Simulcast, short for simultaneous broadcasting, is the process of sending the same Audio on the same radio frequency through more than one transmitter.
Speed of RF is free space = 186300 Miles / sec
Therefore: 1/186300 = 5.3 µ seconds to travel 1 Mile (5.3 µsec)
In locations where multiple simulcast signals are arriving at a receiver, one of two things will happen. If the signal from one transmitter is significantly stronger than the others, the stronger signal will "capture" the receiver and overpower the weaker ones. You may have experienced this with FM radio stations as you're driving between cities. One minute you're listening to a station from the city behind you and suddenly your radio picks up the station from the city ahead of you. This is known as the capture effect.
Complications arise when the receiver is in an overlap area and picks up two equally strong signals. Those signals will add together inside the receiver to produce a signal that is the combination of the two transmitted signals.
You can think of the two radio signals as waves, with peaks and troughs as they travel from the transmitter to the receiver. If the content of the two signals is the same and the signals arrive at the receiver at exactly the same time, the peaks and troughs will match each other and the two signals will reinforce each other. In this condition the signals are described as in-phase and result in constructive interference. Most receivers perform very well with this positive type of interference. If the transmitters are slightly off frequency you will get heterodyning or mixing of the two signals (sum plus the difference and the original two frequencies).
However, if the content of the two signals is the same but the signals do not arrive at the receiver at the exact same time, the peaks and troughs won't match up. This condition is known as out of phase and the results is destructive interference. The resulting signal will be distorted to some degree, depending on how far apart in time the two signals arrived at the receiver. If the signals are far enough out of phase the distortion will be so severe that the recovered audio will be unintelligible.
If the signals are exactly 180 degrees out of phase the peaks and troughs will cancel each other out.
Ideal Time Alignment:
To minimize interference in the overlap areas, simulcast transmitters should be very close in frequency (+/- 1 Hz).
The audio will have to arrive in the center of the overlap area at the same time (+/- 10 µsec) and be close in deviation (+/-.2 db) for the best quality audio.
It takes a radio signal about 5.3 µsec to travel one mile therefore all the transmitters must be the same distance (+/- 1 mile) from the main link transmitter. If not we have to correct for the Audio not arriving in sync.
Example 1 (all identical TX’s and Antennas):
The distance from the main link transmitter to Link-band TX “A” is 24 miles, to Link-band TX “B” is 20 miles, and to Link-band TX “C” is 28 miles.
It will take a signal 127 µsec to arrive at Link-band TX “A”, 106 µsec to arrive at Link-band TX “B” and 148 µsec to arrive at Link-band TX “C”. If Link-band TX “A” Audio is delayed 21 µsec and Link-band TX “B” Audio is delayed 42 µsec, all the signals will arrive at the center at the same time. This time delay is known as Time Alignment and this can be accomplished with a digital delay circuit or delay lines. In many systems this mathematical method is close enough but on air testing may be required.
Transmit Frequency Stabilization:
In the past systems could be frequency synchronized using highly stable oscillators based on the radioactive decay of cesium or rubidium.
Now it is cheaper and easier to use GPS stabilized oscillators. The GPS satellites have an extremely accurate clocks (oscillators) that can be used to correct all the master oscillators used in the Link-band TX’s if the GPS signal is lost for a few days the oscillators will not drift fare enough to cause any problems.
Most commercial systems generate PL tones (ctss) at the transmit sites phase locked to a GPS referenced master oscillator. We could do the same or live without the PL tone (ctss) on the 147.12 MHz transmitters.
Most Simulcast systems also make use of voting receivers. We can use the existing voter and receive sites without change.
Linking the Simulcast Transmitters (TX):
If we link the new Link-band TX sites with 220 MHz we will not require complicated duplexers. But all the 220 MHz RX’s, 147.120 MHz TX’s, GPS stabilized oscillators, and digital delay circuits would be have to be identical. This would ensure that the Audio level, Audio band pass and Audio Time Delays are the same. We can add Link-band TX sites one at a time but we may have to readjust the Audio delay circuits as we add TX sites.
Continued in Part 2
- October 2000 issue of Monitoring Times http://www.signalharbor.com/ttt/00oct/index.html
- APW Electronics Limited, APW SYSTEMS DIV.
- Personal experience with past simulcast systems. (Good and Bad)
- Online Calculators Time, Delay, Phase:http://www.sengpielaudio.com/calculator-timedelayphase.htmhttp://www.soundandmusicco.com/delay_calculator.htm
- Many months of researching Internet sights.