Ogier Electronics

Applications

There are many different options that enable video to be transmitted from the cameras of a CCTV system to the Control Centre. Coaxial cable can be used if the ranges are very short and there is an easy path. Lasers can be used, although they are very vulnerable to weather and require precise alignment with particularly rigid installations. Alternatively fibre optic cable can be employed if it is currently available and it is reasonably secure.

If however no fibre is available, it is invariably cheaper and quicker to use microwave. In some cases it is the only solution. With the Ogier range of products the video quality of the microwave transmission is equal to that available from the best fibre optic systems.

Even if the decision is made to use microwave, there are still many options, the major ones of which are summarised below. A brief summary of the pros and cons is provided with more details given in the Downloads.

Because we manufacture equipment covering the full range, we believe we can give impartial advice on which technology is best suited to each application. The information provided here is intended as an introduction only. If more details or assistance is required, please contact us through the Contact Us page or directly by phone.

Analogue v Digital

All commercial digital transmissions compress the video data, which means that some data or resolution will be lost. In addition time is taken in compressing and decompressing the signal.

This is not necessary with analogue transmission, which means that it is capable of providing the very highest video quality with no latency whatsoever. This makes it particularly applicable to safety critical monitoring systems where delays or loss of quality cannot be accepted, and to long-range camera systems where loss of clarity or resolution could necessitate longer focal length lenses, or to many city centre systems where real time operation is essential for the efficient tracking of targets.

However the major disadvantage of analogue is that it has a very limited non-line of sight capability. Thus in general, the digital links are the only effective solution for mobile or transportable systems where lines of sight cannot be guaranteed.

DVB-T v Internet Protocol

Digital Video Broadcast Terrestrial (DVB-T) provides the best video quality of all the digital options together with the most resilience. Thus it is the right solution where video quality is important or where the transmission paths are complex. Its disadvantage is that it generally requires more bandwidth than IP, typically four times as much. This means that our single Ethernet link can transmit 20 IP videos in the same bandwidth as required by 5 DVB-T channels.

The other issue is that DVB-T is a more expensive technology, compounded by the fact that it is essentially a one-way system and therefore requires additional hardware to achieve two-way transmission. Thus if price and camera control are equally or perhaps more important than video quality, an IP solution could well be better. General applications include monitoring overall activity or detecting unusual occurrences rather than obtaining precise identification in commercial premises, car parks and supermarkets. Against this, the DVB-T solution will invariably outperform the IP solution in mobile or similar applications where the lines of sight could be complex.

Frequency Selection

The higher frequency bands are generally preferred because there is more capacity and less chance of interference than in the more cluttered lower frequency bands. Conversely, the higher frequency 31 and 58 GHz bands are more susceptible to rain fade. Typically the range of a 31 GHz link is up to 10 km in Europe, 15 km in the Middle East but only 5 km in the tropics. The range of the 58 GHz equipment is even shorter because as well as rainfall, oxygen absorbs 94% of the signal every kilometre.

Thus if very long ranges are required, it is necessary to use lower frequencies. The ideal is the 5 GHz band because it is substantially unaffected by weather. It is also generally protected by a licensing regime and therefore has better immunity against interference than the public bands. The ranges of these links can be as long as 100 km for single channel equipment and 80 km for multichannels.

An alternative is to use the licence exempt bands around 2.4 and 5.8 GHz. Although we manufacture such equipment, we do not recommend their use except for very short ranges in cities or in open countryside. The reason for this is that being a public band system, interference is becoming an increasingly serious and sometimes insurmountable problem see below.

Licenced v Public Band

There are three advantages in using the public bands at 2.4 and 5.8 GHz.

  1. There is generally no licence fee, which makes them easier and cheaper to use
  2. The lower frequencies allow a better non line of sight capability
  3. The equipment is cheap because it is based on consumer products

The major disadvantage is interference because the band can be used by anyone, including all the wireless laptops, microwave ovens and any of number of ISM systems. Its not an issue in short-range domestic applications, but in CCTV the effects can be serious because the ranges are longer and the video is required continuously.

Licenced bands by comparison require regulatory approval, which enables them to be interference free. In addition, there is up to 10 times the spectrum available, which means that they can accommodate far more cameras with higher data rates and hence better images. Their two disadvantages are the higher cost of the professional rather than domestic equipment and the fact that they do not have an effective non line of sight capability because of the higher frequencies that are employed.

The conclusions are that the licence exempt bands should be avoided for safety critical applications, high security systems, or even when high quality video and interference free operation is essential. Such systems have to use licenced frequencies. Licence exempt frequencies could however be considered for commercial areas or in small housing complexes providing the user is made aware of the possibility of interference. They should not be used in a large or significant citywide system because interference is almost inevitable.

Point-to-Point v Point-to-Multipoint

Point-to-Point systems are normally used because the antenna discrimination allows the frequencies to be reused on different cameras. They therefore have higher capacities than Point to Multipoint systems that have to employ a different frequency channel for each camera.

Also the equipment in Point-to-Point systems is simpler if there are few cameras, whereas in Point to Multipoint, a relatively complex Base Station is required irrespective of the number of cameras.

In practice Point to Multipoint is only used in IP based systems. Many companies supply such equipment in the licence exempt bands, whereas we supply equipment in the 10.5 GHz licenced band.

Line of Sight v Non Line of Sight

A non line of sight capability is essential for mobile systems. It is also useful in some fixed systems because it can avoid the need for collecting points or relays.

There are however serious limitations. Non line of sight operation requires the signals to be transmitted through objects or reflected off buildings in which 99% of the signal can be lost, which results in a 10:1 reduction in range. This effect is universally true and is irrespective of the modulation used.

The other limitation in using non line of sight is that the characteristics of the paths can well change, which means that one day there may be good communications, whereas on another the communications can be lost. This is a typical problem with Wi-Fi and Wi-Max and whilst not a major problem in domestic applications, it could be serious for professional CCTV systems where it is unrealistic to move the equipment.

In summary therefore mobile systems have to use non line of sight systems. In fixed systems however there is a choice and our recommendation is to use a line of sight system if reasonable ranges and a high availability are required. Alternatively if these requirements are not particularly important it may be possible to use a non line of sight system.