Resolution of surveillance cameras
The so-called resolution , in a nutshell , is how many things the camera can see . The definition of resolution for televisions and CCTV surveillance cameras is different from lenses and photos . They have their own methods of definition . For example , lenses, One line of black and white lines is counted as 1 line in the photo , while one line of TV is counted as 2 lines ( one line in black and white ). The difference is that the resolution of the lens and photo indicates how much can be counted within 1mm on average . Line , and the TV shows how many lines can be counted in the entire picture .
The following explains the resolution of CCTV surveillance cameras : camera resolution is divided into vertical resolution and horizontal resolution , vertical resolution is called horizontal resolution , and horizontal resolution is called vertical resolution ( as shown in Figure 1 )
Determination of resolution
Figure 1 The EIA-1956 standard resolution map is being placed , and the horizontal and vertical border marks of the test chart are used to fill the effective space of the display ( under scan ) for video recording , as shown in Figure 2 .
Visual inspection The general measurement method is to see where the wedge-shaped black and white stripes are separated from the black and white monitor. When the black and white cannot be distinguished , the common critical point is the resolution . The monitor and lens must use a machine with a performance that exceeds the camera under test ( Figure 3) Visual inspection must be performed under the following conditions :
Measurement of lighting conditions : the color temperature of the light source is 3100k + 100k; the illumination is 2000lx + 100lx.
Camera setting conditions : Gain is 0dB, AGC OFF; Electronic shutter is OFF; White balance is optimal .
Line selection method The visual method is a general detection method.Another method is to select the corresponding horizontal line of the resolution map and use an oscilloscope to confirm the amplitude of the vertical line width ( black and white ) , that is , how much TVL is used to determine the resolution .
The vertical resolution is determined by the number of scanning lines in the TV system. The PAL system indicates that the effective scanning lines for one frame are 625 lines , and the average field has 20 lines of vertical blanking time . Therefore , the actual picture appears only 585 TVL ( as shown in the figure ). Because CCTV uses a progressive scanning method , the resolution is reduced . Its ratio is called the Kyle coefficient . According to experience , a value of 0.7 is used . In this way , the final vertical resolution HV = (625-20x2) x0.7, about 410TVL Left and right are the limits of vertical resolution . As mentioned above , vertical resolution is determined by the TV system and does not represent the performance of the camera .
This is one of the important items for evaluating the performance of CCTV surveillance cameras . In theory , the horizontal resolution is expressed by the following general formula , which is proportional to the image frequency . The specifications of each index are as follows :
N: TVL , f: image signal frequency, A: aspect ratio (3: 4 = 0.75) , Q: blanking rate = (Q = 0.16) , fh: horizontal scanning frequency (f = 15.625k Hz) , use N = 2fA (1-Q) / fh means that in the PAL system , the value of N = f (MHz) x80 (TVL / MHz) is obtained after substituting each value : the image frequency 1 (MHz) = 80 (TVL / MHz) .According to the above relationship , you can obtain high resolution after increasing the image frequency as much as possible , but the CCD actually used determines the clock frequency and sampling frequency . The sampling frequency when using HI-RESO CCD is 14.18MHz, according to the sampling principle ( for continuous The frequency components contained in the signal are correctly reproduced as sampled data.The sampling frequency must be more than twice the upper limit of the frequency of the continuous signal. ) The frequency of the image signal is limited to about 7MHz.
In addition , if there is a frequency component of the image signal component that exceeds 1/2 of the sampling frequency , a folding phenomenon will occur , and a false signal / moiré image will be impure , which is difficult to refer to ( Figure 6-1) . In order to prevent this phenomenon , It is necessary to use a high-efficiency band-pass filter with a frequency of more than 7MHz . However, at the time, it was actually difficult to manufacture such a filter . Therefore , the image signal frequency became about 6MHz and the horizontal resolution reached 480TVL.