2/28/2023 0 Comments Frequency resolution calculatorIn other words, if the boundary was pushed a little further, and no audio was added to the bandwidth, and the active video was allowed to creep into this area of the signal, even more horizontal resolution may be achieved! With the sound carrier at 6.5 MHz was removed this effectively allows for higher horizontal resolution to be achieved. It is useful to know that PAL Standard D at 6 MHz is the highest of the “defined” video bandwidth Standards for PAL systems. It was therefore decided that for PAL Standard I there is no need to use a bandwidth frequency higher than 5.5 MHz, if it is not appreciated by the viewer, as signal bandwidth is a quantity that needs to be minimised.įor PAL Standard D however, they decided in China to settle on 6 MHz, whilst other countries using the PAL Standard decided to settle on 5 MHz video bandwidth. Subjectively equal resolutions (for 625 lines) were obtained for video bandwidths in the region of 5.5 MHz. The reason 5.5 MHz is the defined standard for UK and Ireland has been determined from subjective tests, where viewers, seeing real pictures, were asked to compare horizontal and vertical resolution as the horizontal resolution was varied. Due to the fact the receivers are tuned to a particular standard (sampling 8 MHz to find the information where to the guardband, horizontal blank, start of video, start of sound is located) this cannot be achieved without modifications across the whole of the system. either by lowering the frame rate or 'stretching' the signal baseband of 8 MHz to accommodate a higher spatial video frequency. 7.3 MHz of video will not fit without making changes to the set standards. Therefore, the limiting factor is the baseband of 8 MHz. Of the 6.5 MHz available, PAL Standard I uses 5.5 MHz for video (with sound at 6 MHz) and PAL Standard D uses 6.0 MHz for video (with the sound at 6.5 MHz). That leaves a maximum of 6.5 MHz out of a possible 8 MHz available for the video and sound portion of the signal. The frequency for PAL is defined at 8 MHz for the whole of the signal of whichĠ.25 MHz is a guardband at the beginning of the signal to prevent interference.ġ.25 Mhz is used in the portion for the horizontal blanking period, that resets (or zeros) the components that read the signal, and this includes the front porch, sync pulse and back porch (with colour burst). This maximum Bandwidth of 7.366 MHz does not agree with the defined values used in practice Note: On appearance, this calculation resolves to 766 TV Lines of Horizontal resolution – but does that effectively work in reality? Or to put it another way, the maximum possible temporal frequency (cycles per unit time) is 7.366 MHz. (remember a clock cycle is an interval of time between two pulses – i.e one black line and one white line as depicted in the above image). This time period therefore has to accommodate 383.04 cycles. The aspect ratio of conventional analogue TV is 4:3 therefore the maximum horizontal number of cycles is: 288 x 1.33 = 383.04 There are 576/2 cycles per picture height = 288 cycles per picture height maximum. The maximum spatial frequency (wave number cycles per unit distance) in the vertical direction corresponds to two lines being alternately black and white. So there are 576 active lines per picture. The number of active lines per picture is 625 minus 49 lines due to field blanking / synchronisation, as previously mentioned. We start by assuming that the horizontal and vertical resolutions in the picture should be identical. The bandwidth of a TV signal is determined by the number of picture elements (pixels) necessary to send per unit time. * Visible Line Bandwidth for the PAL (D) Standard = 6 MHz * Colour Clock Cycles per line = 283.75 cycles * Frame Rate / Refresh Rate / Picture Rate = 25 frames per second * Line Frequency = 15625 Hz (625 lines x 50Hz / 2) * Picture Frequency = 50 fields per second (50 Hz) This relationship is used to produce a number of formula to calculate the theoretical TV Line resolution of a camera which I’ll cover in this Chapter.įirst, let us remind ourselves of the characteristics of a PAL Standard. You can see from the previous Chapter that from measuring the active video TV Lines using an oscilloscope, there is a direct relationship between the TV Line value and the spatial frequency of the analogue waveform. This question is hotly debated between CCTV manufacturers, engineers and salesmen!
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