Optical rotary encoder！

Is rotary encoder similar to optical Sine/Cosine encoders?
Only in providing high resolution, but there are three main differences: its "holistic"
nature (see technology section), its much lower profile and its significantly simpler,
more durable, construction. Also it has significantly less electric cycles per revolution
Does having a lower number of cycles per rotation imply inferior performance?  
Not necessarily, the relatively low number of EC/Rs adapts it for both low speed and
high speed applications while its near‐ideal Sine/Cosine outputs with low DC offsets,
minimal harmonic distortion, perfect amplitude matching, and extremely high
temperature stability enables higher depth quantization, resulting in very high
resolution and accuracy.
Is the output of the Electric Encoder absolute?
Yes, the output signals convey the absolute rotary (or linear) position without having to
move to an index position.  
How else does the Electric Encoder differ from the optical Sine/Cosine encoder?
Thanks to the relatively low number of EC/R the output signals are confined to the low
end of the frequency spectrum and are less prone to errors due to PWM noise.  
The absence of temperature, or time, degradation mechanisms ‐ ball bearings, LEDs, etc.
virtually eliminate any failure mechanisms.

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