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This article was more relevant when analogue video cameras were all the rage. Nowadays we have digital sensors in our digital cameras and resolution refers more to the horizontal x vertical storage and display resolutions of the digital video. However, analogue cameras are still common in security video so this article is still useful. When selecting a video camera we are most interested to know how sharp the picture is going to be and to make an informed decision we need some way of making a comparison. The sharpness of a video picture is directly related to its bandwidth, which strictly speaking, is the difference between the lowest frequency (usually 25Hz for PAL or 30Hz for NTSC) and the highest frequency that the device can pass without any significant loss of detail. In other words we want to know the frequency response. In the case of a video device with inputs and outputs, we can simply inject test signals and read the results off a calibrated display. But what do we do with a camera which only has an optical input or a TV monitor which only has an optical output? A method was devised to quantify bandwidth (or sharpness / resolution / definition) as a simple figure of merit referred to as "Lines". The more lines, the wider the bandwidth / the sharper the picture / the finer the resolution / the better the definition... you get the idea, a bigger number is better. OK, so what are these mystical lines? Well, to start with "Lines" was a very unfortunate choice of words. As you probably know, a colour video picture is made up from a series of regularly spaced horizontal lines which are modulated to produce light and dark shades of red, green and blue to fool our eyes into seeing a rectangular picture with a whole rainbow of colours. In the PAL system there are two interlaced fields of 312.5 lines each which effectively gives us 625 lines per frame. The 50Hz field rate reduces the severe flicker we would experience from a 25Hz frame rate (there are two fields per frame). The NTSC system consists of two interlaced fields of 262.5 lines each yielding 525 lines per frame at a 60Hz field rate (30Hz frame rate). These lines are NOT the lines we are talking about when we are talking about sharpness / resolution / definition! The numbers of these horizontal lines CANNOT and DO NOT change! PAL has 625 horizontal lines and NTSC has 525, end of story! Now that we have that out of the way, let's continue. In the PAL system, these horizontal lines are 64uS
(64 microseconds) long, but 12uS are not useable for video because they contain the synchronisation and blanking pulses, so there are 52uS left for video (and we don't get to see all of that due to overscan). Let's say we have a video picture made up from dark and light vertical lines. If we look at such a video signal on an oscilloscope we will see a square wave which shows us that a single vertical "Line" is either a dark or light one. Basically, if there were 104 lines, there would be 52 of these dark/light pairs in the 52uS available, therefore each pair would be 1uS wide, equalling 1MHz (52/.000052 = 1,000,000), so 104 lines would be equal to 1MHz. VHS vcrs were often quoted as having a resolution of 240 lines (120/.000052 = 2.3MHz), and S-VHS vcrs were often about 400 Lines (200/.000052 = 3.84MHz) and so on. How is the number of lines measured? 1. In the case of a camera an accurate printed test chart is photographed to fill the visible area. The video output from the camera is viewed on a waveform monitor and the resolution determined by comparison with calibrated markers on the test pattern. 2. A monitor is fed with a test pattern with calibration markers embedded and where the pattern becomes a mushy grey the frequency is beyond the equipment’s ability to resolve it into discernible Lines. 3. A VCR was tested with a playback tape to determine the frequency response, then a record / play test was performed. The result in MHz can easily be converted to Lines mathematically. So to recap, Lines (of resolution / etc) has nothing to do with how many horizontal lines the picture is made up from, because for PAL or NTSC that can't change. It is determined by observing how many discernible vertical lines are able to be reproduced by the device under test. Lines (of resolution / etc.) is just another way of expressing bandwidth or frequency response that is applicable to opto - electronic and electronic video devices so their frequency response can be compared. (c) 2000 Quest Electronics abn 99 064 323 255 Test chart fragment from
Analogue Video Resolution - Lines vs MHz