The Technology

A Case Study of DTV System Planning with Cable-Sharing

Introduction

DTV digital HD surveillance products are developed by Taiwan IC design house ITE Tech. Inc. along with several foreign and domestic surveillance manufacturers. DTV surveillance products are based on DVB-T digital TV technologies. The product line includes DTV CAM, DTV DVR, DTV accessories such as POC, and other software and hardware.

DTV systems have five features. Among them, the “One x Multiple”, or “cable-sharing” feature means DTV supports multiple 4K video over on coaxial cable, simultaneously. In this article, we will introduce the benefits brought by “cable-sharing” through an international case. We will also show how to plan a DTV surveillance system taking advantage of cable-sharing.

Background Descriptions

This case is from a customer of one ccHDTV alliance member. The member accepted the national-level project and started to design in 2015. The installation starts from 2016 and will take a couple of years to complete. The purpose of this project is to ensure the fairness of national exams and to eliminate the scandal. To achieve this goal, the authorities plan to install or upgrade surveillance equipment in all schools where the national exam takes place. Because the project is on-going and is protected by the non-disclosure agreement, the country’s and the schools’ names are edited. The scale of this project includes all schools assigned to hold the national exams. The contractor of this project is required to install or upgrade surveillance equipment including cameras, DVRs, and storage server. They are also required to build the remote monitoring system for easy management. In this article, since our purpose is to show the application of cable-sharing. We will take only one school for example.

In the school under discussion, the academic building where the exam rooms are located is in long rectangle shape. The length of one floor is about 125 m and there are four floors. The height of each floor is around 3.5 to 4.5 m. There are the stairs and offices near the center of the building, and classrooms at both wings. The classrooms are used as the exam rooms during the national exam. To meet the requirements as an exam site, one office in the third floor are reserved as the control center, and in each exam room, two cameras, one at front, and the other at the rear, are installed.

As shown in Figure 1, a total of 72 cameras are required. The cable goes vertically from the control room to each floor, and to each classroom.

< …………………………………………............................…….....         125 meters公尺         ………………...................................................................……..>

(a) The fourth floor: four exam rooms in the right wing, and five in the left. There are 18 cameras.

(b) The third floor: four exam rooms in the right wing, five in the left. The control room is located near the center. There are 19 cameras, and two cameras in each room. The 19th camera is installed at the entrance of the control room, not in the interior of the room.

(c) The second floor: four exam rooms in the right wing, five in the left. There are 19 cameras, and the 19th one is installed in the office of exam affairs.

(d) The first floor: three exam rooms in the right wing, and five in the left. There are 16 cameras.

Figure 1. The arrangement of the exam rooms and the locations of the surveillance cameras.

Consideration of DTV System Planning

To use the “cable-sharing” feature of DTV system, no complicated IP setups are required. In most small- to medium-size cases, the planning requires no special considerations. However, in large-size or multi-floors cases, proper grouping of cameras can help to minimize number of splitters or shorten the cable length. This will also reduce signal attenuation, optimize video quality, and save the cable and man-power. During the planning, two general rules can be used to check the feasibility of the system.

1. At the DVR, the level of any channel should be greater than the sensitivity of the DVR.
2. At the DVR, the level difference of any two channels should be within certain range.

We will explain in the following case how to convert the above two rules to practical criteria.

We will take the third floor as an example to show how to plan the installation, and analyze the plan to see if it is a valid design. As shown in Figure 1(b), 19 cameras are needed, and 18 of them are located in nine exam rooms and one at the control room entrance. Theoretically speaking, tens of channels can share one coaxial cable, but the more the channels, the more splitters are required to combine the signals at the camera ends, and to separate the signals at the DVR ends. The more the splitters, the larger the loss. In general, we need to trade the channels in one cables for the total numbers of cables. The following are assumptions we made for this case.

(a) We limit the maximum channels in one cable is eight. After calculation, we found the maximum number of cables is Ceiling (19/3) = 3 cables.

(b) To reduce the number of splitters, we will try to equally distribute the channels in each cable.
      In addition, to simplify the design and stock, we also make the following assumption:

(c) although there are 1x2, 1x3, 1x4 splitters available in the market, we choose to use 1x2 splitters only, and its nominal attenuation is 4dB.

(d) the only exception to (c) is where the 3 cables combined at each floor. We will use 1x3 splitter, and the nominal attenuation is 6dB.

The assumptions about the cable and the DVR are:

(e) In this case, the coaxial cables are 5C coaxial cables. To simplify the calculation, although the attenuation is a function of frequency, we will a fixed, worst case attenuation 0.16dB/m wherever attenuation is needed. The is the attenuation at 473 MHz.

(f) We use only 8-channel DVR. The overall attenuation of the splitters to separate signals at the DVR input is 11dB.

Based on the above assumptions, as shown in Figure 2, we group the cameras to red, blue, and orange groups, each with 6, 7, and 6 cameras, respectively.

Figure 2. The grouping of cameras on the third floor.

Under the above grouping and assumptions, we will check the conditions of each group

＊the orange group

The left camera in Room 3-1 goes through the most, 4+1 splitters.

The left camera in Room 3-3 goes through the least, 1+1 splitters.

The right camera in Room 3-1 is the farthest camera from DVR.

The left camera in Room 3-3 is the nearest camera from DVR.

＊the blue group

The left camera in Room 3-5 goes through the most splitters, 4+1 splitters.

The camera at the control room entrance goes through the least, 2+1 splitters.

The left camera in Room 3-6 is the farthest camera from DVR.

The camera at the control room entrance is the nearest camera from DVR.

＊the red group

The left camera in Room 3-8 goes through the most, 4+1 splitters.

The right camera in Room 3-7 goes through the least, 1+1 splitters.

The left camera in Room 3-9 is the farthest camera from DVR.

The right camera in Room 3-7 is the nearest camera from DVR.

Table 1 is a checklist to examine the design of the orange group. Enter the cells in the thick black outlines according to the design, and the result will be calculated. From the floor plan provided by the school, Dmax = 61m, and Dmin = 34m.

Table 1. The checklist of the orange group.

The following explains how the results are calculated.

(i) as mentioned above, the cable is 5C coaxial cable. The attenuation increases with frequency. Because we limit the maximum channels in one cable is eight, we can assume that the maximum channel frequency will not exceed 473 MHz. Therefore, we use the worst-case attenuation of 0.16 dB/m, i.e., the attenuation at 473MHz in 5C coaxial cable, in order to simplify the calculation.

(ii) As mentioned earlier, we use 8-channel DVR, and the input attenuation due to splitter is S_DVR = 11 dB.

(iii) Smax is the maximum splitter loss (“combiner loss” in the table) along all links. From the analysis, the left camera at Room 3-1 goes through the most splitters, including 4 2x1 and one 3x1 splitters. The overall attenuation is Smax = 4x4 + 6x1 = 22dB.

(iv) Assume the output of the camera is -10dBm, and the sensitivity of the DVR is -80dBm. The difference of these two numbers 70dB is called “link budget”. After subtracting splitter and DVR loss, we can calculate the maximum transmission distance.

Dmax = (70-22-11)/0.16 = 231.25m

(v) There is no limitation of minimum transmission distance, therefore, enter 0.

(vi) It is mentioned earlier that at the DVR input, the signal level differences between any two channels sharing the same cable should be controlled within a range. A reasonable number is 35 dB. If the difference exceeds this number, the higher-power channel may interfere the lower-power channel and affect its receiving. Consider the worst case in which the lowest-level channel passes through the most splitters, while the highest-level channel pass through no splitter. After subtracting Smax, the channel can propagate at most (35-22)/0.16 = 81.25m before it becomes 35 dB lower than the highest-level channel.

After the above complicated computing, we find the design of the orange group is valid (the “OK” in the table).

The criteria in the checklist are the same for the blue and red group, and from the floor plan,
Blue: Dmax = 63; Dmin = 15
Red: Dmax = 90; Dmin = 63
The designs of the blue and red group are also valid.

People may ask why not using one channel per cable, which can greatly reduce the complexity of the problem? If considering only the technical feasibility, there are of course more than one solution to this problem. However, if taking budget, easiness to install, frequency to maintain, etc., into consideration, we can narrow down and find the most proper solution.

In this case, do not forget that a line of a couple centimeter long may represent a distance of several meters or even kilometers in countries such as US, Canada, Australia, etc. The feature of “cable-sharing” really save cable and man-power, and make later maintenance easy.

More about Cable-Sharing

The cable-sharing of DTV is easy, and it also has the following characteristics.

Flexible Topology

DTV’s cable-sharing requires neither additional equipment nor complicated setup. The only thing to do is to set the cameras to different channels, like the TV system, at the DVR end. It also supports various topologies such as parallel, serial, star, tree, and hybrid, etc.

Support of Various Transmission Media and POC (Power over Cable)

In addition, DTV’s cable-sharing is not limited to coaxial cable. It can work with twisted pairs, network cables, etc. It can also combine with POC (Power over Cable), providing AC or DC electricity to the cameras. DTV’s cable-sharing can also work with the existing CCTV cameras.

For further questions or information, please contact us.