Digital devices for receiving and transmitting signals via fiber-optic communication lines.
The previous material (TZ No. 1-2010) discussed the possibilities of transmitting an analog signal via fiber-optic communication lines. Some of the basic concepts of transmitting signals via fiber-optic cable were considered. In this issue, we will focus on such a concept as optical budget (or energy potential of the line).
Optical budget– the difference between the optical power of the transmitter and the sensitivity of the receiver, expressed in dB. This is the passport information that the manufacturer must indicate in the accompanying technical documentation for its equipment.
The optical loss budget is calculated at the final stage of design. Its purpose is to determine the amount of loss for the worst signal path (the longest or with the greatest number of connections). This is one of the most important design stages, allowing to identify the necessary technical characteristics of the equipment used and other elements of the fiber-optic communication line. The calculation is quite simple and consists of summing up the losses in decibels of all components of the path, including attenuation in the cable and on all types of connections. Depending on the result obtained, it may be necessary to replace the equipment selected for the transmission path with more powerful equipment or with a longer operating wavelength. If the total attenuation of the line is greater than the optical budget, the designed system will not work. Therefore, it is often necessary to make an assessment in advance, taking into account each component of the line.
In simplified form, this can be represented as the following diagram:
Typical values of losses in passive components of the path are given
For example, when connecting a TV camera and a monitor with a 1 km multimode cable with one splice connection and terminated with ST connectors, the calculated loss at a wavelength of 850 nm will be 1.0 + 1.0 + 2.5 + 0.5 = 5 dB. The system usually provides a reserve of 3 dB for the inevitable decrease in the emitter power and aging of the line over time. Thus, the predicted losses in the communication line will not exceed 8 dB, which allows using a relatively low-power transmitter with an optical budget of 10 dB. Increasing the cable length to 2 km with the same transmitter power will entail the need to switch to a longer wavelength.
Calculators, often posted by manufacturers and trading organizations on their websites, can help you calculate the optical budget.
Digital Transmission
Transmitting and receiving signals over fiber-optic communication lines using analog equipment can be quite difficult. The use of digitalization of signals allows us to achieve much better results.
The use of digital receivers/transmitters ensures the transmission of the following types of signals:
video;
audio;
video + audio;
control signals;
video + control signals;
video + control signals + audio;
video + control signals + 10/100 M;
video + control signals + contact status + telephony + 10/100 M.
Both multimode and single-mode fibers can be used for transmission.
The fundamental difference between digital and analog equipment is the presence of the function of analog-to-digital conversion in the transmitter and digital-to-analog conversion in the receiving device. ADC (analog-to-digital converter) converts a continuous analog signal into a sequence of «0» and «1». The reverse operation is performed by a digital-to-analog converter (DAC). The quality of the transmitted signals is greatly influenced by the quality of the digitalization devices (ADC and DAC) used in the assembly of receivers and transmitters over fiber-optic communication lines.
Fig. 2. The principle of transmitting a digital signal over a fiber-optic communication line
Fig. 3. Representation of analog (a) and digital (b) signals
One of the most fundamental differences between an analog and digital signal, besides the actual form, is immunity to noise. A digital signal in electronic form is also susceptible to noise, just like an analog signal. But digital signals can only have two values: zero and one. Noise will only affect the signal if its magnitude reaches levels that can exceed the noise immunity of the digital circuits that determine whether the signal is equal to zero or one. This means that digital signals allow noise to accumulate to a higher level than analog video signals, so we consider digital signals to be virtually immune to noise. Ultimately, this results in increased transmission distance, high noise immunity, and no signal distortion, i.e. higher image quality.
Another important advantage of digital video signal is the ability to process and store information digitally. This means compression, various corrections, etc. without deteriorating the image quality. It is extremely important that the copy and the original do not differ in image quality. No matter how many copies of a digital image we make, the quality will always remain the same as the original. Another advantage of digital video is the ability to verify the authenticity of a copy. This function is often called watermarking and allows you to protect information recorded in digital form from counterfeiting, which is extremely important when using CCTV systems.
The wide application of various multiplexing methods can also be attributed to the advantages of transmitting signals in digital form.
Multiplexing technologies
Multiplexing allows transmitting a large number of signals (video and audio information, alarm sensor status, camera control signals) over a single fiber optic cable.
The main types of multiplexing in digital signal transmission:
WDM (WDM – wavelength division multiplexer) – wavelength division multiplexing – a technology that allows for the simultaneous transmission of several information channels over a single optical fiber at different frequencies.
DWDM (Dense Wave Division Multiplexing) – systems that increase the transmission speed over a single fiber to terabit values. The DWDM system is based on the principle of wave multiplexing with a fixed wavelength grid with a “step” of 10 nm.
CWDM (coarse wavelength division multiplexer) – a simplified version of DWDM. Channel separation is not dense (coarse). Currently, CWDM systems with 16 channels (in the wavelength range from 1310 to 1610 nm) with a “pitch” of 20 nm are produced.
Lasers used in DWDM systems have greater selectivity (useful power) and, as a result, a longer transmission distance, a smaller «step» between transmitted channels, and higher requirements for spectral characteristics. In video surveillance systems built on the basis of fiber optic technology, CWDM systems have become widespread as they meet CCTV requirements and are less expensive.
1-channel digital video + transmitter/receiver (Fiber System)
The products are designed to transmit 1 video channel and 1 data channel over one or two single-mode optical fibers in real time, using 8/10-bit digital encoding/decoding at a laser wavelength of 1310/1510 nm, high quality at a distance of up to 50 km. The data channel supports RS-232, RS-422, RS-585 (2- and 4-wire) and is compatible with Biphase, Manchester, NRZ. Both the video channel and the data channel have lightning protection at the input. The optical budget is 17 dB, and the power consumption is 10 W. The 220 V power supply has built-in protection against voltage surges. The products are available both in a flat case with an external power supply, and in the form of compact rack-mount (for mounting in a 19” rack) units 1U high. A power transformer and one or two boards are installed inside. The performance characteristics allow the use of devices in various temperature conditions, from -45 °C to +70 °C. These models are mainly used in video surveillance systems for transmitting video and control signals from PTZ cameras over long distances.
CFO-OPX System (Teleste)
The CFO-OPX platform with Coarseutilises Coarse Wavelength Division (CWDM) technology is a modular system for transmitting video (up to 64 channels and up to 72 when using 1310 nm), data, audio, as well as contact information and Ethernet data in single-mode fiber over distances of up to tens of kilometers.
The CFO-OPX comprehensive system includes a number of devices designed for various purposes — active devices for transmitting video and audio over distances of up to 100 km of the CEV series, passive devices of the COM series, optical repeaters and Ethernet switches.
The modular structure of the CFO OPX allows for flexible configuration of the system to solve a wide variety of problems. The reliability of the CFO-OPX meets all requirements for professional video surveillance systems.
VDT/VDR 14100-WDM (IFS)
The VDT/VDR14100-WDM series of video transmitters/receivers and data transceivers support simultaneous broadcast-quality video transmission with 10-bit analog-to-digital conversion and bidirectional data transmission over a single multimode or single-mode fiber. The modules are universally compatible with products from major stand-alone TV camera manufacturers and support RS-232, RS-422 Sensormatic Sensornet, and 2- or 4-wire RS-485 interfaces and all major data transmission protocols. The plug-and-play design ensures easy installation without the need for electrical or optical alignment. Each module has LED status indicators for monitoring normal system operation. The modules are available in stand-alone or crate-mounted versions. The equipment has a wide operating temperature range (from -40 °C to +75 °C) and a full lifetime warranty.
KBC FDVA (KBC) series
Using advanced digital processing technology, it provides good performance characteristics for CCTV. Supports transmission of one analog composite video channel via multimode (up to 4 km) or single-mode (up to 42 km) fiber. One, two or four modules can be placed in one device, which provides significant savings in rack space and on-site power consumption. This series is available in various configurations: compact, wall-mounted or as a board for a 3U crate. PAL, NTSC and SECAM standards are fully supported. Transmission of uncompressed video signal with 8-bit digital encoding. 4-channel module for wall mounting or as a board. Up to 56 receivers in one 3U crate.
Operating temperature -40 °C to +70 °C. Operating humidity 0 to 95% without condensation. Mean time between failures (MTBF): > 100,000 hours.
Fiber optic receiver SF42A2S5R/W-N and fiber optic transmitter SF42A2S5T/W-N (SF&T)
The devices provide transmission of 4 video signals using high-quality digital coding, 1 audio signal (can be increased to 8 audio channels) and 1 bidirectional control signal (maximum 3 channels) via single-mode optical fiber of the 9/125 µm standard over a distance of up to 40 km.
The devices have a wide optical dynamic range, optical attenuators are not required. Compatible with any CCTV cameras of NTSC, PAL or SECAM systems. Bandwidth — 5 Hz — 10 MHz. The devices comply with NEMA & CALTRANS Traffic Signal Control Equipment Specifications.
Multiplex transmission in real time, plug-and-play support.
The equipment does not require additional settings, can be used as separate modules or for rack installation (optional). There is LED indication, built-in lightning protection. Operating temperature from -40 ° C to + 70 ° C. Warranty — 3 years.