The simplest method of recording a two-dimensional display is to
focus a TV camera on the oscilloscope image and record it on a standard
video tape recorder. However, this does not give very good image
quality, partly because of the double conversion from electrical
to visible image, then back to electrical before obtaining the final
visible image, and also because of interaction between the radial
scan lines forming the sector image and the parallel lines used
for TV. Another possibility is to use a movie camera, but film is
expensive and processing it takes time. Furthermore, unless the
movie camera shutter is synchronized to the oscilloscope scan rate,
there will be unsightly flickering or blank bars across the picture.
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| Fig. 9 |
A much better method is to change the sector image into a TV format
by means of a digital scan converter (Fig.
9). This comprises a matrix of electronic memory cells (typically
512 x 512 elements). The image intensity at each point on the waveform
is assigned a specific number by which is it expressed in digital
form on a scale of 0-64 grey levels and its value stored in the
appropriate memory cell. The memory matrix can then be "read" in
any sequence, for example as a series of horizontal lines to form
a TV image, or as vertical lines to enable it to be printed by a
fiberoptics strip chart recorder. Digitizing the image also permits
a certain amount of manipulation, for example addition of alphanumeric
characters, changing contrast, or elimination of low-level "noise".
Images from a scan converter can be transferred directly onto videotape.
For quantitative analysis, selected scans can be photographed or
measured directly from the oscilloscope screen.
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| Fig. 8 |
It is sometimes useful to be able to record M-mode and two-dimensional
images simultaneously. This makes it possible, for example, to analyze
in detail the motions of structures whose precise spatial orientation
is defined by a two-dimensional image. An electronic cursor superimposed
on the display is adjusted to the desired position and the appropriate
B-mode lines are printed on an M-mode strip-chart recorder. Unfortunately,
with a mechanical sector scanner only one such line is available
for each two-dimensional scan field (typically 60 per second) and
the resulting M-mode recording quality is poor. With an electronically
steered system, however, the beam direction can be changed in any
desired sequence. For example, it is thus possible to allocate 500
pulses a second for transmission along a selected axis to form a
good quality M-mode recording, alternating these with pulses whose
direction is changed sequentially to form the two-dimensional image
(Fig. 8).
2004