So ... umm ... why not leave hi-res mode on all the time? When would "normal" mode be preferable, and is it really most of the time?
Because more often than not, the noise is the thing you're worried about. If High-Res mode destroys the little peaks of noise that, for example, causes your comparator to produce glitch pulses, that's a bad thing! An oscilloscope is a device to display truth, not a device for displaying beautifully thin lines with no respect for the truth. Leaving your scope in Hi-Res mode by default leaves you truly blind to a big part of the picture.
High-resolution acquisition trades off scope bandwidth for an effective increase in vertical resolution.
True, although keep in mind that the bandwidth that is lost does not manifest itself as the visible shape of the curve on the screen, but in the fuzziness of the curve itself.
What I ended up doing was set the scope to HighRes mode.... and.. severely restricting the sample memory to like 16k points. With so few points, the high res mode was able to pull the signal out of the noise to an amazing clarity. Too high a sample memory and the HighRes averaging wasn't as effective.
What kind of scope do you have? My Rigol has completely different behaviour: the ADC runs at the sampling rate as determined by the sample memory (so, exactly like normal), but at
display time all the points that belong to the same column on the screen are averaged together. So if I do a single-shot capture, it looks noise-free, but as I horizontally zoom in (without doing any further captures, just zooming into the original waveform), the averaging is changed to the new size of the pixels, and more noise is revealed. I think this is rather nice, easy-to-use behaviour, because it's very explicitly de-fuzzing the signal in the way that is most appropriate to how you're currently looking at it.
In HiRes mode, what is the relationship between memory depth, scan speed, and number of boxcars that are averaged per pixel. More to the point, what is the range of number of samples/boxcars per pixel?
This is easy to figure out by playing with your scope, and you'd best do that because different scopes behave very differently (see the different between Smokey's scope and my Rigol above). Connect to the 1kHz square wave test signal on your scope, set your timebase to about 1 second per division, and mess with different memory depths, hi-res vs normal mode, and changing the horizontal timebase (both with live captures, and on stored single shot captures). You should be able to make a 5V 1kHz square wave turn in 2.5V DC, but exactly how you achieve that will depend on the scope (on Smokey's scope, changing the memory depth; on my Rigol, simply changing the timebase).
( Also, pedantically, the boxcar is the shape of the convoluting impulse. So you mean the number of ADC samples per boxcar, not the number of boxcars per pixel. )