JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards
Section: Broadcast Systems
Chapter: Wireless Channels
Section: Multipath Fading, Delay Spread


Ghost Cancellation in Television Reception

Effect of multipath on reception

Multipath reception causes copies of the image to appear on screen, with a right shift proportional to the excess delay of reflection. Ghosts can appear due to multipath reception of wireless TV signals, but also because of improper cable terminations in cable distribution systems.


Figure: TV screen with ghost caused by multipath reception against mountains in Austria . Photo courtesy J. Tichelaar, Philips Research.

Exercise

The Austrian broadcaster ORF in the above image uses 50 Hz PAL transmission (625 lines).
Estimate the impulse response of the channel.

Ghost Cancelling and Equalization

A ghost cancelling training sequence (GCR signal) is transmitted in the vertical blanking interval of an NTSC, PAL or Secam signal. A digital channel equalizer is used to cancel to the ghost images.


Figure: TV screen after ghost cancelling caused by digital equalization. Photo courtesy J. Tichelaar, Philips Research.

History



JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz, 1993, 1995.