Username:
Password:
Central Arkansas Simulcast - Amateur Radio Repeater System

N5LSN

Central Arkansas Simulcast

442.700 MHz (+5.0MHz) CTCSS 118.8Hz

A growing wide-area analog 70cm amateur radio simulcast repeater system aimed at providing efficient auxiliary and backup communications throughout Central Arkansas.

CAS Coverage Map

Want to help expand coverage and fill the gaps in order to reach the goal? You can always donate or contact me if you want to help!

Repeater System Information

Simulcast Example Diagram

This repeater system combines multiple transmitters and receivers in Central Arkansas onto one single frequency by simulcasting and uplink voting, increasing coverage and ease of use while decreasing frequency band usage.

By programming this one frequency pair into your radio, you have programmed all of the repeaters in this system. When you use this system, you will automatically be using the repeater closest to you!

Think of this system like the towers that your cell phone operates on. You can move between areas without having to worry about changing the channel on your radio!

Features

  • AllStar link capable (see DTMF controls below).
  • Echolink capable (see DTMF controls below).
  • SkywarnPlus automatic weather and government-issued alerts.
  • Broadcastify audio livestreaming & archiving.
  • Zello remote RX / TX access access.
  • Google Assistant integration for weather forecasts, news, and more.
  • Full autopatch and reverse-autopatch capabilities (make & receive telephone calls - see DTMF controls below).
  • Unique courtesy tones for local traffic, link traffic, and active local alerts.
  • Tail-messages for announcements and local alerts.

Courtesy Tones

This repeater system is equipped with a variety of courtesy tones to help users understand the current state of the system and the source of transmissions. The following tones are used:

Link State Alert State Source Tone
None None Local
None Alert Local
Active None Local
Active Alert Local
Active None Link
Active Alert Link

DTMF Controls

Please always identify yourself via callsign before executing commands.

Code Description
*1 + [NODE]
  • Disconnect specified AllStar node (e.g., *19876 to disconnect from node 9876)
  • Disconnect specified EchoLink node (prefix with a 3 and pad to 7 digits, e.g., *33001234 for node 1234)
*2 + [NODE]
  • Connect specified AllStar node -- monitor only (e.g., *29876 to monitor node 9876)
  • Connect specified EchoLink node -- monitor only (prefix with a 3 and pad to 7 digits, e.g., *33001234 for node 1234)
*3 + [NODE]
  • Connect specified AllStar node -- transceive (e.g., *39876 to connect to node 9876)
  • Connect specified EchoLink node -- transceive (prefix with a 3 and pad to 7 digits, e.g., *33001234 for node 1234)
*4 + [NODE] Enter command mode on specified Allstar node (ex. *49876 to enter command mode on node 9876)
# Exit command mode
*6 + [PN] Autopatch up - place an outgoing phone call to [PN] (ex. *6911 to dial emergency services)
*0 Autopatch down - hangup all phone calls
*700 Say time of day
*701 Force local ID
*702 System status
*703 Disconnect all links
*704 Reconnect links disconnected with "disconnect all links"
*705 Say last node to key up
*706 + [NODE] Connect specified link -- local monitor only (ex. *7069876 to local-only monitor node 9876)
*707 Parrot - enter *707 + speak while still holding PTT - repeater will parrot the transmission
*708 + [DIGITS] + # Touchtone pad test: command + Digit string + # to playback all digits pressed
*720 Play the current SkywarnPlus tailmessage (if there is one)
*72 + [1-9] Use TTS to read out the description of one of the alerts in the current list of alerts (in the list [Tornado Warning, Tornado Watch, Flood Watch], Tornado Warning would be alert #1 - so *721 to read the Tornado Warning)

Google Assistant Integration

This repeater system is equipped with a Google Assistant integration that allows you to ask for weather forecasts, news, and more. To use this feature, simply key up the repeater and say "OK Google", and immediately unkey. You should hear a short sound effect letting you know that Google is now listening. Quickly key back up and clearly state your query.

Example: "OK Google - what's the weather going to be like in Little Rock tomorrow?"

Please note that this is an experimental feature and may not always work as expected. Even when everything is working correctly, you will need a strong signal into the repeater along with good modulation for Google to be able to understand you.

System Considerations

This system relies on a data connection between each site location and a single master controller. This inherently introduces a noticeable delay between the time at which the system receives your audio, and the time at which the system re-transmits (repeats) that audio. This results in the user being able to hear a slight "echo" of the last bit of their transmission. This is completely normal, and to be expected on a system of this magnitude and complexity.

Given that this system is made up of several transmitters operating simultaneously, you may experience some noises on your receiving end that are not present on conventional single-site repeater systems. These noises may present as a "buzz" or a "whine", and are also completely normal. The following video showcases an example of these noises that you might encounter.

Failover & Redundancy

Given that this system relies on a data connection between each site location and a single master controller, there are failover processes in place in the event that any of these data connections are severed. If any site location in the CAS 700 system loses its connection to the master controller, that site location will automatically enter a “failover mode”, in which the machine will continue to operate as a basic, unlinked repeater on the same 442.700 frequency but different CTCSS tones until connection is restored. This means that if you are in Little Rock and you try to key up CAS 700 and notice it does not seem to be working, you can switch from the normal CAS 700 CTCSS tone to the Metro backup CTCSS tone to see if it has gone into failover mode. If you can, you should program each of the sites "failover mode" into your radio (for example "METRO BACKUP") so that you can still use them if the simulcast system encounters issues. The "failover" CTCSS tones for each site are listed below. Failover CTCSS is REQUIRED on the backup inputs and is available on the backup outputs as well.

CAS 700 BACKUP CTCSS

Location CTCSS Tone
Hot Springs 67.0Hz
Benton 77.0Hz
Metro 88.5Hz

Each node in this system also employs some form of backup power. This may be in the form of battery, generator, or both.

N5LSN's Central Arkansas Simulcast Repeater System

The Central Arkansas Simulcast (CAS) repeater system is a growing wide-area analog 70cm amateur radio simulcast system designed to provide efficient auxiliary communications throughout Central Arkansas. Originally launched as a GMRS repeater system in 2022, the project has since transitioned to the amateur radio 70cm band and is now coordinated with the Arkansas Repeater Council on 442.700 MHz.

The Goal

My goal for CAS is to provide a single-frequency pair repeater system that enables communications between any two or more points in the central Arkansas region using an affordable mobile or base station.

The Beginning

My journey into repeater building began in 2021 when my dad and I constructed a GMRS repeater for our family to use during camping and off-roading trips. Recognizing its potential, we decided to share the repeater with the community by making it free and open to the public through a cooperative use agreement facilitated by the Arkansas GMRS Repeater Group. This became the Paron 575, the first free, public GMRS repeater in central Arkansas.

Encouraged by its success and the positive feedback from local users, we expanded the system by building three additional GMRS repeaters in Little Rock, Benton, and Hot Springs. The repeaters were linked together using full-duplex VoIP trunks, creating a system that allowed seamless communication across the coverage areas of all four repeaters.

The Problem

This multi-site setup presented some challenges. Users often had to manually switch channels while moving through varying terrain to maintain the best signal, which was inconvenient. Additionally, the overlapping coverage of the four repeaters meant that in some areas, the same traffic could be heard on multiple frequency pairs. This not only wasted valuable RF spectrum but also created interference for other GMRS licensees.

The Solution

To address these issues, I began researching simultaneous broadcasting, or simulcasting. Simulcasting involves transmitting the same signal on the same frequency from multiple locations simultaneously. When combined with receiver voting—a technique that dynamically selects the best signal from a group of receivers—it effectively merges multiple repeaters into a single, unified system. This appeared to be the perfect solution, especially for GMRS, as it enabled the benefit of combining the coverage from multiple machines while only using a single frequency pair.

After months of research, development, and testing, I successfully converted three of the repeaters (Little Rock, Benton, and Hot Springs) into a full-duplex simulcast system, which I named the Central Arkansas Simulcast (CAS) 700. This system utilized Jim Dixon’s (WB6NIL, inventor of the "Allstar" linking system) VOTER (Voice Observing Time Extension for Radio) protocol. I hand-built VOTER control boards, essentially custom radio-over-IP adapters, and interfaced them with analog UHF radios to bring the system to life.

The Network

The VOTER system relies on IP data links between sites to function. While the internet can be used for this purpose, I wanted to avoid dependence on it for a system intended for emergency or backup communications. To achieve this, I began constructing a 5 GHz wireless data network to connect the sites. So far, I’ve established wireless links between Little Rock and Benton, as well as to my QTH, where the master control server for CAS is hosted.

The Transition

The CAS 700 GMRS system was well-received and widely used for a short time. However, clarifications from the FCC regarding interconnected GMRS systems prompted a reevaluation of the project. Rather than dismantle the advanced simulcast functionality I had worked so hard to develop, I decided to transition the system to the amateur radio 70cm band. The Central Arkansas Simulcast is now officially coordinated with the Arkansas Repeater Council and operates on 442.700 MHz.

The Future

While I have the ability to build additional CAS nodes (repeater machines), time and resources are currently limited. I became a father in 2024, which has shifted my priorities, but I remain deeply committed to continuing the project.

Of the three nodes currently online, I have a fourth ready for deployment if I can secure a suitable high-profile site. As any repeater builder knows, finding an ideal location with the necessary elevation and infrastructure is one of the most challenging aspects of the process.

The 5 GHz wireless links I’ve established so far are functional but come with their own set of challenges. To keep costs manageable, I’ve relied on older, more affordable used equipment rather than investing in new gear. Additionally, the geographic limitations of my QTH make it difficult to establish reliable links. Trees growing across the street from my location have begun to obstruct the signal, causing timing issues, audio drops, and other problems in the VOTER system. I’m exploring options to raise the height of my 5 GHz antenna or find a nearby tower that can serve as an intermediary hop to improve connectivity to the other sites.

I am also hoping to, at some point in the future, increase the resiliency of the system by establishing redundant connections between sites utilizing more advanced routing equipment and protocols.

The Nerdy Stuff

The VOTER system consists of a server and its clients. The server, a Linux machine running the Asterisk VoIP telephony system with the app_rpt and VOTER extension modules, acts as the repeater controller and voting comparator. Clients connected to the server can function as receivers, transmitters, or both, providing a highly flexible and scalable system.

Simulcasting and voting require extremely precise timing to ensure that signals are transmitted and received synchronously across all sites. To achieve this, the system leverages GPS receivers, which utilize the cesium atomic clocks onboard GPS satellites for high-accuracy timing.

Operation Overview

  1. Receiving Signals: When a user keys up on the CAS input frequency, one or more receivers within range send a COR (Carrier Operated Relay) signal and an analog audio stream to the VOTER board. The board chops the audio into 20-millisecond frames, time-stamps each frame with the exact moment it was received, and includes the average RSSI (Received Signal Strength Indicator) for that frame. These packets are then sent over IP to the server.

  2. Voting and Reassembly: At the server, the packets are deconstructed and reassembled in order based on their GPS timestamps. The system compares the packets from all receivers during each 20ms interval and selects the one with the highest RSSI. The best frames are repackaged into a final analog audio stream, which becomes the system’s receive audio.

  3. Transmitting Signals: To make the system full duplex, the server takes the receive audio stream, chops it into 20ms frames, and time-stamps each frame with the exact GPS time it should be transmitted. These frames are sent to all transmitters, where they are held until the specified moment, ensuring that all sites broadcast the same signal simultaneously.

There is a lot more that goes into the proper simulcasting and voting, but this a general overview of how the VOTER system functions.

IP Challenges

Some readers might wonder why high-accuracy timing information is embedded into the VOTER packets and why buffering is required on both the RX and TX paths. One of the key challenges in this system is the unpredictable latency of IP networks. Since packet arrival times can vary and cannot be predicted, the system must account for the worst-case scenario. By calculating the expected maximum latency and tuning the RX and TX buffers accordingly, the system ensures that packets have enough time to travel to and from the slowest client, maintaining synchronization across all sites.

These buffers result in a slight delay in the repeated audio path. This delay can be heard as an echo in your voice when listening to a radio and transmitting at the same time. The length of this audio delay is directly related to the latency of the slowest data connection in the VOTER system.

Conclusion

The Central Arkansas Simulcast represents a significant achievement in amateur radio repeater technology in Arkansas. While the journey began with GMRS, the transition to the 70cm amateur band has allowed the system to thrive and continue serving the community. As the project grows, I look forward to expanding its capabilities and exploring new ways to enhance amateur radio communications in Arkansas. Despite the challenges of balancing family life, limited resources, and technical hurdles, I remain passionate about this project and committed to its future growth.

Contributing

If you are interested in contributing to the growth of the system, please do not hesitate to contact me by any methods listed at N5LSN.com.