UBC Theses and Dissertations
Cancellation/reduction and measurements of some impairments in cable television pictures Lau, Jose Alberto
As technology improves, customers' expectations become higher every day. With such technologies as the Super-VHS video recording and laserdisc player, customers compare these against the quality of the video received over the cable system. Distortions in the image are mainly caused by the nonlinearity of the underground cascaded amplifiers in the cable TV system. Moreover, the addition of new channels on the congested transmission system will increase present interference levels or create new interferences causing picture quality degradation. There are two approaches for combatting impairments. First, these can be prevented by modifying hardware components in the cable TV system such as amplifiers, converters among others. The other approach is to deal with the distortion itself through the use of image processing and filtering techniques. The idea of this approach is, if not to totally cancel the distortion, to reduce it until it becomes less visible to the human eye. This thesis deals with the second approach that consists of reducing/cancelling and measuring impairments such as the single frequency intermodulation, narrowband and wideband CSO (Composite Second Order) distortion. This would complement the automatic detection for a complete monitoring process of impairments. Our approach would fit in an automatic system since it employs non-intrusive methods that do not involve the use of a test signal and hence, no interruption in the transmission would be needed. In this thesis, several filtering methods for cancellation and reduction of single frequency intermodulation, narrowband and wideband CSO are studied and compared. Among these methods are the FIR Linear Filtering, Interpolation in the frequency domain and the Alpha Trimmed Filter. Results show that a combination of the last two methods reduces the single frequency intermodulation considerably. In addition, results also show that the interpolation in the frequency domain perform better than the other methods for narrowband CSO cancellation and reduction. In the case of wideband CSO, a combination of the interpolation in the frequency domain and the Alpha Trimmed filter reduces the impairment considerably. In the area of SNR measurements, different algorithms are proposed for each impairment. For the single frequency intermodulation, the interpolation in the frequency domain algorithm provides acceptable SNR measurements of less than 0.5 dB margin error. For the narrowband CSO, algorithms consisting of the interpolation in the frequency domain and the block averaging are proposed. These two algorithms provide acceptable results. For the wideband CSO, several algorithms that involve the one-dimensional and two-dimensional Discrete Fourier transform are proposed. The two-dimensional algorithms provide more accurate results than the one-dimensional approach giving a margin error of less than 2 dB for SNR measurements of up to 53 dB.