How Does Simulcasting Actually Work?

[CanesFan95]

If 2 CBers on Channel 6 both key up at the same time, they will double up and interfere with each other. Anyone trying to listen will prolly hear a mess and garbled transmissions depending on location. So lemme ask you this. How can simulcasting even work? If you have 2 repeaters on the same frequency at the same time, aren't they gonna interfere with each other and screw up each other's signal? Why don't 2 or more repeaters simulcasting on the same frequency interfere with them selves, similar to how the 2 CBers will interfere if they both key up?

 

[mmckenna]

Yes, they would interfere with each other.

The way a basic single channel simulcast works is that the two transmitters must be on the exact same frequency and transmitting the exact same signal at the exact same time.
This usually involves GPS for accurate timing and delay to get the audio to be transmitted at the same time. If the timing is off, it doesn't work very well.

Also, AM (CB radio) and FM (LMR/Public Safety) work differently, so comparing CB and LMR won't work.

 

[CanesFan95]

But regardless of the audio, what about the radio waves? If the radio waves are on the same frequency, aren't they gonna interfere?

 

[AZDon]

Simulcast

When two or more sites transmit at the same time their waves are out of phase and thats what causes the distortion. Simulcast simplifies this, each transmit site has a GPS antenna and a highly stable clock which identifies exactly where each site is. Lets say for example there are 4 transmit sites which are connected to the system comparator (system computer). The sysem comparator can tell exactly how far each site is from the the comparator. All received mobile audio is sent to the comparator then the comparator decides when each simulcast site precisely when to transmit. This is all done at the speed of light and the transmit sites will vary in miliseconds allowing the waves of each site to be in phase.

 

[Thunderknight]

And commercial receivers can tolerate a small amount of difference in the signal arrival time, as a perfectly timed simulcast system will only be perfectly in phase at certain spots. Outside of those spots, there are small differences (that increase with distance). Commercial radios can tolerate a small difference (allowed in the P25 standard). Beyond that area, they have issues too.

 

[CanesFan95]

Remember back in the day when agencies were conventional on VHF / UHF and the repeaters got out like gangbusters all over the place? The audio was great and there was no simulcasting distortion.

Then 800 MHz trunking came along and the range totally sucked so they had to spend a lot more money putting up lost of sites and simulcasting them all. That's why I've never been a fan of trunking, especially on 800 MHz.

 

[Golay]

Heterodyne

I wanted to mention Homeboy:
CB is AM. Two AM stations transmitting at the same time will create a heterodyne t a listener, even if one is considerably weaker than the other. I was told that this is the reason aviation uses AM. Air traffic control can tell by a heterodyne that another aircraft was also trying to contact them, and can ask for a repeat.

FM works differently. The strongest signal "captures" the frequency, and a listener will not hear, or even realize there was also a weaker station out there transmitting at the same time.

 

[CanesFan95]

Well, I hear FM stations on ham repeaters double on each other by accident all the time and they sound like a mess and you can't hear either one.

 

[dlwtrunked]

I wanted to mention Homeboy:
CB is AM. Two AM stations transmitting at the same time will create a heterodyne t a listener, even if one is considerably weaker than the other. I was told that this is the reason aviation uses AM. Air traffic control can tell by a heterodyne that another aircraft was also trying to contact them, and can ask for a repeat.

FM works differently. The strongest signal "captures" the frequency, and a listener will not hear, or even realize there was also a weaker station out there transmitting at the same time.

There is only a heterodyne if they are not exactly on the same frequency and differ by an amount in the audio bandwidth or the receiver. The real advantage is that even if on exactly the same frequency, the audio spoken by both stations can be heard, alerting the ground station. With FM, generally only the stronger station will be heard.

 

[CanesFan95]

But usually on FM, you can't hear either station and it's all a garbled mess when 2 hams double on each other on a repeater.

 

[buddrousa]

But that is not GPS timed and planned for by a controller

 

[ScannerSK]

FM works differently. The strongest signal "captures" the frequency, and a listener will not hear, or even realize there was also a weaker station out there transmitting at the same time.

When our local police used conventional repeaters very often dispatchers would state there was "double traffic" when two stations keyed up at the same time. That's a flash from the past. Typically, the voice from the strongest station would be heard however it was very easy to tell there was another weaker station broadcasting at the same time due to the garbled heterodyne in the background.

Why don't 2 or more repeaters simulcasting on the same frequency interfere with them selves, similar to how the 2 CBers will interfere if they both key up?

All the transmit frequencies are kept within a very tight tolerance to prevent any noticeable heterodyne effect (along with GPS timed audio to prevent any type of echo in the audio). Although the heterodyne effect may be reduced it cannot be entirely eliminated due to the picket fencing effect caused by signals being reflected off other objects (buildings, trees, planes, vehicles, etc.)

I have monitored the 5 MHz AM time signals during solar storms and have received up to a dozen stations simultaneously all transmitting at the same time without interference to each other. The reason for this as well is due to the very tight tolerance required for the transmit frequency by these stations to prevent the heterodyne effect.

Shawn

 

[ofd8001]

As a scanner listener, I'm aggravated by simulcast distortion. At times, my $500 scanner has either garbled audio or no audio.

As a public safety responder, I love our reasonably new (4 years) simulcast system. I can talk-in to the system pretty much everywhere I want to. The clarity of the audio is remarkable. Unfortunately the price tag for these radios can be as much as $3K.

That's on the "2-Way" side.

We also have a 5 site simulcast system for our VHF pagering system. When it was installed, the technicians had to be pretty fussy to ensure all transmitters were in perfect "sync". I believe it also required a certain type of transmitter with a high stability oscillator.

 

[kayn1n32008]

But usually on FM, you can't hear either station and it's all a garbled mess when 2 hams double on each other on a repeater.

Likely because the signal strength of the two signals is similar. If one signal was 20db stronger the stronger signal would be 'captured', although it would be obvious that there was a second, weaker station transmitting as well.

Simulcast systems must control the phase of the RF(using a very accurate time source like GPS), so that with in the intended coverage of the simulcast system, the signals from the transmitters are arriving with in the allowable delay spread(the maximum amount of time difference between two or more transmitters signal arrival at any point with in the coverage area)

I live near a city that has a 4 site simulcast EDACS system, and with in the intended coverage area, on analogue it sounds outstanding, and the little bit I have listened to Provoice on this network, it sounds good(as good as Provoice sounds)as well.