A logic gate interprets certain input voltages as high (logic 1) or low (logic 0).
TTL (transistor-transistor-logic): A signal above 2 V is interpreted as logic 1 and a signal less than 0.8 V is interpreted as logic 0. The output voltage ranges between 0-5 V.
HTL (high-threshold-logic): A signal above 3 V is a logic 1 and a signal less than 1 V is a logic 0. The high output signal level is dependent from the supply voltage. Because of the higher voltage difference between logic 0 and 1, the HTL logic is more immune to interference and more resistant against electrical noise.
Mechanical degree is the actual rotation of the shaft in degrees. Electrical degree is used for electrical signals. The required time for completing one alternating voltage/current cycle is defined as 360 electrical degrees (el°). For incremental encoders, one cycle is equal to one complete pulse. With a given PPR the electrical degree can be converted to mechanical degree for any incremental encoder.