Chroma Phase Error Correction Circuitry for NTSC PAL Video
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Solution Overview
Problem
Existing video processing systems, such as NTSC and PAL standard systems, face anomalies like color shifts and Hanover Bars due to phase errors between the received color burst signal and locally generated sine and cosine signals during demodulation of composite video signals, which existing techniques fail to address effectively, especially in dynamic color regions.
Innovation Solution
The implementation of chrominance phase error correction circuitry that demodulates the color burst signal into corrected sine and cosine signals, using phase correction circuitry to adjust the demodulating signal and minimize phase differences, thereby ensuring accurate demodulation of chrominance information in both NTSC and PAL systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase locked loop generates cosine and sine signals locked to color subcarrier frequency, then demodulation of chrominance component is achieved, but phase error of 3 degrees remains causing color shifts and Hanover Bars
Solution Approach 1:
The patent implements a feedback mechanism where demodulated chrominance signals are fed back to a phase correction circuit. The phase error is detected from the relationship between the color burst signal and the locally generated sine/cosine signals, and this error information is used to adjust the phase of the demodulating signals. This closed-loop feedback continuously reduces the phase error from 3 degrees to near-zero, eliminating color shifts and Hanover Bars while maintaining accurate demodulation.
2Object-generated harmful factors
If existing techniques average chrominance data for even and odd display lines, then phase error is partially addressed, but U and V components are desaturated and technique only works for constant color regions
Solution Approach 1:
The patent transitions from static averaging techniques to dynamic phase correction. Instead of averaging chrominance data for even and odd lines (which desaturates colors), the system dynamically adjusts the phase of the demodulating sine and cosine signals line-by-line or frame-by-frame. This dynamic adaptation allows the system to correct phase errors without sacrificing color saturation, and it works effectively for both constant color regions and dynamic color changes in the video signal.
3Speed
If color burst signal is locked within 3 degrees of phase, then frequency locking is achieved, but remaining phase error introduces display anomalies
Solution Approach 1:
The patent applies preliminary action by using the color burst signal (which has accurate phase information) to pre-correct the phase of the locally generated sine and cosine signals before they are used for demodulation. The phase correction circuit processes the phase error detection and correction signals in advance, so that by the time the corrected signals are used for chrominance demodulation, the phase error is already minimized. This preliminary correction ensures both rapid phase locking and high phase accuracy, eliminating display anomalies.
Data Source
AI summary
Chrominance phase error correction circuitry includes a demodulator for demodulating a received video color burst signal into first and second demodulated signals and signal generation circuitry for providing to the demodulator a demodulating signal for demodulating video color burst signal. Phase correction circuitry detects a phase error from the first and second demodulated signals and varies a phase of the demodulating signal to provide a corrected demodulating signal for demodulating a video chrominance signal with the demodulator during an active video period.