The TV lines value is derived from the number of horizontal pixels – it measures the number of vertical lines that can be resolved across a TV screen. It is a popular method of measuring analogue equipment as the resolution of tubed cameras and monitors tends to be poor at the corners. Pixels are perhaps a better measurement as it describes the overall detail recorded or displayed.
Nonetheless, both measures of record resolution are widely quoted and generally accepted as the measure for image quality, the belief being that the more TV lines and pixels the greater the image quality will be.
The assessment of image quality would therefore seem to be a simple question of looking at the quoted figures on the datasheet. However, it is not quite as cut and dried as that. First, it is important to understand how digital multiplex record systems process the camera images and identify any new factors that can affect image quality. Second, the value of record resolution as quality definition needs to be looked at in a little more detail as, strictly speaking, it is a measure of quantity and not quality.
The digital difference
Digital multiplex recorders differ to traditional multiplexer and VCR systems in that they digitally compress, or reduce, the size of the camera images and record them to computer storage media such as hard disk, CD ROM or digital tape.
Compression is used by all CCTV digital recording systems, as uncompressed video requires such a large amount of storage space it would not be practical. To explain compression in simple terms, it works by removing the redundant data from an image that the human eye cannot pick out and thus reduces the size of the video data to be stored. Compression is used along with camera record rate to manage the amount of information to be stored. There are also different types of compression, and these affect the quality of digital images.
Digital compression methods
Most CCTV manufacturers use industry-standard compression formats such as JPEG, MPEG, H.261 and H.320. This means that images captured using these compression formats tend to be more transportable and are more likely to be viewable over standard networks using existing PCs and software. Another widely used compression format is Wavelets. This is what is known as a proprietary method as it is not an internationally accepted standard.
Each method of compression offers different image 'qualities'. Broadly speaking, these are: MPEG Originally developed for video conferencing and video phones. It works by storing or transmitting a complete image after which only the moving parts are updated. It may therefore exhibit a 'blocky' effect when there is a lot a movement.
JPEG Developed by the graphics industry where high-definition images are important. It uses a uniform compression on all areas of the image and is generally accepted to have good colour and tonal gradation.
Wavelets These divide the image into high- and low-definition areas, applying more compression to the low-definition areas. The advantage is that it makes it easier to pick out edges. The disadvantage is that it tends to have poor tonal gradation. Low-definition areas such as flat surfaces tend to be compressed more, giving a 'vaseline' effect.
When it comes to making a choice, you should remember that no two manufacturers' implementation of any one standard will be the same and so should be examined on their own merits. The other important factor is whether or not you want to go for a graphics and computer industry standard, such as JPEG and MPEG, or the non-standard proprietary method. At the end of the day, image quality is subjective and depends on what you find acceptable.
A trade-off needs to be made between image quality and the cost of digital storage
Measuring max record resolution
It is advisable to pick a digital recording system with the highest maximum record resolution, as this will give a much better chance of getting a good quality image once it has been compressed and stored.
The figures on the datasheets do need to be checked though, as many manufacturers' quoted figures refer to the live image resolution and not the recorded resolution, which is often less. Make sure you ask the question and establish exactly what you are looking at – after all, it is the recorded image that will be used for evidence and identification.
Be wary of using just pixels and TV lines as an absolute measure of quality: they simply describe the maximum quantity of information gathered and displayed; they do not actually define the quality of the image reproduced. The overall quality of the hardware is the primary factor in determining image quality.
A system can have a high number of TV lines and pixels, but the quality of the image comes down to the quality of the video introduced to the compression process. Badly designed hardware will introduce 'noise' to the video, which in turn creates errors in the compression process.
Once you have chosen the system with the maximum, uncompressed image quality, the actual quality of the recorded image on digital systems comes down to two, user-definable settings: the compression rate and overall camera record rate.
The less compression that is applied, the higher the quality of each image will be. Similarly, just as with traditional time-lapsing, the more images recorded, the greater the overall visual quality will be.
However, a trade-off needs to be made between image quality and the cost of digital storage, as the higher the quality of images and rate of record the more storage media is required. Deciding on the trade-off will vary from project to project and really comes down to how much the end user is willing to spend on storage, which in turn determines the recorded image quality.
What is the answer?
Good quality digital recording is determined by far more than the resolution quoted on the datasheet. Probably the best way to ensure that you get the right system for your customer is to compare the results of the recorded images produced by various manufacturers' systems for yourself.
A quick test is to check the quality of the live images on screen – high-standard live images means that the images used to form the recordings will also be of a high standard. However, there is no substitute for having a thorough demonstration of the playback from a range of different products.
Source
Security Installer
Postscript
By Nicola O'Dwyer, Marketing Manager of Dedicated Micros.