Information Technology Reference
In-Depth Information
The computer screen has more cycles-per-millimetre than the cinema screen, but in this example has the same
number of cycles-per-picture- height.
Spatial and temporal frequencies are related by the process of scanning as given by:
Temporal frequency = spatial frequency x scanning velocity
Figure 2.5
shows that if the 1024 pixels along one line of an SVGA monitor were scanned in one tenth of a
millisecond, the sampling clock frequency would be 10.24 MHz.
Figure 2.5:
The connection between image resolution and pixel rate is the scanning speed. Scanning the above
line in 1/10 ms produces a pixel rate of 10.24 MHz.
Sampling theory does not require regular sample spacing, but it is the most efficient arrangement. As a practical
matter if regular sampling is employed, the process of timebase correction can be used to eliminate any jitter due to
recording or transmission.
The sampling process originates with a pulse train which is shown in
Figure 2.6
(a) to be of constant amplitude and
period. This pulse train can be temporal or spatial. The information to be sampled amplitude- modulates the pulse
train in much the same way as the carrier is modulated in an AM radio transmitter. One must be careful to avoid
over-modulating the pulse train as shown in (b) and this is achieved by suitably biasing the information waveform
as at (c).
Figure 2.6:
The sampling process requires a constant-amplitude pulse train as shown in (a). This is amplitude
modulated by the waveform to be sampled. If the input waveform has excessive amplitude or incorrect level, the
pulse train clips as shown in (b). For a bipolar waveform, the greatest signal level is possible when an offset of half
the pulse amplitude is used to centre the waveform as shown in (c).
In the same way that AM radio produces sidebands or identical images above and below the carrier, sampling also
produces sidebands although the carrier is now a pulse train and has an infinite series of harmonics as shown in
Figure 2.7
(a). The sidebands repeat above and below each harmonic of the sampling rate as shown in (b). The
consequence of this is that sampling does not alter the spectrum of the baseband signal at all. The spectrum is
simply repeated. Consequently sampling need not lose any information.
