SDI 4K signals

Emelec VíasCom has tested its digital video cables for SDI-4K signals up to the maximum distances indicated in the data sheets. Below we explain how the tests were carried out.

STANDARDIZED SIGNALS

The 4K is a signal that presents a resolution with 4 times more pixels than Full HD (also known as 3G-SDI). This means that 4 times more information is needed to represent a 4K signal than for a 3G-SDI signal, forcing the cable to carry much more information in the same time interval.

Like HD signals that can be transmitted on two frequencies (HD-SDI and 3G-SDI), 4K signals are also divided into two types: if they are transmitted at 30 frames per second (fps or frames per second) or twice that, 60 fps. We define 6G-SDI as the 4K resolution signal at 30 fps and 12G-SDI as the 4K signal at 60 fps which requires twice the speed. The 6G and 12G indicates the bitrate of the signal, 6 or 12 Gigabits per second which is the amount of bits per second that pass through the cable.

The standards that define 4K signals are SMPTE 2081 for the 6G-SDI signal and SMPTE 2082 for the 12G-SDI signal. Both standards limit the attenuation that the signal must experience at the output of the cable to a maximum of 40 dB for a frequency that must be half its bitrate. Thus, the 6G-SDI 6Gbps signal should not exceed 40 dB of attenuation at a frequency of 3 GHz. In the case of the 12G-SDI signal, the attenuation cannot exceed 40 dB at a frequency of 6 GHz.

TESTS PERFORMED

Based on the limits set by the standards themselves, the process followed to calculate the maximum cable length to transmit both signals was to measure the cable attenuation at 3 GHz and 6 GHz frequencies. First, we started from an initial distance which we reduced until we found the 40 dB of attenuation at a frequency of 3 GHz. This gave us the maximum recommended length of the cable to transmit the 6G-SDI signal. Then, the same process of reducing the length of the cable has been carried out until finding the 40 dB at a frequency of 6 GHz, which has resulted in the maximum recommended length to transmit the 12G-SDI signal.

Below are the graphs obtained when testing with the Q11-3007 cable.

40 dB attenuation at 3 GHz (6G-SDI) for a distance of 110 meters from the Q11-3007 cable.

40 dB attenuation at 6 GHz (12G-SDI) for a distance of 70 meters from the Q11-3007 cable.

A Keysight N9916A network analyzer has been used to calculate the attenuation, which allows the generation of signals with frequencies up to 14 GHz. The cable is connected at both ends. A sweep has been done between 100 MHz and 6 GHz to observe the cable attenuation within these frequencies. With the help of the markers that allow us to place the analyzer, we see exactly the attenuation that is generated at both 3 GHz and 6 GHz.

Carrying out the test for the cable reel Q11-243.

Calculations have been made for the single core cables Q11-3005, Q11-3006, Q11-3007, Q11-3008 and Q11-3009, and for the multi core cables Q11-234 and Q11-243.

The results obtained in metres of cable were as follows:

All the measurements have been carried out in the laboratory of the Department of Electronic Engineering of the Technical University of Catalonia on the Terrassa Campus.