The LED electronic display screen is composed of a plurality of mutually independent pixel points (light-emitting elements). Due to the separation of the pixel points, the control and driving of the light-emitting is determined only in a digital manner. The illumination states of these pixels are controlled synchronously by the controller and driven independently. The true color display of the video means that the brightness of each pixel is separately controlled and is completed synchronously within a prescribed scan time. The large screen is composed of tens of thousands of pixels, which makes the complexity of the system much larger than that of the two-value display large screen, and puts higher requirements on the overall data transmission speed. Setting a regular D/A for each pixel is obviously unrealistic, and you must find a solution that minimizes system complexity and maximizes performance.
It is known from the visual principle that the average brightness perception of a person's pixel point may depend on its light/off duty cycle. That is to say, the brightness can be controlled as long as the pixel lighting/de-duty is adjusted. For the LED electronic display, this means that as long as the number representing the brightness of the pixel is converted into the time (D/T conversion) of the pixel illumination, the D/A conversion of the luminance is realized.
Let the screen data refresh period be, the data controlling the brightness of any pixel point is n-bit binary number
D=bi2i (where bi=0 or 1)
Ton is the illumination time corresponding to D, and the duty ratio of the pixel on/off is:
d=Ton/Ts=D=bi2i
This expression can be implemented with a pre-settable subtraction counter, but a counter per pixel will make the display circuit extremely complex. The above formula is rewritten as: Ton=Tsbi2i, which means that Ton can be divided into several time periods, because when it is small enough, the Ton synthesized in several separate time periods has the same visual effect as the continuous Ton of the same total length. Thus, generally, for n-bit binary data D=bi2i, the Ts is divided into n segments, and the appropriate time division function f(i) is selected such that the i-th segment Ti=Tsf(i), where 0Tibi=Tsf(i) )bi, then?ヤ=Ton/Ts=Tibi/Ts=f(i)bi
That is, the on/off duty ratio of this pixel. Since the function f(i) can be common to all the pixels, the above equation shows that as long as f(i) is used to control each pixel uniformly, it is possible to realize D/ for each pixel of the full screen independently and synchronously. T conversion. The above equation can be implemented with the circuit of Figure 1 for a single pixel. In the figure, the SFR is an 8-bit shift register, and the figure shows the waveform of the time division function f(i).
The large-screen display driver circuit usually adopts a "serial shift + latch + drive" structure in order to minimize the data transfer line. To achieve the above formula on the full screen at the same time, it is only necessary to uniformly control all ST signals by f(i). Of course, the premise of this is to require the shift register to store the same weight in each pixel control data, which can be done by prior data processing.
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