HD-SDI Systems for CCTV

Both the original analog technology and then compressed digital technology on which IP systems are ultimately based came out of the broadcast industry.  Now another broadcast industry originated system is being applied to CCTV: Serial Digital Interface (SDI).  This is a series of standards developed by the Society of Motion Picture and Television Engineers (SMPTE) that started many years ago with uncompressed Standard Definition (SD) video running at 270Mbps over coaxial cable.

This was extended so that it is also now available at a variety of data rates the most important of which for CCTV applications are High Definition (HD) at 1485Mbps and at 2970Mbps. And, it is very likely that the next SDI standard will operate at around 10Gbps. All SDI digital signals operate at a peak to peak voltage level of 800mV ±10% over 75Ω coaxial cable and employ BNC connectors. It is possible with high quality coaxial cable to transmit reliably over at least 400 meters at 270Mbps, 220 meters at 1485Mbps and 200 meters at 2970Mbps, roughly comparable to what is attainable with analog signal transmission over coaxial cable.

Note however that achieving these sorts of distances does assume good quality coaxial cable: attempting to make SDI work over significant distances with some of the cheap cable sometimes used in CCTV installations is not a good idea.The great technical feature of SDI is that it is about as pure a digital video signal as it is possible to get so that with good cameras, lenses and transmission technology there are very few impairments in the pictures with the actual source picture being delivered across the SDI link unaltered.  Thus, for some higher end CCTV applications it is a very useful technique.

An example might be where centralised video analytics are being used in high risk sites such as airports where there is little margin for the distortion and noise that might accompany even high quality IP systems.Other attractive features of HD-SDI are:There are no network issues to worry about so that it is easily installed and commissioned by non-IT installers and technicians. There is no transmission delay (i.e. latency) so camera control is as easy as for analog cameras. In some ways the SDI solution looks like an analog solution and as already noted can often use the same copper cabling infrastructure already in place for conventional analog CCTV.

There are now many surveillance oriented cameras that have a native HD-SDI (1.485Gbps) or 3G-SDI (2.97Gbps) interface.  Of course, distances of around 100 to 200 meters are too short to be practical in many CCTV networks, so fiber optic transmission systems are often essential and there is a growing range of SDI fiber products available for the CCTV industry. It is certainly best to use singlemode fiber with these systems but operation over limited lengths of multimode fiber (eg, 300 meters at 3G-SDI) is also possible using optical components operating at 850nm.So, while HD-SDI can offer superb video quality and can be very convenient to install and to use, what are the downsides?

A minor one is that displays with native SDI inputs tend to be expensive so the usual solution is to use SDI-HDMI converters with HDMI monitors .  These are readily available, generally work well but are an additional expense and yet another bit of equipment in the control room. A more important concern is that of storage.  There are many 4-channel and some 8-channel DVRs which accept HD-SDI signals, compress them (generally using H.264) and store on an internal hard drive. Note three issues here:

1. This is a DVR so the advantages of NVRs are not possible.
2. Currently, only a restricted number of channels are available compared to the 16 to 64 typical of conventional DVRs and NVRs.
3. The signal gets compressed and the level of compression determines the quality of the recorded image, just as it does in conventional IP systems.

Finally, it is important to note that HD-SDI technology is being adapted and customised for CCTV via a consortium of suppliers and users known as the HDcctv Alliance (see www.highdefcctv.org).  The Alliance is developing standards for several areas such as the transmission technology itself (based on the broadcast industry’s SMPTE standards) and the transmission and integration of duplex data and audio signals onto the same coaxial cable carrying the HD-SDI signal.

In addition, lossless compression techniques (such as Dirac) will probably be incorporated to provide greater transmission distances over standard CCTV quality coaxial cables.As often happens with the introduction of new technologies, there is a fair amount of hype about HD-SDI with some extremely optimistic projections for its take up in the marketplace.  The reality is that it does have some very nice features that have appeal for different markets ranging from very high end applications down to retrofits of existing simple analog systems. OSD expects that the trend to IP will continue, particularly in larger systems where the flexibility and built-in redundancy of well designed networked CCTV is of paramount importance, but that HD-SDI provides a very useful option for many applications.

For further information on HD-SDI systems and the options available for optical signal transmission please contact any of OSD’s systems engineers who will be very happy to assist you.