There is some confusion in the marketplace as which technology is most appropriate for different applications. While there is a lot of marketing hype about IP technology the fact remains that for many applications the “old fashioned” AM, FM or even Digital technologies offer anything from very adequate performance to stunning performance at extremely competitive pricing. The following attempts to give a flavour of the trade offs between the main technologies available today.
Amplitude Modulation (AM) Systems
AM is a very simple technology which can provide extremely good performance for many situations. It is easy to get wide bandwidths (at least 15MHz for our OSD381/383 pair, much more for some specialty products) without excessive circuit complexity. Also, over low optical loss budgets typical of most security type CCTV which only occasionally operates over more than 2 to 3 km the signal-to-noise ratio attainable with AM can be very good. This does degrade from some maximum value (typically 55 to 70dB measured over a short fiber length) with increasing fiber length (and loss), so some care is required to ensure that your optical loss budget is under control. However, linearity as measured by parameters such as differential gain and phase, while perfectly adequate for single hop CCTV links is not usually good enough for systems where 3 or 4 links may have to be put in for the series. It can be difficult to achieve the linearity obtained with multimode AM systems on singlemode AM links because the optical devices required for the latter are usually not as linear as those for multimode. Typically, OSD’s AM products can be considered very mature and are unlikely to be developed much further. Having said that, several of them such as the OSD365A micro-miniature video transmitter and the OSD553 triple receiver cards continue to be very popular owing to their attractive combination of performance, cost and reliability.
Frequency Modulation (FM) Systems
FM has two key advantages over AM.
- Performance is not so strongly dependent on optical loss: the signal to noise ratio will degrade slowly with increasing loss to a threshold point after which it degrades very quickly.
- Performance is not dependent on the linearity of the optical components since it is almost completely dominated by the FM modulator and demodulator.
This improvement comes at a small cost of an increase in circuit complexity and cost and a large increase in fiber bandwidth required. Generally the latter is not too critical but must be factored in if you are considering long runs on multimode fiber (e.g. greater than 10 to 20km) due to modal bandwidth limitations or even longer runs on singlemode fiber (eg greater than 30 to 100km) due to material dispersion effects. OSD’s application engineers can advise on the best design choices in these cases.
Typically FM systems offer good SNR (60 to 65dB) which is maintained over substantial optical losses and good linearity which is typically 2 to 3 times better than AM. Consequently, multiple loops become more practical.
However, FM occupies the middle ground between AM and Digital systems in terms of performance but is fairly comparable with Digital in terms of pricing. Consequently, in new systems it tends to be mostly used in multimode systems operating over several kilometers where Digital is not technically practical because of the great demands it places on fiber bandwidth.
“Digital” in this context refers to uncompressed digital transmission systems where the analog signal is digitised in an Analog to Digital Converter (ADC) and transmitted in serial form over the fiber at data rates from 100Mbps for fairly low performance products up to more than 300Mbps for high end products. The system video performance is dominated by the analog to digital conversion at the transmit end and the digital to analog conversion at the receive end. The actual transmission path will have virtually no visual effect on the picture if the Bit Error Rate is better than 1 error in 1 billion bits (ie a BER of 1 X 10-9). In fact, BERs down to as low as 1 X 10-4 still allow quite useable signals to be received and monitored on screens.
These excellent characteristics come at the cost of:
- Increased circuit complexity and cost.
- Greatly increased transmission bandwidth requirements.
The basic SNR is determined by the number of quantization levels, which are in turn fixed by the number of bits per digital word. Typically systems use between 8 and 12 bits leading to weighted SNR values of between about 56dB for 8bits and >70dB for 12 bits.
Be careful about the headroom allowed in digital systems: typically if you exceed the nominal level of AM and FM systems some distortion takes place but it may not be obvious. Exceeding the limit in a digital system results in fairly obvious limiting effects. Unlike its competitors, OSD includes an extra 3dB headroom in its input circuits to allow for out-of-specification cameras, matrix switches and distribution amplifiers which are often exist in the real world. We achieve this by using ADCs which are specified at a very high level: for example our 10 bit products all use 12bit ADCs, even though we only transmit 10 bits over the fiber, to ensure true 10bit performance. Likewise, our 9-bit products use 10 bit ADCs.
As noted earlier, digital systems require far more bandwidth than FM or AM systems. For example, OSD’s 4 channel OSD870, 880 and 890 systems offer studio quality 10MHz video performance and the OSD860 8-channel system offers 6MHz bandwidth at a slightly lesser performance level with all of them operating on the fiber at 1.35Gbps. This significantly reduces the distances achievable in multimode fiber: contact OSD Application Engineering for advice when using multimode or when transmitting more than 50km on singlemode fiber.
Internet Protocol (IP) Systems
The previous three technologies are often lumped under the generic term “analog systems” whereas systems based on IP transmission are often called “digital systems” so there can sometimes be confusion as to what people are referring to when talking about digital systems.
IP solutions typically involve the digitisation of an analog video signal then using some sort of compression to reduce the hundreds of megabits per second transmission rates referred to above down to data rates that can be easily transmitted over relatively low bandwidth media such as the Internet, corporate local networks or even leased lines. The speeds available range from less than 10kbps to well over 10Mbps. Clearly, the lower the speed the more compression is required and the lower the ultimate quality is likely to be. There are many so-called “IP Cameras” available which are basically an analog camera with built-in digital compression which typically uses MPEG4 or H.264 encoding. Most (not all) of these tend to have fairly average quality so the second way IP systems are implemented is to use a standard analog camera with a standalone encoder which, again, is usually MPEG4 or H.264.
The very best IP systems provide excellent subjective performance although very few actually measure very well (Please see “Video Quality”). However, the main driver behind these systems is not picture quality but system convenience and control. The fact that most organisations have an Ethernet Local Area Network (LAN) somewhere in the background means that (theoretically) one can place a camera wherever there is access to the LAN and connect it back to the security control center without the need to install any dedicated CCTV cabling. Many possibilities arise once the pictures are on the network:
- Storage on multiple Network Video Recorders (NVR) is possible anywhere in the world
- Multiple display sites are possible
- Integration of CCTV with other aspects of security and organisational management such as access control, perimeter protection, building management systems, etc becomes practical
Despite these features, as of 2010 only about 30% of CCTV systems worldwide are IP based with many users preferring to maintain an analog camera and transmission system (although these will often employ uncompressed digital video systems such as OSD’s OSD8XX and OSD8XXX product ranges) and then perform all IP related functions at the security control center, usually via multi-channel Digital Video Recorders (DVR).
There is no question that with continual improvements in IP technology (LAN speed, compression techniques, storage technologies, etc) a majority of CCTV systems will be IP based by 2015.
Most CCTV systems over the next several years will rely on so-called “analog” transmission technology whether it be AM, FM or Digital with a growing proportion becoming all IP.
The transmission can be AM in many simple applications because of its good performance at cost effective pricing. AM can also be considered where long lengths of multimode cable must be covered.
Many multimode fiber based commercial and professional applications can be well served by FM based systems which can offer excellent video, audio and data performance but, in general, digital systems would be better, particularly if singlemode fiber is being used.
Overall, OSD would recommend digital systems for most applications where high performance or long distances are involved. Digital systems can also be used over multimode fiber but be careful to consider multimode’s bandwidth limitations.
Finally, don’t hesitate to contact OSD if you would like some advice on any of these topics.