That is because Keysight uses full memory always but still manages to get high waveform update rates through a clever parallel acquisition system.
No, it doesn't. There is no "clever parallel acquisition system".
which avoids the need for the acquisition system to "wait" for the processing to finish. Only when the acquisition is stopped and the processing of the previous cycle data has completed, the whole memory is then used for the last acquisition, which is the only time the full amount of memory is actually used.
ANd considering the complexity of the different memory allocations it does make sense that the user doesn't get any memory information whatsoever.
Yes it does use all the memory from the user's point of view so what happens under the hood is just semantics.
It's not, it's a technical reality. You can choose to ignore it of course but it's a simple fact. In repetitive mode, only the last acquisition will use the max available memory.
Also, in terms of MegaZoom, the user has no idea how much memory is actually used because the scope shows no information whatsoever.
And there is definitely something clever going on. Think about the situation where a new trigger comes before the entire memory is filled in combination with needing to have pre-trigger data as well.
So what? Nothing really changes except that some part of the sample memory is used for pre-trigger data (as per horizontal delay setting) and the rest for post trigger data, just with the latter being smaller as some part of the memory is used for pre-trigger.
In repetitive mode, the scope will still only use as much memory as needed to fill the screen, and only use the full available memory for its last acquisition before a stop.
You make it sound as if by some magic MegaZoom uses all the memory (for sampling data of the current acquisition) all the time, but this is, plain and simple, wrong. Also, MegaZoom is not magic, it's actually a pretty simple, although clever, idea to maintain very high update rates - something which was valued by people coming from analog scopes.
The PDF you linked to is a simplified explaination for what is really going on under the hood. But for the user it doesn't really matter.
It does matter, because it explains how your tool works, and understanding your tool (and its limitations!) is pretty important for any serious work.
But to some extend I agree, what's going on inside MegaZoom doesn't matter all that much because the user doesn't even have to care about memory, as there is no manual control or even information about memory usage. Which has been the idea behind MegaZoom all the time - designed for users who came from analog scopes, to give them an as much analog-like experience as possible.
But that still doesn't mean the scope is always using full memory (for acquisition data of the current acquisition cycle), and a user should really not assume this to be the case.
Since when does NI make bench scopes?
They kind of do if you buy the modules to build a self contained system.
Even if you put NI Digitizers in a NI PXI chassis and use NI LabView, it's still a Digitizer system where one has to write the software for it and not a general purpose bench scope like the SDS2kX+. Aside from costing an insane amount of money.
So I'd say let's just not go there
Well, you brought up scientific use cases and I disagreed Lecroy didn't have any competition in that market.
Fair enough, but digitizers are not normally used where scopes are, because they are digitizers and not scopes (and even what NI sells as "PXI scopes" are really just PXI digitizers, although with some scope capabilities).
And none of it is competitive with a bench scope for application where a bench scope would suffice. For example, a NI "PXI Scope" which is at least somewhat comparable to the SDS2kX+ in specs (i.e. 8bit ADC, less than 500MHz BW) starts at $7,846 (PXI-5152, 2ch 300MHz, 2GSa/s, 8M/ch). If you need a bench scope then that's a lot of money for the privilege to also have to write your own software (plus to pay for PXI chassis, controller and LabView, which easily lifts this 300Mhz 2ch "scope" over $13k). And it only gets more expensive from there.
Digitizers have their place in specific use cases, for example where very high precision is required (i.e. true 16bit), or where continuous data streams are needed (scopes are generally incapable of streaming data for an extended amount of time). Outside specific niches however, it's real bench scopes hands down.
I told you, just let's not go there
I don't think any of the points which make the WavePro a poor general purpose scope apply to the SDS2kX+ - or any Siglent scope as of today.
Currently the way the memory is managed. I was just curious on why (how Siglent got the idea) the SDS2000X doesn't allow to record outside the screen and much to my surprise (yes, that did surprise me) the Wavepro 7200A works in the same way. Since all other DSOs I have come across so far do allow recording outside the screen without needing work-arounds I take the liberty to assume that this is something Siglent copied from Lecroy.
Just curious, what settings did you use?
But yes, it's quite normal that in repetitive mode and short time bases a DSO will not use all available sample memory.