Fundamentals - The MPEG - page 43

In some computer graphics systems these extremes represent black and peak white respectively. In television
systems the traditional analog video waveform must be accommodated within this number range. Figure 2.34 (b)
shows how this is done for a broadcast standard luminance signal. As digital systems only handle the active line,
the quantizing range is optimized to suit the gamut of the unblanked luminance and the sync pulses go off the
bottom of the scale. There is a small offset in order to handle slightly misadjusted inputs. Additionally the codes at
the extremes of the range are reserved for synchronizing and are not available to video values.
Figure 2.34: The unipolar quantizing range of an eight-bit pure binary system is shown at (a). The analog input
must be shifted to fit into the quantizing range. In component, sync pulses are not digitized, so the quantizing
intervals can be smaller as at (b). An offset of half scale is used for colour difference signals (c).
Colour difference video signals are bipolar and so blanking is in the centre of the signal range. In order to
accommodate colour difference signals in the quantizing range, the blanking voltage level of the analog waveform
has been shifted as in Figure 2.34 (c) so that the positive and negative voltages in a real signal can be expressed
by binary numbers which are only positive. This approach is called offset binary and has the advantage that the
codes of all ones and all zeros are still at the ends of the scale and can continue to be used for synchronizing.
Figure 2.35 shows that analog audio signal voltages are referred to midrange. The level of the signal is measured
by how far the waveform deviates from midrange, and attenuation, gain and mixing all take place around that level.
Digital audio mixing is achieved by adding sample values from two or more different sources, but unless all the
quantizing intervals are of the same size and there is no offset, the sum of two sample values will not represent the
sum of the two original analog voltages. Thus sample values which have been obtained by non-uniform or offset
quantizing cannot readily be processed because the binary numbers are not proportional to the signal voltage.
Figure 2.35: 0 dBFs is defined as the level of the largest sinusoid which will fit into the quantizing range without
clipping.
If two offset binary sample streams are added together in an attempt to perform digital mixing, the result will be that
the offsets are also added and this may lead to an overflow. Similarly, if an attempt is made to attenuate by, say,
6.02 dB by dividing all the sample values by two, Figure 2.36 shows that the offset is also divided and the