If they can interleave ADCs, they shouldn't have any problem interleaving external RAM. Adding a few extra RAM chips wouldn't add that much cost. Maybe you've looked at the board, but I would guess layout would be the only believable bottleneck with having to get all of those extra traces to and from the FPGA. I think it's not something they really cared that much about, just like with the firmware that I've read complaints about.
If it is so easy everyone do it.
It is extremely easy as long as do not need do it in practice in real life with real components and with all manufacturing realities.
Also, Owon do not use this cheapiest way, interleaved "simply method" ADC's. It use better not interleaved ADC system.
Also Interleaved system can be good but not so that take cheapest slow ADC and drop these to PCB and then some poor clocking system.
Can you explain why Hantek do not have full speed deep capture memory? There is only maximum 125MSa/s in one ADC and or Rigol, there is maximum 100MSa/s in one ADC, why they still have slow memory, why they do not interleave this memory also. (

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Maybe it is better to first design it
and think all things what are related this... then need not tell in every next phases....oops I forget this... then next problem... and agen oops... I forget this... and soon it can find that "I'm not interesting" or find that need do lot of lab works and also price have jumped to exponential rising curve. Well it is not so simple.
It can do, but after then all need do different and sorry but it is not cheap.
maybe it can think that it is easy to copy 10000 dollar RTO to 150 dollar scope. No way.
can you send this 3.2MSa/s scope HW principle to me so that there is also full speed deep 10M or 100M memory with acceptable speed in display refresh rate and/or waveforms/s rate. Even if it is conventional DSO principle. Even better if it is DPO.
How you handle this splitted memory, with what kind of bus and processing system and also do other things. I want see it on the paper.
Tell me even some practical principle what is also easy to implement to cheap production.