There has been a lot of chatter on the club nets recently about “doubling” and transmissions where one person is talking over someone else. While it’s easy to get frustrated about this behavior over linked networks because both feeds of audio are hearable, this is no different than the historical challenges of FM signal doubling.
First, let’s define a few terms:
Full-Duplex – A communication between two points where transmissions can flow simultaneously in both directions between those two points.
Half-Duplex – A communication between two points where transmissions can only flow in one direction at a time between those two points.
Doubling – An amateur radio term for two transmissions that occur at the same time on the same transmission path.
FM Capture or Capture Effect – Capture effect is defined as the complete suppression of the weaker signal at the receiver’s limiter (if present) where the weaker signal is not amplified but attenuated. When both signals are nearly equal in strength or are fading independently, the receiver may rapidly switch from one to another and exhibit flutter. [From Wikipedia]
A repeater system using a single FM radio pair with no other features, including linking, is a Full-Duplex system. This is the de facto nature of being a repeater – the repeater must receive a transmission and send a transmission at the same time. Repeaters would not be very useful if they could not do that. A personal FM radio transmitting to and receiving from a repeater is a Half-Duplex transmission. The standard personal radio can only send or receive at one time. This has been the default operating mode with repeaters for decades.
Linking between repeaters has always been of interest to the amateur community. Historically this was done with RF links on a different band between repeaters. This linking could be both Half-Duplex (one receiver “wins” through a voting process based on signal strength) or Full-Duplex (two repeaters sending and receiving all transmissions). With digital network-linked repeater such as those using Allstar, the same characteristics apply – some links are Half-Duplex, and some are Full-Duplex.
In general, Allstar-linked nodes are Full-Duplex connections. This is whether the node is directly connected to a radio unit or if it’s a “radio-less” node. An example of a radio-less node operating in Full-Duplex mode is hub.w8wky.org Allstar Node 48496. That node sits at the nexus of other connections so, to work, must both simultaneously send and receive transmissions. A personal hotspot, as a counter example, is a Half-Duplex system. Since it only has one radio, it can only send or receive transmissions at one time. An application on a computer or phone is exactly the same as a single radio – the app cannot send and receive transmissions at the same time, or the audio would echo. However, these Half-Duplex systems link to Full-Duplex systems for normal operations.
So, what does that mean? While a personal node – RF or app – is Half-Duplex, the systems it’s talking to is still Full-Duplex. While transmitting, the Half-Duplex nodes are deaf to any received communications but that does not convert the whole system to Half-Duplex. All other sites will be simultaneously receiving the transmission and may also transmit at the same time.
In RF-only environments, simultaneous transmissions would be subject to FM capture effect. In general, this is called Doubling and, on a repeater, sounds like one of two things – either one signal is weakened and the other sounds like a buzz in the background or both signals are completely wiped out and the repeater simply transmits noise. Anyone who has used repeaters for any amount of time has no doubt experienced this phenomenon. In those cases, someone comments that the transmission was doubled and asked one or more parties to repeat their transmission. This is a well-established situation and, in general, is not considered malicious because it happens by accident with well-meaning operations.
With network-based digital linking, everything is the same except for the fact that digital audio streams are designed to be mixed perfectly and thus in Allstar (and other) linked networks will hear both audio transmissions clearly and at the same time – like two people talking over each other in a room. There’s no capture effect at play. In fact, it’s just like what you hear on AM/SSB HF transmissions – many people talking at once. Our brains are hard-wired from in-person experience to consider this rude – and it is when done intentionally or due to poor etiquette! However, on network-linked systems, this behavior – when done unintentionally with reasonable setups – is no different than historical RF Doubling that’s handled with people backing off.
When this occurs on the W8WKY Hub System or any other network, all parties should keep the following in mind:
- Doubling is going to happen and, over networks, both (or all) transmissions may be perfectly hearable. Give everyone the benefit of the doubt and …
- Always obey the common amateur rule of listen-before-transmitting. While one may “lose out” in favor of someone making a transmission first, it does help alleviate congestion. It’s also important to…
- Avoid quick-keying. On large nets and large networks – and SARA is blessed to have one – all of the linked systems may have different transmit tails, may be IDing, etc. If one keys up really quick after the last transmission there a good chance a doubling will occur, especially on nets. Therefore, one should….
- Don’t assume malice where accidental interference has occurred. If you’re operating as a net controller or talking in a round table, use it as a teaching moment to remind people not to quick key and to take their time. And everyone should….
- Obey the golden rule – treat others as you’d like to be treated.
- If a net controller or someone else calls out particular interfering behavior, don’t take offense but rather take it as advice and help being offered in good-faith. Any persistent problems or interference should be referred to the repeater owner, network owner, or club leadership as appropriate.
On additional point – It’s incumbent upon “non-traditional” transmitters – notably users of non-radio connection types such as Echolink, DVSwitch Mobile, IAXRpt, and the Allstar Phone Portal – to make sure they are not interfering. In many cases, those connection types have significant lag relative to RF transmissions and full-time linked connections due to how they connect. Users of those apps must listen for obvious breaks in the communications where lagged transmission can fit in. If it’s a net, ONLY check in with your callsign and DO NOT provide other commentary – that way if it doubles with someone else, the transmission is not completely disruptive to others.
By keeping the above in mind and following some good, civil practices we can all have a great on-air community. SARA is fortunate to have a large community and an even larger net footprint! Keep it going and create a culture where everyone has a place but everyone is challenged to maintain good operating practices.