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VDP3108 Datasheet(PDF) 8 Page - Micronas |
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VDP3108 Datasheet(HTML) 8 Page - Micronas |
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8 / 61 page  VDP 3108 ADVANCE INFORMATION MICRONAS INTERMETALL 8 2.2. Color Decoder In this block the entire luma/chroma separation and multi standard color demodulation is carried out. The color de- modulation uses an asynchronous clock, thus allowing a unified architecture for all color standards. Both luma and chroma are processed to an orthogonal sampling raster. Luma and Chroma delays are matched. The total delay of the decoder is adjustable by a FIFO memory. Thus, including the delay of the display proces- sing, exactly 64 µsec processing delay can be obtained. The output of color decoder is YCrCb in a 4:2:2 format. 2.2.1. IF-Compensation With off-air or mistuned reception any attenuation at higher frequencies or asymmetry around the color sub- carrier is compensated. Three different settings of the IF-compensation are possible: flat (no compensation) 6 dB /octave 12 dB /octave Fig. 2–4: Frequency response of chroma IF-com- pensation 2.2.2. Demodulator The entire signal (which might still contain luma) is now quadrature-mixed to the baseband. The mixing frequen- cy is equal to the subcarrier for PAL and NTSC, thus achieving the chroma demodulation. For SECAM, the mixing frequency is 4.286 MHz giving the quadrature baseband components of the FM modulated chroma. After the mixer, a lowpass filter selects the chroma com- ponents; a downsampling stage converts the color dif- ference signals to a multiplexed half rate data stream. The subcarrier frequency in the demodulator is gener- ated by direct digital synthesis; therefore substandards such as PAL 3.58 or NTSC 4.43 can also be demodu- lated. 2.2.3. Chrominance Filter The demodulation is followed by a lowpass filter for the color difference signals for PAL/NTSC. SECAM requires a modified lowpass function with bell-filter characteristic. At the output of the lowpass filter all luma information is eliminated. The lowpass filters are calculated in time multiplex for the two color signals. Three bandwidth settings (narrow, normal, broad) are available for each standard. The filter passband can be shaped with an extra peaking term at 1.25 MHz. dB MHz 0 01 2 3 4 5 –10 –20 –30 –40 –50 dB MHz 0 01 2 3 4 5 –10 –20 –30 –40 –50 PAL/ NTSC SECAM narrow broad normal Fig. 2–5: Frequency response of chroma fil- ters 2.2.4. Frequency Demodulator The frequency demodulator for demodulating the SE- CAM signal is implemented as a CORDIC-structure. It calculates the phase and magnitude of the quadrature components by coordinate rotation. The phase output of the CORDIC processor is differen- tiated to obtain the demodulated frequency. After a pro- grammable deemphasis filter the Dr and Db signals are scaled to standard CrCb amplitudes and fed to the cross- over-switch. |
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