Acronym for "Proportional Pitch Control", a system devised by ARP Instruments as an alternative to the conventional pitch and mod wheels. It appeared on many of ARP's later-production synths, including the Odyssey and Axxe (but not the 2600) starting around 1976. It consisted of a row of three pressure-sensitive pads. Pressing on the rightmost pad bent the pitch upwards, by an amount proportional to the force applied to the pad. The leftmost pad bent pitch downwards. The center pad applied modulation, which on most of the synths that used this system resulted in vibrato. The exact motivation behind the creation of PPC has not been established by historians, but ARP generally did many things to distinguish their products from Moog's (such as using sliders for panel controls instead of rotary pots) for marketing purposes, as well as to try to find better alternatives for the performer. An advantage for synth design and packaging was that the PPC pads, unlike pitch and mod wheels, did not require a lot of depth under the panel.
PPC was mostly disliked at the time it was introduced. Early production examples suffered from lack of consistency in the sensitivity of the pads; some required a lot of force, and some much less. Performers found that it was difficult to achieve and hold precise intervals with the pitch pads. The early ones also had problems with wear. These problems were mostly corrected in later production. Today, the system is generally regarded with somewhat more respect than it was at the time.