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VPC3233D Datasheet(PDF) 11 Page - Micronas |
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VPC3233D Datasheet(HTML) 11 Page - Micronas |
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11 / 78 page  ADVANCE INFORMATION VPC 323xD, VPC 324xD Micronas 11 Fig. 2–5: Frequency response of chroma filters 2.3.4. 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 differ- entiated to obtain the demodulated frequency. After the deemphasis filter, the Dr and Db signals are scaled to standard CrCb amplitudes and fed to the cross- over-switch. 2.3.5. Burst Detection / Saturation Control In the PAL/NTSC-system the burst is the reference for the color signal. The phase and magnitude outputs of the CORDIC are gated with the color key and used for controlling the phase-lock-loop (APC) of the demodula- tor and the automatic color control (ACC) in PAL/NTSC. The ACC has a control range of +30 ... –6 dB. Color saturation can be selected once for all color standards. In PAL/NTSC it is used as reference for the ACC. In SECAM the necessary gains are calculated automatically. For SECAM decoding, the frequency of the burst is measured. Thus, the current chroma carrier frequency can be identified and is used to control the SECAM processing. The burst measurements also control the color killer operation; they are used for automatic stan- dard detection as well. 2.3.6. Color Killer Operation The color killer uses the burst-phase/burst-frequency measurement to identify a PAL/NTSC or SECAM color signal. For PAL/NTSC, the color is switched off (killed) as long as the color subcarrier PLL is not locked. For SECAM, the killer is controlled by the toggle of the burst frequency. The burst amplitude measurement is used to switch-off the color if the burst amplitude is below a programmable threshold. Thus, color will be killed for very noisy signals. The color amplitude killer has a programmable hysteresis. 2.3.7. Automatic standard recognition The burst-frequency measurement is also used for automatic standard recognition (together with the sta- tus of horizontal and vertical locking) thus allowing a completely independent search of the line and color standard of the input signal. The following standards can be distinguished: PAL B,G,H,I; NTSC M; SECAM; NTSC 44; PAL M; PAL N; PAL 60 For a preselection of allowed standards, the recogni- tion can be enabled/disabled via I2C bus for each stan- dard separately. If at least one standard is enabled, the VPC32xxD checks regularly the horizontal and vertical locking of the input signal and the state of the color killer. If an error exists for several adjacent fields a new standard search is started. Depending on the measured line number and burst frequency the current standard is selected. For error handling the recognition algorithm delivers the following status information: – search active (busy) – search terminated, but failed – found standard is disabled – vertical standard invalid PAL/NTSC SECAM MHz dB MHz dB |
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