This post is prompted by some discussion on another thread where the question was asked: what is your scope's sample rate at 50msec/div timebase?
The straight forward answer in my case, where my scope has 500k memory per channel, is 1MS/sec giving a nominal bandwidth of 500kHz.
But then it struck me that my scope only displays 500 points on the screen (if persistence mode is turned off) and I realised that I don't know how it selects those 500 points from the 500k it has sampled. If, as I suspect is the case, it just selects from the samples then the effective sampling rate is a 1000 times worse at only 1kS/sec with an effective bandwidth of only 500Hz! (I'm assuming that peak detect is not being used either.)
If this is the case then if you don't use persistence mode or peak detect the fact that extensive memory allows fast sampling will not protect you from seeing aliasing effects on the screen.
Given the drive for high waveforms per second rates I would have thought that only one point per horizontal pixel is used; so are the samples sampled for display?
Or do scopes average in some way every 1000 stored samples to produce one displayed sample - I suspect that they don't because it would introduce an overhead and slowdown the display rate.
It's true that on some scopes, only every
n-th data point is displayed: the DSO just uses decimation (throwing out) of the unneeded sample points. This technique allows the fastest waveform update rates, but as the display data is decimated, important signal details can be lost, because (as you pointed out) the sample rate is effectively reduced.
A much better way is to do a peak to peak assessment: if a number of samples occur within one pixel of the display, then the extreme values are displayed vertically and joined together; then all of the samples per pixel are visible. This, of course, requires more processing - so slower wfrm/s rates.
One other thing I might point out in this discussion that is relevant - but not understood by some DSO users: deep memory is not just important in SINGLE SHOT mode; it has a direct and
important impact on running the scope in NORMAL mode (at slower time base settings) = because the memory length determines the sampling rate.
Look at the two attached images showing the exact same signal at 50ms/div: the first with sample length set to 14k, so the rate is 20kSa/s (thus missing some of the 20us spikes); the second with sample length set to 56MB, so the rate is 50MSa/s.