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
- Invention of GCR Ghost Cancellation Reference signal by Philips.
- FCC allows the use of line 19 of the NTSC vertical blanking interval
- ghost cancelling demonstrated at NAB in Las Vegas, 1993
