The Engineer's Guide to Standards Conversion - Snell

2.2 Aperture effect
The sampling theory considered so far assumed that the sampling clock contained
pulses which were of infinitely short duration. In practice this cannot be achieved
and all real equipment must have sampling pulses which are finite. In many cases
the sampling pulse may represent a substantial part of the sampling period. The
relationship between the pulse period and the sampling period is known as the
aperture ratio. Transform theory reveals what happens if the pulse width is
increased. Fig 2.2.1 shows that the resulting spectrum is no longer uniform, but has
a sinx/x roll-off known as the aperture effect. In the case where the aperture ratio is
100%, the frequency response falls to zero at the sampling rate.
Max
0.64
Level
0 F s
Frequency
2F s
3F s
F b = F s/ 2
11
Fig 2.2.1
Aperture effect. An aperture ratio of 100% causes the frequency
response to fall to zero at the sampling rate. Reducing the aperture
ratio reduces the loss at the band edge.
This results in a loss of about 4dB at the edge of the baseband. The loss can be
reduced by reducing the aperture ratio. An understanding of the consequences of the
aperture effect is important as it will be found in a large number of processes related
to standards conversion. As it is related to sampling theory, the aperture effect can
be found in both spatial and temporal domains. In a CCD camera the sensitivity is
proportional to the aperture ratio because a reduction in the AR would require
smaller pixel area. Thus cameras have a poor spatial frequency response which
begins to roll off well before the band edge. Aperture effect means that the actual
information content of a television signal is considerably less than the standard is
capable of carrying. Fig 2.2.2a) shows the vertical spatial response of an HDTV
camera, which suffers a roll-off due to aperture effect.