What confuses some users is that what they DO understand is that a PSW (pure sine wave) inverter that changes 12V DC battery power to 120V AC power is better than a MSW (modified sine wave) inverter for 'sensitive electronics'...which can be anything from a microwave to computer to a c-pap machine.
But gasoline powered portable generators (or frame mount RV generators) have never been either one, they are ALL sine wave generators. Yes, the quality of the waveform varies but it is never square wave, modified square wave, or modified sine wave.
Its just 'sine wave'.
Saying 'sine wave' when talking about generators is referring to a class and style of output waveform, not a statement of purity....UNLESS you include the word 'pure' as in 'pure sine wave'.
Sorta like saying that a golf ball is a 'sphere'. But it's not a 'perfect sphere'.
What can mess up the quality of the sine wave with a non-inverter type of genset is the AVR...this is the module in the generator head that actually controls the voltage of the waveform, and the quality can vary from one genset maker to the next. Most of these gensets have a simple mechanical governor linkage that actually controls the throttle to regulate the RPM, usually at a rated 3600 rpm for portable gensets.
But inverter gensets, which vary widely in RPM but usually have fairly 'clean' waveforms, can also produce a 'dirty' (as used above) sine wave, usually when stressed to their limit, or when they are struggling at high elevations where the engine is sputtering, or when they are running out of gas and the inverter module is struggling to keep up with the load.
But because with either type of generator, the basic waveform is still 'sine wave', so we can operate microwave ovens, air conditioners, radios, lamps, toasters, electric skillets, battery chargers, laptops, circular saws, sound systems, and an entire list of appliances that RVers have been using for decades with non-inverter generators.
Again, the output waveform from either type 'simulates' the power from a household outlet, and most of these common electrical products easily tolerate a little bit of THD, or noise, on the AC signal.