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GMRS / FRS Discussions related to GMRS (General Mobile Radio Service) and FRS (Family Radio Service) communications

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  #1 (permalink)  
Old 03-11-2013, 9:46 PM
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Default CTCSS Vs. DCS

Seems to me that if two radios transmit a CTCSS code at the same time, a 3rd radio would hear both. This is because CTCSS is just a frequency and two sources of the same frequency won't interfere much with each other.

However, DCS being a digital code might interfere with another radio transmitting the same code such that the 3rd radio won't hear anyone.

Is this true?
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Old 03-11-2013, 11:04 PM
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Strongest signal at the receiver wins regardless of CTCSS or DCS as long as the receiver and transmitter are using the same CTCSS or DCS.
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Old 03-11-2013, 11:09 PM
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I think what he's getting at is that CTCSS won't matter, but if two radios were transmitting a DCS code at the same time, the bit stream might get garbled and the receiving radio would not open the squelch. But practically, the signal strengths would be different, the FM capture effect would also help and one radio would present the better signal and a good bitstream.
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Old 03-12-2013, 5:35 AM
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How can the receiver tell the difference between two signals overlaid on the same frequency? Once the squelch opens wouldn't all present signals come in?
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Old 03-12-2013, 8:07 AM
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What's the point...or maybe I should say, what is the practical application you are a trying to achieve?

This is only done reliably with simulcasting, where a GPS clock signal syncs both TX to to allow the RX to hear both without any issues.
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Old 03-12-2013, 8:13 AM
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Quote:
Originally Posted by n9mxq View Post
Strongest signal at the receiver wins regardless of CTCSS or DCS as long as the receiver and transmitter are using the same CTCSS or DCS.
This is what is known as the "FM Capture Effect." Works, but only if two incoming signals vary in signal strength by about 6 dB. If they are closer in comparative signal strength, you will get a superheterodyne clash, and anything could result.
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Old 03-12-2013, 8:15 AM
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Originally Posted by dnoyeb View Post
How can the receiver tell the difference between two signals overlaid on the same frequency? Once the squelch opens wouldn't all present signals come in?
True in theory, and in practice using AM product detectors. However, the "capture effect" is such that if one incoming signal is sufficiently stronger than the other, the FM product detector will follow the zero voltage crossing points of the stronger signal and essentially not see those of t he weaker signal.
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Old 03-12-2013, 11:09 AM
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Interesting, thanks.
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Old 03-12-2013, 9:21 PM
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Originally Posted by dnoyeb View Post
How can the receiver tell the difference between two signals overlaid on the same frequency? Once the squelch opens wouldn't all present signals come in?
If I understand DCS correctly, it's a continuous data stream, so even if the receiver unsquelches, a second signal of nearly identical strength with the same bit stream could conceivably mess up the data stream being received and the receiver would squelch up.

Practically, it probably wouldn't happen...
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Old 03-13-2013, 2:31 PM
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Quote:
Originally Posted by dnoyeb View Post
How can the receiver tell the difference between two signals overlaid on the same frequency? Once the squelch opens wouldn't all present signals come in?
The squelch merely "mutes" the audio. You have to consider what is going on ahead of the squelch circuit and actually what is being fed into the CT/DC decoder. If one of the signals is strong enough, it wins. If there are multiple signals of "equal" strength, it's a mess. It's not like they'd be cleanly mixed to provide the scenario you describe; although theoretically possible if carriers are exactly identical, in sync, etc. Highly unlikely.
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Old 11-27-2013, 4:52 AM
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Gentlemen, I am new at this.
What's the difference CTCSS Vs. DCS. I have a GMRS radio. Does it make a difference which codes I use in distance, clarity etc?.
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Old 11-27-2013, 5:02 AM
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Gentlemen, I am new at this.
What's the difference CTCSS Vs. DCS. I have a GMRS radio. Does it make a difference which codes I use in distance, clarity etc?.
Try the RR wiki. It should have full descriptions of both technologies.
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Old 11-27-2013, 7:39 AM
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Quote:
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Does it make a difference which codes I use in distance, clarity etc?.
No. They have no effect on range or audio quality.
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Last edited by nd5y; 11-27-2013 at 9:33 AM..
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Old 11-27-2013, 7:56 AM
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Default agreeing with davedaver1

However, with dcs, it would only open the squelch of participating radios with the same code programmed. This allows you to select groups on the same frequency without alerting all radios on same frequency. Signal strength, covered by other poster still stand true. I don't know any FRS/GMRS radios that use DCS, but only CTCSS. Have fun.
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Old 03-26-2014, 12:45 PM
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Lightbulb Closely Related Question?

Can a transmitter or in this case a repeater transmit both ctcss and dcs be transmitted at the same time? In this case what I'm trying to achieve is using the dcs as an identifier being decoded and used to display what repeater was being used while the ctcss tone is used to open the squelch on the receivers.
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Old 03-26-2014, 12:53 PM
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Transmitting both at the same time would not be a problem but you might not be able to receive both.
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Old 03-26-2014, 1:10 PM
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We would have a dcs decoder on the receiver and depending what dcs code was received would translate into a identifier.
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Old 03-26-2014, 1:48 PM
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Never heard of this at all. I'm not sure you could transmit both at the same time, they both occupy the sub-audible space in the carrier, never seen equipment that would do it, but I suppose one could stack a secondary encoder on the repeater Tx.

I think it would be much easier to do it either by 1. DTMF - send short burst, even 2 digits at the beginning or end of the keyup of the repeater, then you can decode that at the control station, as you describe. Each repeater having it's own DTMF code. Or 2. A voice controller that actually announces which repeater it is. (and I have seen a #3....send MDC1200).
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Old 03-26-2014, 2:04 PM
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Yeah I thought about DTMF or MDC1200 but was trying to keep the signaling to a minimum, not so annoying.
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Old 03-26-2014, 4:44 PM
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Quote:
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Yeah I thought about DTMF or MDC1200 but was trying to keep the signaling to a minimum, not so annoying.
Select the option to mute them then.
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