Tech Focus: Long Distance/Low-Cost Video Transmission using Compressed 3G/HD-SDI
Transmitting high-quality video signals over low-cost cable or long distances is becoming increasingly desirable in vision systems.
The integration of compressed 3G/HD-SDI technology has made it possible to transmit high-quality video signals over longer distances or low-cost twisted pair cables without compromising on performance. When combined with autofocus-zoom cameras, this creates a powerful solution for a wide range of applications, including tethered ROVs, surveillance and remote monitoring.
Understanding 3G/HD-SDI
HD-SDI is a professional low-latency digital video interface standard for transmitting uncompressed HD video signals at up to 1080p resolution (1920×1080) and 30 frames per second; this requires a bit rate of 1.485 Gb/s. 3G-SDI is an upgraded version of the HD-SDI standard, supporting 1080p resolution at 60 frames per second; this requires a bit rate of 2.97 Gb/s.
Coaxial cables and their video transmission range and cable loss
When a digital video signal passes through a cable, it is attenuated by the loss in the cable (also by changes of cable impedance and each of the cable connections). Coaxial cables have a linear amount of loss per meter, so a cable that is 2x the length will have 2x the signal loss. The quality of a coaxial cable (and usually price/meter) is indicated by the loss per meter (lower is better, see Fig. 1). A better quality, more expensive cable will have lower loss and be able to carry a video signal over a longer distance.
The loss in a coaxial cable is also frequency dependent; a higher frequency signal will usually have a higher loss (see Fig. 2). Cables are often rated by their maximum frequency/bandwidth capability as an indication of this property and cable quality. A better quality cable will have lower losses at higher frequencies, and hence a higher frequency/bandwidth rating. Therefore, to transmit video signals at higher bandwidth, higher resolution or higher frame rate choose a quality cable.
Longer distance transmission is achieved by using higher quality cable or a lower bandwidth video signal. Conversely the cost of the coaxial cable is reduced by using shorter lengths or a lower bandwidth video signal.
The bandwidth rating of coaxial cable used for analogue/CVBS/SD video is typically 270 Mb/s, making it unsuitable for HD video transmission (unless the video has been significantly compressed using a technology such as HD-VLC).
Twisted pair cable
Compared to standard 75 Ohm coaxial cable, twisted-pair cables are extremely cost effective and are more accommodating in terms of flexibility, and signal loss with bending. However, the attenuation per meter and with increasing frequency, are much worse than with coaxial cable, hence in comparison the range of video signal transmission is much shorter. The impedance of a typical twisted pair cable is 100 Ohm leading to additional losses when connecting to a traditional 75 Ohm video source.
However, these twisted pair cables can be used to transmit video if the cable length and signal frequency are low enough. With a suitably compressed and impedance matched video signal, 1080p video can be transmitted over distances up to 150m (see below). This enables the re-use of existing cable infrastructure/composite cables or upgrades to existing twisted pair video systems.
HD-VLC video compression
HD-VLC is a standard that compresses SDI video by approximately 5x. This means that a HD video signal can be transmitted over longer distances or lower cost cables. At the receiving end the HD-VLC signal is converted back to uncompressed SDI so standard HD-SDI equipment can be used to receive the video.
HD-VLC enabled devices can output and receive 100 Ohm differential signal enabling easy support for transmission over twisted pair cables. This enables HD-VLC based systems to transmit HD-video (1080p30 compressed) up to a distance of 150m over Cat-5e/6 UTP cable. HD-VLC encoding and decoding is entirely hardware based so the latency of the decoding/encoding latency is imperceptible. The HD-VLC compression algorithm preserves the quality of the original image because it only eliminates data that is invisible to the human eye (VLC = visually lossless compression). The use of HD-VLC video also enables the selection of a cost-optimized coaxial cable.
Best practices for implementation
When transmitting digital video ensure that there are a minimum number of connectors/joints in the cable, reduce the number of cable bends and protect the cable from flexing and mechanical or environmental damage. If you are using slip rings, ensure that they are rated appropriately for the video signal data rate being carried. Overspecify the cable/connection as there are often unexplained losses (usually due to connectors) and deterioration over time. For extremely long distances you may want to consider using signal repeaters.
Autofocus-zoom cameras for long-reach video transmission
Harrier autofocus-zoom cameras have SDI and HD-VLC output capabilities; they can offer powerful solutions for a wide range of applications. For example, the Harrier 40x AF-Zoom 3G-SDI Camera provides real-time 3G/HD-SDI/HD-VLC video output with a 40x optical zoom and a 2MP Sony CMOS sensor. These cameras can be integrated with systems that require long-reach high-definition digital video transmission solutions that use SDI or HD-VLC video signals.
Our HD-VLC product range includes the Harrier 3G-SDI Camera Interface Board (which can be fitted to various Harrier AFZ cameras) and the BlueBird SDI Adapter, which provides options for conversion from HD-VLC back to 3G/HD-SDI, USB 3.0 and HDMI.
Applications
Our SDI – HD-VLC product range is particularly well-suited to a wide variety of applications. This is due to the ability to combine 3G-SDI, long distance HD-VLC, low-cost twisted pair cables or cost-optimized coaxial cables with no visible compression artifacts or latency. Plus, engineers can choose from a wide range of autofocus-zoom cameras.
For example, the SDI – HD-VLC solution is ideal for sectors where long-distance pipe inspection is a key activity. Other applications include tethered remote vehicle inspection, legacy SD camera installation upgrades and other systems where remote monitoring is required (e.g. medical applications).
Read our white paper for more information on long-reach video technologies and contact us to find the right solution for your system.
