Tuesday, June 12, 2018

A Tale of Two Time Slots

Not everyone who has been grabbed by the DMR evolution may understand what two time
slots accomplish.

DMR is based on TDMA technology which allows one 12.5 kHz wide channel to host two simultaneous discussions or data streams at the same time, on the same frequency, without interfering with each other.

Comparing FM to TDMA based DMR.
More details can be found here: https://hvdn.org/whatsdmr

There are also benefits of TDMA compared to standard FM that are not spectrum related. Things like better battery life on a handheld radio is just one. This is accomplished due to the fact that the transmit cycle is not full duty cycle, but rather keyed on and off rapidly, which benefits on less batter load and overall longer life between recharges.

Another benefit is reduction of the cost for repeaters since some components like antennas, feed lines, controllers and more can be shared. The cost for two traditional single channel FM repeaters is cut down greatly by replacing it with just one DMR repeater.

Dual Time Slot and Full Duplex Hotspots

Many repeater operators and owners are currently using a single time slot repeater, even though many specify to use a certain talk group on time slot and other talk groups on time slot 2 which may seem misleading.

Its currently not possible with 99% of amateur radio repeaters to take advantage of true dual time slot features. Part of this reason is due to cost.  A repeater would need two sets of RF equipment in order to make use of true two time slot capabilities. This prevents repeater operators from upgrading equipment. Often times, analog FM repeaters are re purposed as DMR repeaters with out board controllers and voice modems and its hard to do this to create a dual time slot system without great frustration.

Toronto based VA3XPR repeater system and its various components
showing how complex repeaters of any time can become

Since operating a repeater can be costly for both equipment and site related needs such as internet access and leases at great wide coverage area sites, a dual time slot hot spot fills a certain gap and need.

Enter Inexpensive Dual Time Slot Hotspots

Many hot spots are one time slot at a time and also semi-duplex.  This means that while the hotspot is transmitting, its not possible to receive signals.  This is often referred to as semi-duplex operation.

Its usually hard for most people to be involved in two discussions at the same time, so this is not usually an issue, but from a control perspective of a hotspot - a full duplex hot spot allows parameters to be changed while its in transmit since its receiver is on full time.

An example of this application would be using a TG 4000 disconnect to drop active users from the hotspot at any time, rather than waiting for a pause in transmission or by using the Brandmeister dashboard to drop active calls.

You can also have time slot 1 set to say, World Wide 91 and time slot two set for USA Nationwide 3100 at the same time, on the same frequency.  Here is a sample video of that.

As covered in the "DMR Code Plug Concepts" article, you can get very creative with code plugs thanks to a dual time slot hot spot.

Sound off in the comments on what you think a dual time slot hot spot or repeater can accomplish that a semi-duplex repeater or standard JumboSpot, OpenSpot, DVmega and others.

Welcome to 2018, the break through year for DMR and digital voice evolution in ham radio.

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