Why Simulcast?

So before we ask the question of why simulcast, let us pose the question of what it is. There are a couple different definitions, but in our context simulcast is when you have multiple transmitters on the air over a wide geographic area. These transmitters are all on the same frequency, typically within a couple of Hz of each other. Now before we go any further, let use define what other types of repeater system that are typical in commercial, amateur and public safety use. 
Single Site Repeater
  This is a single repeater that receives and transmits from only one site. Most ham repeaters are of this type. This is the simplest setup there is for a repeater. The site is typically the best that can be had for the intended coverage area of the repeater. These repeaters are typically duplexed (Transmit and Receive) off of a single antenna and feedline. This is accomplished using a set of filters know as a duplexer. The duplexers main job is to keep the energy and noise from transmitter away from the receiver and also provide a narrow filter for the receiver to prevent the front end of the receiver from overloading due to strong adjacent signals while passing your intended receive frequency. 

Single Site Transmitter with Voting receivers 
  The setup for this configuration is a single transmitter (often paired with a corresponding receiver) at a site with the best coverage for the intended area. Along with that, it will have 2 or more receive only sites in other areas. The reason for this is the transmit site has the advantage of running high power and a high gain antenna on a tower in a high location. This puts the radios in the field at a disadvantage where they may be able to hear the repeater very well, but since they are often running lower power and lower gain antennas, the repeater may not hear them as well or at all. The multiple receivers deployed are used to balance at the coverage of both inbound and outbound transmissions. These multiple receivers are connected to a type of receive audio selector called a Voter. 
  A Voter, in a basic sense is the decision maker on which receivers' audio in the field gets chosen to be repeated. The reason for this is that if the audio from all the receivers were mixed together and sent out over the air, the result would be distorted audio due to the slight time differences of when the audio arrives back to the transmitter site. A basic Voter will look at the noise above 5 KHz and turn this noise into a reference voltage. These voltages will be compared to each other and the Voter will select the receiver that has the least amount of noise assuming it is the strongest of the signals arriving at the voter. The receiver with the least noise will have the lowest rectified voltage due to having the least noise, the voter sees this and decideds this receiver is the best audio to use. This particular receiver audio will then be sent to the transmitter to be sent out over the air. This selection happens on a continuous basis and for the most part, you do not hear the switching between the different receivers making it sound like one unbroken transmission. 
  With a voted system, you can expand receive coverage wherever you have the ability to get your receive audio back to to the Voter device. This back haul can be an RF Link, telephone leased line (sometimes called an RTNA circuit), Ethernet leased line or the Internet. Unfortunately in this type of system architecture you are limited to a single transmit site and have very limited options for expanding your coverage from that site. The main options you would have are to put the antenna up higher, put up a higher gain antenna or put more power into the antenna. 

A Simulcast Transmitter system with voting receivers
  This system contains all of the aspects of a single site TX voted system with an big added benefit, the ability to add transmitters as well! Here, you utilize multiple transmitters who's carriers are locked to very high stability oscillators (often GPS disciplined) and transmit out over the air on same frequency. This system requires the TX audio being transmitted out of each transmitter to be exactly the same in every respect. This requires some decent test equipment or it can be done with some basic equipment with and a good working knowledge of electronics and radio theory to accomplish. The control for the synchronization of the transmitters is accomplished by some type of control system. This system varies between manufacturers and the types of back haul being used. For our intent and purpose we will focus on the control system we will be using and that is the Voter module contained within the AllstarLink software. 
  Now to answer the big question that is the title of this page, why simulcast? Well, the biggest and best answer is, it gives you the ability to massively expand the coverage of a repeater system using a single frequency pair while appearing to the end users like a single site system. Now with that question answered, it's time to move on to the button below to take you to "Into to Simulcast".