The Engineer's Guide to Standards Conversion - Snell

More documents

Recommendations

Info

The horizontal lines in the drawing are the boundaries between the quantizing intervals, and the curve is the input waveform. The vertical bars are the quantized samples which reach to the centre of the quantizing interval. The quantizing error waveform shown at b) can be thought of as an unwanted signal which the quantizing process adds to the perfect original. If a very small input signal remains within one quantizing interval, the quantizing error becomes the signal. As the transfer function is non-linear, ideal quantizing can cause distortion. The effect can be visualised readily by considering a television camera viewing a uniformly painted wall. The geometry of the lighting and the coverage of the lens means that the brightness is not absolutely uniform, but falls slightly at the ends of the TV lines. Input Output Quantisng error Fig 2.6.2 Quantizing error is the difference between input and output waveforms as shown here. After quantizing, the gently sloping waveform is replaced by one which stays at a constant quantizing level for many sampling periods and then suddenly jumps to the next quantizing level. The picture then consists of areas of constant brightness with steps between, resembling nothing more than a contour map, hence the use of the term contouring to describe the effect. As a result practical digital video equipment deliberately uses non-ideal quantizers to achieve linearity. At high signal levels, quantizing error is effectively noise. As the depth of modulation falls, the quantizing error of an ideal quantizer becomes more strongly correlated with the signal and the result is distortion, visible as contouring. If the quantizing error can be decorrelated from the input in some way, the system can remain linear but noisy. Dither performs the job of decorrelation by making the action of the quantizer 17
unpredictable and gives the system a noise floor like an analogue system. All practical digital video systems use so-called nonsubtractive dither where the dither signal is added prior to quantization and no attempt is made to remove it later. The introduction of dither prior to a conventional quantizer inevitably causes a slight reduction in the signal to noise ratio attainable, but this reduction is a small price to pay for the elimination of non-linearities. The addition of dither means that successive samples effectively find the quantizing intervals in different places on the voltage scale. The quantizing error becomes a function of the dither, rather than a predictable function of the input signal. The quantizing error is not eliminated, but the subjectively unacceptable distortion is converted into a broadband noise which is more benign to the viewer. Dither can also be understood by considering what it does to the transfer function of the quantizer. This is normally a perfect staircase, but in the presence of dither it is smeared horizontally until with a certain amplitude the average transfer function becomes straight. 2.7 Digital Filters Except for some special applications outside standards conversion, filters used in video signals must exhibit a linear phase characteristic. This means that all frequencies take the same time to pass through the filter. If a filter acts like a constant delay, at the output there will be a phase shift linearly proportional to frequency, hence the term linear phase. If such filters are not used, the effect is obvious on the screen, as sharp edges of objects become smeared as different frequency components of the edge appear at different times along the line. An alternative way of defining phase linearity is to consider the impulse response rather than the frequency response. Any filter having a symmetrical impulse response will be phase linear. The impulse response of a filter is simply the Fourier transform of the frequency response. If one is known, the other follows from it. Fig 2.7.1 shows that when a symmetrical impulse response is required in a spatial system, the output spreads equally in both directions with respect to the input impulse and in theory extends to infinity. However, if a temporal system is considered, the output must begin before the input has arrived, which is clearly impossible. 18
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: 2.6 Quantizing error It is possible
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 and 38: Input samples A B C D Sample value
Page 39 and 40: a) Data in Phase select b) FILTER R
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

The Engineer's Guide to Standards Conversion - Snell

Create successful ePaper yourself

Delete template?

Save as template?