Why does my HD signal play in fast motion, with regular audio on my standard cable?

While watching HD channels on my HDTV with Standard cable, the signal jumps in fast motion, then slows but the audio never changes. It's not over the air or with a converter box so there should be no interference.Why does my HD signal play in fast motion, with regular audio on my standard cable?Most cable systems do not transmit HD over standard cable, but require you to subscribe to digital cable service. There are exceptions, however, but what you are describing is unusual--it may be an encrypted signal.