner said that b/c of the vocoder only being 512K, that the more chans I program, the less memory I have to use for other functions. I'm looking to program a total of 160 chans in the radio, will I still be able to scan, page, etc...THANKS AGAIN!!

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Ok is there a difference from "APCO 25 CAI" and "APCO 25 IMBE?" If so, what are the differences? I always thought that if it has APCO 25 or CAI that it is IMBE as well.

Thanks in advance!

[Wowbagger]

CAI stands for "Common Air Interface" - it is the set of specifications for APCO-25, defining everything from the way you turn bits into RF (modulation schemes), to how those bits are organized (protocol definition), to how, once you've pulled the appropriate bits out of the protocol stream, how you turn them into audio (vocoder specifications).

The vocoder specifications for the CAI call out the Digital Voice Systems, Incorporated (DVSI) Improved Multi-Band Excitation (IMBE) vocoder for APCO-25 phase I. For APCO-25 Phase II half rate, the CAI calls out the DVSI Advanced Multi-Band Excitation (AMBE) vocoder. The implementation of the AMBE vocoder is such that the vocoder can also encode and decode IMBE.

So you are correct in that anything which implements the APCO-25 CAI will implement at least the IMBE vocoder, and if it implements the Phase II half-rate stuff will implement the AMBE vocoder.

Now, the AMBE vocoder is also used in other digital voice protocols, such as D-Star and the AOR HF digital voice protocol - however, that does NOT mean that an APCO-25 radio can decode those protocols nor does it mean that a D-Star radio can do APCO-25: while the vocoders are the same the protocols and modulation schemes are not.

[fail999]

Wowbagger,

That was nicely done......

fail999

[themedic66]

Ok, that was a beautiful explanation..the only thing is that I didn't understand half of the terms used....In any case, THere is an AS that's 512K and that has CAI, but the owner can't produce the flashcode. I want to buy this radio to monitor LAPD frequencies. Since from my knowledge their freqs are IMBE, I need IMBE to RX them right?? I also heard something about their freqs having "NACS"? Also the owner said that b/c of the vocoder only being 512K, that the more chans I program, the less memory I have to use for other functions. I'm looking to program a total of 160 chans in the radio, will I still be able to scan, page, etc...THANKS AGAIN!!

[Batwings21]

You need to know host and dsp, flashcode is easily manipulated. If he can't give that info to you, he's a rat. Its available during the entry into test mode( pushing one of the side buttons five times after turning it on). on a astro saber, if it does astro cai, that is the same as IMBE. But flash alone will not tell you if it will do IMBE. And size of vocoder does not affect how much stuff you can program in with cps/rss it only affects what host/dsp it can hold, as the newer host/dsp combos need 1 meg vocons to hold them.

[Wowbagger]

OK, once again:

IMBE is not a modulation scheme. It is a vocoder. Saying that a given radio channel is "IMBE" is like saying "this song is purple". Or, as I have said in the past, "Saying IMBE or P25 is like saying 'bacon' or 'purple'".

CAI means "C4FM modulation, APCO-25 protocol, IMBE vocoder".

So yes, if the radio does CAI, it will decode P25 - that's what CAI means.

The LAPD is using APCO-25 CAI.

You have said the radio does CAI.

Assuming the radio will tune in the frequency, it will receive their signal. Assuming they aren't using encryption, you should be able to receive them.

HOWEVER:

1) If you have the radio programmed to actually monitor the control channel, so that it will be able to automatically follow the conversations as they are assigned to different channels, you will have a problem, in that your radio will attempt to register with the network. This is a REALLY BAD THING, as your radio won't be authorized to do so, and the system will tell your radio to FOAD, and your radio will die.

If you are lucky.

If you are unlucky, your radio WON'T die, and will continue to try to register with the system. That will be bad, as then the system administrator will be motivated to hunt you down via radio direction finding, and will bust you for "unauthorized access to a public safety system", which is a crime.

2) If you program your radio to act as a dumb scanner, you won't have the above problem. However, you will also have to program your radio to ignore any Network Access Code (NAC) being used, so that it will unmute on any conversation.

The best advice you will receive, both from me and, I suspect, from other members of the board, is "If you want to monitor, get a scanner." Scanners can be smart enough to monitor the control channel without trying to register with the system (as they CANNOT register, as they don't have a transmitter.)

[smokeybehr]

Wowbagger: The good thing is that LAPD is not a trunked system. They're still 100% conventional.

There's quite a few people out there that are using Astro Sabers to monitor LAPD, so we know that it works. Monitoring other systems in the LA area is a different story (ICIS, for example). And as we all know, there are flashcodes that are restricted by Motorola, but with a "whoreflash", you get it all...

TheMedic66: If the guy won't give you the basic radio information (serial number, flashcode, firmware rev, and DSP rev) I wouldn't walk away, I'd RUN away. Generally, the AS models with a 512K vocon are all either analog-only, or are VSELP vocoders, which are both completely incompatible with the APCO Project 25 standard.

[PhillyPhoto]

HOWEVER:

1) If you have the radio programmed to actually monitor the control channel, so that it will be able to automatically follow the conversations as they are assigned to different channels, you will have a problem, in that your radio will attempt to register with the network. This is a REALLY BAD THING, as your radio won't be authorized to do so, and the system will tell your radio to FOAD, and your radio will die.

If you are lucky.

If you are unlucky, your radio WON'T die, and will continue to try to register with the system. That will be bad, as then the system administrator will be motivated to hunt you down via radio direction finding, and will bust you for "unauthorized access to a public safety system", which is a crime.

2) If you program your radio to act as a dumb scanner, you won't have the above problem. However, you will also have to program your radio to ignore any Network Access Code (NAC) being used, so that it will unmute on any conversation.

The best advice you will receive, both from me and, I suspect, from other members of the board, is "If you want to monitor, get a scanner." Scanners can be smart enough to monitor the control channel without trying to register with the system (as they CANNOT register, as they don't have a transmitter.)

The LAPD is on a Digital CONVENTIONAL system. Most of their information can be found here, including the NAC. The NAC has nothing to do with control channels, and I believe you're thinking of connect tones.

I do agree that if you aren't authorized on any system, be it conventional or trunking, you should get a scanner instead, and not complain if your radio gets shut off or worse.

Also, if you still want to go ahead and monitor any digital conventional system, I'd suggest looking at XTS300s/XTS5000s. They're more up to date, and you'd have a better chance at getting one with CAI. Not to mention a lot less wacker-like, which is always a plus to me.

[DPL]

CAI means "C4FM modulation, APCO-25 protocol, IMBE vocoder".

I am curious: does CAI not include the use of CQPSK modulation?

[Wowbagger]

CAI means "C4FM modulation, APCO-25 protocol, IMBE vocoder".

I am curious: does CAI not include the use of CQPSK modulation?

OK, we have somebody who is confusing a vocoder with a modulation scheme, and you want me to mix in a completely DIFFERENT modulation scheme?

Technically, yes, the CAI does include CQPSK on 6.25 channel spacing, but that's not being used, as everybody has a hard-on of some form of TDMA system and a half-rate vocoder rather than going to a complex modulation scheme like CQPSK (and I use the term "complex" in the sqrt(-1) sense of the term).

[w2sjw]

Now, the AMBE vocoder is also used in other digital voice protocols, such as D-Star and the AOR HF digital voice protocol - however, that does NOT mean that an APCO-25 radio can decode those protocols nor does it mean that a D-Star radio can do APCO-25: while the vocoders are the same the protocols and modulation schemes are not.

Yep, it's all the the modulation scheme. While the CAI formats are using either C4FM or CQPSK, D-STAR uses GMSK. What I find quite unusual with my XTS5K is when I set a 2M channel in the radio as 'digital-only' on the TX & RX, the transmissions from my Icom IC-92 don't even register a flicker on the LED of 5K!