Digital storage oscilloscopes are well known and have many capabilities for displaying waveforms of interest to a user and information related to those waveforms. For example, the envelope of a waveform is sometimes of interest to a user. Oscilloscopes exist which have an operating mode, termed `envelope mode`, in which the minimum and maximum excursions of a signal (termed the envelope of the signal) over a predetermined number of preceding signal acquisition periods is displayed. At each acquisition, the acquired signal is analyzed to determine the new envelope waveform, and that waveform is displayed on the screen. During display of the envelope waveform, the oscilloscope is unable to display the waveform of the underlying signal. In such an oscilloscope, the envelope waveform is occasionally reset, and the signal being monitored is briefly displayed to provide the user an idea of the underlying signal. Such oscilloscopes display the envelope waveform as a fully filled-in waveform. Such a display, however, provides no information concerning the underlying signal.
For example, FIG. 1 is a display screen 2 of a prior art oscilloscope screen displaying the envelope waveform 4 of an underlying signal (not shown). In FIG. 1, the background 5 of the display screen is dark, and the displayed signal 4 is represented by a white display, in the known manner. The signal being monitored has the envelope 4 illustrated in white on in FIG. 1. As can be seen, the fully filled-in displayed waveform 4 gives no indication of the underlying signal, but only displays the minimum and maximum excursions of that signal over the period of time over which the underlying signal is enveloped.
Oscilloscopes also have the ability to rasterize an acquired waveform using sparsely populated vectors with pixels displayed at a reduced display intensity. For example, see U.S. patent application Ser. No. 09/026 185, filed &lt;&lt;filing date&gt;&gt; by Sullivan et al., incorporated by reference herein. By rasterizing using sparse vectors, and with reduced intensity, the rasterizer can process more acquisitions, giving a more accurate representation of the signal being observed.
Though the envelope mode is useful, a user may wish simultaneously to view the underlying signal and its envelope. An oscilloscope which can display the envelope waveform, while simultaneously displaying sufficient acquisitions of the underlying signal to provide an accurate indication of that signal is, thus, desirable.