High resolution color GUI, lots of simultaneous timing-critical and bulk I/O, a bunch of math operations, some sound processing, running a script interpreter,...
Yep, I know exactly what goes into one of those things. However, these days, I wouldn't design one of those around just an MCU.
The SoCs are getting _very_ cheap these days. In quite a lot of cases, you can get a smallish MCU+SoC+DDR for less than the price of MCU+SDRAM.
In this particular use-case, I'd stuff the timing and safety-critical stuff into a small MCU (STM32F030 or low-density STM32F103). In this case, it would be sampling of the inputs, mixing, PCM modulation and fail-safe functions.
In addition to that, I'd stuff something like an AllWinner A20 SoC + 512MB of DDR (A20 can be bought for <$5 in 1-pcs), driving the display, audio, sd-cards, wifi, dealing with model management, etc.
In total, it would be cheaper (BOM-wise) than a F429+SDRAM for example..
( Btw, flew fixed-wing for 10+ years, and rotary for 5+.
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I think we're seeing the lines being blurred right now. MCUs are encroaching on SoCs, horse-power wise, and SoCs are encroaching on MCUs, prize-wise...