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TDA9178 Datasheet(PDF) 8 Page - NXP Semiconductors |
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TDA9178 Datasheet(HTML) 8 Page - NXP Semiconductors |
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8 / 36 page  1999 Sep 24 8 Philips Semiconductors Preliminary specification YUV one chip picture improvement based on luminance vector-, colour vector- and spectral processor TDA9178 Smart sharpness controller The smart sharpness controller (see Fig.16) is a fader circuit that fades between peaked luminance and step-improved luminance, controlled by the output of a step discriminating device known as the step detector. It also contains a variable coring level stage. The step detector is basically a differentiator, so both amplitude of the step and its slope add to the detection criterion. The smart sharpness controller has four user controls: • Steepness control, performed by the I2C-bus DAC: bits SP5 to SP0 • Peaking control, performed by the I2C-bus DAC: bits PK5 to PK0 • Video dependent coring, switched on or switched off by the I2C-bus bit VDC • Coring level control, performed by the I2C-bus DAC: bits CR5 to CR0. The steepness setting controls the amount of steepness in the edge-correction processing path. The peaking setting controls the amount of contour correction for proper detail enhancement. The envelope signal generated by the step improvement processor modulates the peaking setting in order to reduce the amount of peaking for large sine wave excursions. With video dependent coring, it is possible to have more reduction of the peaking in the black parts of a scene than in the white parts, and therefore automatically reducing the visibility of the background noise. The coring setting controls the coring level in the peaking path for rejection of high-frequency noise. All four settings facilitate reduction of the impact of the sharpness features, e.g. for noisy luminance signals. COLOUR DEPENDENT SHARPNESS The colour dependent sharpness circuit increases the luminance sharpness in saturated red and magenta parts of the screen. Because of the limited bandwidth of the colour signals, there is no need to increase the high frequencies of the colour signals. Instead, the details in the luminance signal will be enhanced. In this circuit a limited number of colours are enhanced (red and magenta). Contrary to normal peaking algorithm, extra gain is applied for low frequencies (2 MHz at 1fH). This is needed, because the information that is lacking below 2 MHz (at 1fH) is most important. In large coloured parts the normal peaking is still active to enhance the fine details. The smart peaking algorithm has been designed such that the luminance output amplitude will never exceed 110% of the luminance input signal amplitude. Therefore the normal peaking range (12 dB) will be reduced at large transients, and in case of colour dependent sharpness there is even more reduction. However, by setting bit OSP (Overrule Smart Peaking) one can undo the extra peaking reduction in case of colour dependant sharpness. It must be emphasized that setting OSP may lead to unwanted large luminance output signals, for instance in details in red coloured objects. COLOUR TRANSIENT IMPROVEMENT The colour transient improvement circuit (see Fig.17) increases the slope of the colour transients of vertical objects. Each channel of the CTI circuit basically consists of two delay cells: an electronic potentiometer and an edge detector circuit that controls the wiper position of the potentiometer. Normally the wiper of the potentiometer will be in position B (mid position), so passing the input signal B to the output with a single delay. The control signal is obtained by the signals A and C. When an edge occurs the value of the control signal will fade between +1 and −1 and finally will become zero again. A control signal value of +1 fades the wiper in position C, passing the two times delayed input signal to the output. A control signal of −1 fades the wiper in position A, so an undelayed input signal is passed to the output. The result is an output signal which has steeper edges than the input signal. Contrary to other existing CTI algorithms, the transients remain time correct with respect to the luminance signal, as the algorithm steepens edges proportionally, without discontinuity. SCAVEM A luminance output is available for SCAVEM processing. This luminance signal is not affected by the spectral processing functions. Colour vector processor The colour processing part contains skin tone correction, green enhancement and blue stretch. The colour vector processing is dependent on the amplitude and sign of the colour difference signals. Therefore, both the polarity and the nominal amplitude of the colour difference signals are relevant when using the colour vector processor facility. |
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