themedic66 wrote:Hello all,
I'm getting just a little confused. What is, and what are the differences between IMBE, APCO, DIGITAL, ASTRO, and P25??? The flashcode on my AS3 is 100004-000000-4, so my radio is ASTRO DIGITAL(according to the flascode decoder), but i'm not sure if it is APCO, or IMBE. Any help would be greatly appreciated!!!
Some others here got it almost right, but not quite.
Trunked radio system: a system by which a few RF channels may serve a much larger number of users, by exploiting the fact that very few users will want to use the system at any given time. LTR, EDACS, MPT-1327, TETRA, APCO-25, iDEN, and even the cellular telephone systems (MTS, IMTS, AMPS, CDMA, GSM) are all examples of trunked radio systems.
Protocol: This describes the complete set of methods used to encode and transmit voice from one radio to another. This includes the modulation scheme, the coding standards, the format of the information on the control channel, the format of the information on the voice channel, any vocoders used, and many other aspects of the system.
ANALOG: a modulation scheme whereby the information in a signal is represented by a continuous signal variation. Plain old FM, as used in the ham bands, is analog, as is single sideband (HF) and AM (CB).
DIGITAL: a modulation scheme whereby the information in a signal is represented by a small set of discrete states or "symbols". APCO-25, DSTAR, and some forms of Smartnet are digital systems.
Control channel: a specific RF channel in a trunked radio system which carries no voice data, but rather carries information about what voice calls are going on at any given time, what radios would like to enter into voice calls, what voice channels are available to carry those calls, and many other pieces of information needed to make the system work. Control channels are almost ALWAYS digitally modulated.
Voice channel: a specific RF channel in a trunked radio system which is carrying voice, as opposed to a control channel. A voice channel may carry the voice as either an analog modulation scheme or a digital modulation scheme. A voice channel will usually ALSO be carrying some form of information besides the voice to manage the call, this information is usually encoded in some form of digital scheme (but not always: CTCSS (a.k.a. PL™) is an analog form of call management.)
Vocoder: a software algorithm used to reduce the number of bits needed to represent a voice signal. It is to voice what MP3 is to music. AMBE, IMBE, and VSELP are examples of vocoders. The algorithm may be implemented in the main firmware of the radio, or on a separate chip. Just because two protocols use the same vocoder DOES NOT MEAN THEY ARE COMPATIBLE - they can be using different modulation schemes, coding protocols, or half a dozen other things that can make them different.
Symbol: in a digital system, an instance of the discrete states the channel can be in - the minimum quanta of information that can be transmitted on that channel. A symbol can contain any number of bits of information: in a simple 2 level signaling system like RS-232 one symbol contains one bit of information (hence why most people wrongly assume BAUD==bits per second). In a system like C4FM, a symbol may take on 4 values, and thus encodes 2 bits of information. In a CDMA system, a symbol (or "chip") doesn't even contain one bit of information - it takes MANY chips to make up a bit of information.
Baud: the rate of transmission of symbols (NOT BITS) on a channel, in symbols per second. A 4800 baud system transmits 4800 symbols per second - and this tells you NOTHING about how many bits per second are being sent without knowing more about the modulation scheme being used.
C4FM: Compatible 4 level frequency modulation: a digital modulation scheme wherein the carrier is allowed to take on 4 different frequency deviations from nominal to represent 4 levels (2 bits) per symbol. This is one of the modulation schemes used in the APCO-25 protocol.
ASTRO: A marketing term used by Motorola to refer to their family of trunked radio solutions. In and of itself it is pretty meaningless, as Motorola uses this for several different systems.
APCO: the Association of Public safety Communications Officers - a professional body who (among other things) sets standards for the communications systems used by law enforcement, fire, and other emergency responders.
APCO Standard Project 25 (APCO-25, P25): a trunked radio system protocol adopted by APCO, and selected by the US federal government as the future standard for all emergency responder radio systems. This is a digital protocol, the first implementation of which uses C4FM at 4800 baud to transmit 9600 bits per second on both the control channel and the voice channel, with the voice channel using the IMBE vocoder to encode the voice data. APCO-25 is an "open" standard in that any company can implement the protocol - any proprietary items like the IMBE vocoder must be licensed to any company at a fixed rate.
SmartNet/SmartZone: A Motorola proprietary trunked radio protocol. Originally it used analog transmission for the voice, but later an encrypted version (SecureNet) was introduced that was a digital system using VSELP as the vocoder (and if I recall correctly, 7200 baud one bit per symbol). The control channel is a digital system at 3600 baud, one bit per symbol. SmartNet is being phased out in support of APCO-25.
VSELP: Vector sum encoded linear prediction: an older vocoder algorithm, used in Securenet. Its use is being phased out as it is no where as efficient as more recent vocoders.
IMBE: Improved Multi-Band excitation: A newer vocoder algorithm, used in APCO-25 phase I. Proprietary, owned by Digital Voice Systems Incorporated (DVSI).
AMBE: Advanced Multi-Band excitation: an improvement to the IMBE vocoder, AMBE can represent the same voice data in half as many bits. Also proprietary and owned by DVSI. Used in the newer version of the APCO-25 protocol. An AMBE vocoder can encode and decode using the IMBE algorithms as well as the AMBE algorithms, as such an AMBE vocoder IS compatible with IMBE.
So, to sum it up:
Vocoders != protocols != modulation schemes
Saying "APCO or IMBE" is like saying "Bacon or purple". APCO (APCO-25 being the implied meaning) is a protocol, IMBE is a vocoder.
Motorola uses some very confusing terminology for their radios - sometimes a radio is called "ASTRO DIGITAL" to mean that it is capable of using the APCO-25 protocol on the voice channel, but using the SmartNet protocol on the control channel. Sometimes it means the radio is truly APCO-25 trunking capable (using APCO-25 on both the control channel and the voice channel). Sometimes they use the term to refer to their own SecureNet protocol.
At the level I work with the stuff, I don't deal with the Motorola marketing-speak, so I don't know how to translate it into anything meaningful. However, I do know the APCO-25 protocol (I design APCO-25 equipment for a living), and I know a little bit off the top of my head about the SmartNet protocol (but most of the time I have to refer to the SmartNet design specifications I have at my desk at work, where I am NOT at the current time so I could be getting a few minor details about SmartNet wrong).