What is pixel clock rate?
Pixel Clock Rate is the speed at which pixels are transmitted (over an AV / video signal such as HDMI or DVI for example), in order for a full frame of pixels to fit within a single refresh cycle.
The Pixel Clock Rate (or Pixel Clock Frequency) runs on the TMDS of the HDMI / video signal.
The pixel clock defines parameters that determine whether or not a particular resolution/refresh rate can be transmitted over a particular video cable.
What is TMDS?
TMDS is other wise know as Transition Minimized Differential Signaling. It was developed by Silicon Image Inc, a technology for transmitting high speed digital data used in HDMI signal.
What is Horizontal Scan Rate?
Horizontal scan rate is the frequency that horizontal lines are being transmitted/displayed. You can calculate it once you have the refresh rate and the gross number of horizontal lines in the image.