Getting Started

Motorola apx7000 parts

What is P25 or APCO 25 radio standard?

Project 25 (P25 or APCO-25) is a digital voice mode designed to meet the high demands of pure voice waveform replication for public safety and public works dispatch and push-to-talk radio systems.  This mode has long been used in Amateur Radio and there is growing interest to use P25 amateur repeaters as more used equpiment is surplussed by agencies.  

The P25 digital mode like other digital modes uses a vocoder with a specific codec to convert audio to data. The P25 voice-coding (codec)is the most complex of all digital modes and also why it's so expensive with the many hours devoted to it's development.  Think of it like music audio on a CD.  The CD has packets of audio data that is decoded into music.  P25 is (data) voice packets that are known as frames of data.  

It is the standard digital mode for police, fire, EMS, and public works because its vocoder codec provides the best audio correction and voice rendering.  This means better audio fidelity and the correction of voice packets on the fringe of coverage will be more reliable and readable.

The row of radios above are various models of the Motorola APX line.  

Can I still use analog on this system?

Absolutely!  Our system is in mixed mode meaning both digital P25 and analog can be used.  This way the analog users are not left out.  The analog portion of the system is linked to the Southern California Repeater Network or SCRN. A southern state Los Angels Basin, San Diego, San Bernardino and Las Vegas wide area analog repeater network. At this time, SCRN does not have digital P25 capability, only 21 analog linked repeaters. 

What Radios Can I Use?

There are several models of radios you can use.  While there is a rumor that P25 equipment costs a lot, this is far from the truth.  Yes there are radio models that costs upward of several thousand dollars but, it is not necessary to use these radios if your budget is limited.  There is plenty of used public safety radios that were used in the early to mid 1990s and 2000's.  These include the Motorola XTS and XTL series, astro saber or spectra series, and other brands such as Bendix King, Kenwood and ICOM. ICOM makes a P25 radio that costs about the same price as a top tier legacy portable  ham radio.  We recommend purchasing a radio that has FPP or front panel programming enabled as this will allow you to program frequencies, PL tones, DCS tones, and NAC codes from the front panel without software.  

How is P25, DMR, Nexedge and (European) Tetra similar and different?

They are similar in a sense that they are digital modes that use a vocoder to convert audio into digital packets. However, the codec on the vocoder are different between the many modes. P25 is the only fully interoperable codec between all manufactures. All other digital modes are limited to one or few manufactures. P25 codec provides better correction on the fringe of coverage and also provides a better audio quality and fidelity that most would describe as full voice waveform. DMR on the other hand has lower true voice fidelity than P25.  Therefore P25 is the preferred codec for this system.  

The Quantar repeater stations and links in use for the N6LYE system are all made by Motorola. We chose to use the Quantar for its P25 error correction within the repeater station. The REPEATER WILL ERROR correct up to 3 received frames of data and send them out as a 100% tranmission without loss of audio. So, not only do the portables and mobiles have error correction but the Repeater system hardware will patchup missing frames as your transmission is repeated.  

How is the system linked?

Our P25 system is linked to each other without reliance on a central server run by outside companies. We use RF linking since we feel this is a highly reliable mode of linking. Least reliant on outside resources for support. While many use IP backhauls, there is a requirement that the internet is in service through out the paths at all times. Whereas with 70 cm RF linking, there is no dependence on internet services, multiple routers, microwave paths or land based fiber or copper.

Battery Backup Power

All sites are either backed up by battery or generator power for many days.  

© N6LYE 2020