Pioneer 109D - What's the difference between CAV & PCAV?

I noticed in the discussions about Pioneer 109D, the terms CAV and PCAV are used. I know what CAV is: Constant Angular Velocity. But what does P in PCAV stand for? Thanks in advance.
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falberni wrote:I noticed in the discussions about Pioneer 109D, the terms CAV and PCAV are used. I know what CAV is: Constant Angular Velocity. But what does P in PCAV stand for? Thanks in advance.