# Technical background

In the previous sections, I tried to explain the basic concepts of the DMR operation. That is, the repeater independent private and group calls. This section concerns the more technical details of the DMR mode. In particular the Time Slot and Color Code.

As mentioned before, DMR is a digital mode. The speech signal is first digitized and compressed with a lossy compression codec. The latter is also called VOCODER. Modern codecs are very efficient and allow to transfer two independent speech signals within a single 12.5kHz wide channel. This is exploited in DMR using a technique called TDMA.

TDMA means time-division media access and describes that two independent calls can happen simultaneously on one physical channel. To achieve this, each call is assigned a Time Slot (time slot 1 & 2) and both are transmitting and receiving only within their assigned time slot. These time slots are very short. For DMR they are only 30ms long. This short time, however, is sufficient to transfer audio for at least 60ms. Therefore, DMR allows for two independent and simultaneous calls on a single physical channel.

When time slot 1 or 2 happens, is determined by the repeater. The repeater defines the beat. This also implies that time slots are irrelevant for the simplex operation. Thus you can ignore the time slot settings when programming simplex channels. (see introCodeplug).

What happens on each time slot, is a convention defied by the repeater. A general suggestion is that regional communication happens on the time slot 2 while trans-regional communication should happen on time slot 1.

Color Codes are a technical tool to avoid conflicts between repeaters operating on the same frequency. This usually happens in commercial applications of DMR. One company usually gets only a small number of frequencies assigned. To cover the entire campus, much more repeaters are needed than frequencies are available. Consequently, overlapping repeater ranges on the same frequency will occur and two repeaters may receive the call of a participant. Then, the color code allows the repeater to detect whether a call was intended for it. Only if the color code of a call matches the color code of the repeater, the repeater will react on that call.

This issue usually does not arise in amateur radio applications. We just have enough channels. Hence the color code is usually set to 1. In very densely populated areas, however, overlapping repeater ranges may still occur and different color codes might be used there.

To use a repeater, you not only need to know the input and output frequencies but also the color code of the repeater.