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VPC3233D Datasheet(PDF) 9 Page - Micronas |
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VPC3233D Datasheet(HTML) 9 Page - Micronas |
9 / 78 page ADVANCE INFORMATION VPC 323xD, VPC 324xD Micronas 9 2.2. Adaptive Comb Filter The 4H adaptive comb filter is used for high-quality luminance/chrominance separation for PAL or NTSC composite video signals. The comb filter improves the luminance resolution (bandwidth) and reduces interfer- ences like cross-luminance and cross-color. The adap- tive algorithm eliminates most of the mentioned errors without introducing new artifacts or noise. A block diagram of the comb filter is shown in Fig. 2–2. The filter uses four line delays to process the informa- tion of three video lines. To have a fixed phase relation- ship of the color subcarrier in the three channels, the system clock (20.25 MHz) is fractionally locked to the color subcarrier. This allows the processing of all color standards and substandards using a single crystal fre- quency. The CVBS signal in the three channels is filtered at the subcarrier frequency by a set of bandpass/notch fil- ters. The output of the three channels is used by the adaption logic to select the weighting that is used to reconstruct the luminance/chrominance signal from the 4 bandpass/notch filter signals. By using soft mix- ing of the 4 signals switching artifacts of the adaption algorithm are completely suppressed. The comb filter uses the middle line as reference, therefore, the comb filter delay is two lines. If the comb filter is switched off, the delay lines are used to pass the luma/chroma signals from the A/D converters to the luma/chroma outputs. Thus, the processing delay is always two lines. In order to obtain the best-suited picture quality, the user has the possibility to influence the behaviour of the adaption algorithm going from moderate combing to strong combing. Therefore, the following three para- meters may be adjusted: – HDG ( horizontal difference gain ) – VDG ( vertical difference gain ) – DDR ( diagonal dot reducer ) HDG typically defines the comb strength on horizontal edges. It determines the amount of the remaining cross-luminance and the sharpness on edges respec- tively. As HDG increases, the comb strength, e. g. cross luminance reduction and sharpness, increases. VDG typically determines the comb filter behaviour on vertical edges. As VDG increases, the comb strength, e. g. the amount of hanging dots, decreases. After selecting the combfilter performance in horizontal and vertical direction, the diagonal picture perfor- mance may further be optimized by adjusting DDR. As DDR increases, the dot crawl on diagonal colored edges is reduced. To enhance the vertical resolution of the picture, the VPC provides a vertical peaking circuitry. The filter gain is adjustable between 0 – +6 dB and a coring filter suppresses small amplitudes to reduce noise artifacts. In relation to the comb filter, this vertical peaking widely contributes to an optimal two-dimensional reso- lution homogeneity. 2.3. Color Decoder In this block, the standard luma/chroma separation and multi-standard color demodulation is carried out. The color demodulation uses an asynchronous clock, thus allowing a unified architecture for all color standards. A block diagram of the color decoder is shown in Fig. 2–4. The luma as well as the chroma processing, is shown here. The color decoder also provides several special modes, e.g. wide band chroma format which is intended for S-VHS wide bandwidth chroma. Also, filter settings are available for processing a PAL+ helper sig- nal. If the adaptive comb filter is used for luma chroma separation, the color decoder uses the S-VHS mode processing. The output of the color decoder is YCrCb in a 4:2:2 format. Fig. 2–2: Block diagram of the adaptive comb filter (PAL mode) 2H Delay Line 2H Delay Line CVBS Input Chroma Input Bandpass Bandpass/ Luma Output Chroma Output Filter Notch Filter Bandpass Filter |
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