The Engineer's Guide to Standards Conversion - Snell

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3.3 Fractional ratio interpolation In the vertical axis of a 525/625 converter, there is a periodicity in the relationship between the two line structures which means that an output line occurs in one of 21 different places between input lines. This allows the use of an interpolator which is similar to the rate doubler above, but which is capable of computing the value of impulse responses at more places between input samples. As a practical matter it is possible to have a system clock which runs at a common multiple of the two rates. One way of considering the operation of a fractional ratio interpolator is that it may consist of two integer ratio converters in series. This is shown in Fig 3.3.1a). Clearly this is inefficient as many of the values computed in the first stage are discarded by the second. Once more it is more efficient to combine the two processes into a single filter as shown at b). Here only wanted output values are computed. It will be evident that fixed coefficients are not suitable. The location or phase of each output sample varies and Fig 3.3.1c) shows that the filter coefficients must come from a ROM which can be addressed by the required phase. a) 4 X increase 3 X reduce Fig 3.3.1 a) A fractional ratio converter can be thought of as two integer ratio converters in series. b) It is far more efficient to combine the two. Each sample now requires coefficients of a different phase (overleaf). c) A ROM is required as shown which can be addressed by the phase to produce the correct coefficients (overleaf). 35
a) Data in Phase select b) FILTER ROM Data out Coefficients 3.4 Variable interpolation In converters which need to change the aspect ratio, and in motion compensated converters, it becomes necessary to compute sample values which have an arbitrary relationship to the input sample lattice. Thus in theory an infinite number of filter phases and coefficients will be required. This is not possible in practice, and the solution is to have a large but finite number of phases available. The position of the required sample is used to select the nearest available interpolation phase. The ideal continuous temporal or spatial axis of the interpolator is in practice quantized by the phase spacing, and a sample value needed at a particular point will be replaced by a value for the nearest available filter phase. The number of phases in the filter therefore determines the accuracy of the interpolation. The effects of calculating a value for the wrong point are identical to those of sampling with clock jitter, in that an error occurs proportional to the slope of the signal. The result is program-modulated noise. The higher the noise specification, the greater the desired time accuracy and the greater the number of phases required. The number of phases is equal to the number of sets of coefficients available, and should not be confused with the number of points in the filter, which is equal to the number of coefficients in a set (and the number of multiplications needed to calculate one output value). 36
Page 1 and 2: The Engineer’s Guide to Standards
Page 3 and 4: INTRODUCTION Standards conversion u
Page 5 and 6: SECTION 1 - INTRODUCTION TO STANDAR
Page 7 and 8: One solution to large area flicker
Page 9 and 10: A further advantage of digital circ
Page 11 and 12: changing. In short it is a form of
Page 13 and 14: To prevent aliasing, a band limitin
Page 15 and 16: The theoretical vertical bandwidth
Page 17 and 18: 2.4 Kell effect In conventional tub
Page 19 and 20: 2.6 Quantizing error It is possible
Page 21 and 22: unpredictable and gives the system
Page 23 and 24: clairvoyant powers. Shortening the
Page 25 and 26: 2.8 Composite video For colour tele
Page 27 and 28: Line n received with phase error
Page 29 and 30: 2.9 Composite decoding The reason f
Page 31 and 32: Vertical spatial frequency Temporal
Page 33 and 34: A single pixel has meaning over the
Page 35 and 36: 3.2 Line doubling Input samples Out
Page 37: Input samples A B C D Sample value
Page 41 and 42: Vertical frequency Stop band Tempor
Page 43 and 44: Vertical axis Time axis Fig 3.5.4 b
Page 45 and 46: Vertical frequency Unsuppressed = M
Page 47 and 48: a) Drama aperture +525 c/ph Vertica
Page 49 and 50: 3.7 Motion compensated standards co
Page 51 and 52: This is because relative motion res
Page 53 and 54: a) Input fields Interpolation axis
Page 55 and 56: SECTION 4 - APPLICATIONS 4.1 Up and
Page 57 and 58: It is possible to identify the 3:2
Page 59 and 60: Published by Snell & Wilcox Ltd. Du

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