Fundamentals - The MPEG - page 22

Digital signals of this form will be used as the input to compression systems and must also be output by the
decoding stage in order that the signal can be returned to analog form. Figure 2.3 shows the stages involved.
Between the coder and the decoder the signal is not PCM but will be in a format which is highly dependent on the
kind of compression technique used. It will also be evident from Figure 2.3 where the signal quality of the system
can be impaired. The PCM digital interfaces between the ADC and the coder and between the decoder and the
DAC cause no loss of quality. Quality is determined by the ADC and by the performance of the coder. Generally,
decoders do not cause significant loss of quality; they make the best of the data from the coder. Similarly, DACs
cause little quality loss above that due to the ADC. In practical systems the loss of quality is dominated by the
action of the coder. In communication theory, compression is known as source coding in order to distinguish it from
the channel coding necessary reliably to send data down transmission or recording channels. This topic is not
concerned with channel coding but details can be found elsewhere. [ 2 ]
Figure 2.3: A typical digital compression system. The ADC and coder are responsible for most of the quality loss,
whereas the PCM and coded data channels cause no further loss (excepting bit errors).
[ 2 ] Watkinson, J.R., The Art of Digital Audio , third edition, Oxford: Focal Press (2001)
2.5 Sampling
Sampling is a process of periodic measurement which can take place in space or time and in several dimensions at
once. Figure 2.4 (a) shows that in temporal sampling the frequency of the signal to be sampled and the sampling
rate F s are measured in Hertz (Hz), the standard unit of temporal frequency. In still images such as photographs
there is no temporal change and Figure 2.4 (b) shows that the sampling is spatial . The sampling rate is now a
spatial frequency. The absolute unit of spatial frequency is cycles per metre, although for imaging purposes cycles-
permm is more practical.
Figure 2.4: (a) Electrical waveforms are sampled temporally at a sampling rate measured in Hz. (b) Image
information must be sampled spatially, but there is no single unit of spatial sampling frequency. (c) The acuity of
the eye is measured as a subtended angle, and here two different displays of different resolutions give the same
result at the eye because they are at a different distance. (d) Size-independent units such as cycles per picture
height will also be found.
If the human viewer is considered, none of these units is useful because they don't take into account the viewing
distance. The acuity of the eye is measured in cycles per degree. As Figure 2.4 (c) shows, a large distant screen
subtends the same angle as a small nearby screen. Figure 2.4 (c) also shows that the nearby screen, possibly a
computer monitor, needs to be able to display a higher spatial frequency than a distant cinema screen to give the
same sharpness perceived at the eye. If the viewing distance is proportional to size, both screens could have the
same number of pixels, leading to the use of a relative unit, shown in (d), which is cyclesper-picture-height (cph) in
the vertical axis and cycles-per-picture-width (cpw) in the horizontal axis.