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How to detect echos in a video signal ?

Started by Guy Eschemann October 8, 2003
Hi,
I'd like some advice on how to determine if a video signal has some
echos (ghost pictures) in it or not.
What I have is a digital video stream (ITU-656), and I'm looking for
some kind of DSP algorithm (line-based or frame-based) to solve this.
Any suggestions ?

Many thanks,
Guy.
Guy Eschemann wrote:

> Hi, > I'd like some advice on how to determine if a video signal has some > echos (ghost pictures) in it or not. > What I have is a digital video stream (ITU-656), and I'm looking for > some kind of DSP algorithm (line-based or frame-based) to solve this. > Any suggestions ? > > Many thanks, > Guy.
Look for weak correlations in horizontal lines that have the same delay in all lines. (Easier said than done, I know.) Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." Barbara Smuts, U. Mich. ���������������������������������������������������������������������
On Wed, 08 Oct 2003 12:25:45 -0400, Jerry Avins <jya@ieee.org> wrote:

>Guy Eschemann wrote: > >> Hi, >> I'd like some advice on how to determine if a video signal has some >> echos (ghost pictures) in it or not. >> What I have is a digital video stream (ITU-656), and I'm looking for >> some kind of DSP algorithm (line-based or frame-based) to solve this. >> Any suggestions ? >> >> Many thanks, >> Guy. > >Look for weak correlations in horizontal lines that have the same delay >in all lines. (Easier said than done, I know.) > >Jerry
The best system I've seen used a chirp signal embedded in the vertical blanking interval. This allowed some processing in the receiver to build a very good estimate of the channel response for ghost cancellation. Since it changed the signal a bit from the standard transmission it's a little bit of a cheat, but it worked quite well! Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
eric.jacobsen@delete.ieee.org (Eric Jacobsen) writes:

> The best system I've seen used a chirp signal embedded in the vertical > blanking interval. This allowed some processing in the receiver to > build a very good estimate of the channel response for ghost > cancellation.
This is/was the Philips Ghostbuster Echo-cancellation technology. Although I didn't work on it, the small consulting company I worked for did this chip for Philips and the chip and method used by the chip is described in our then-president Neil Weste's book "Principles of CMOS VLSI Design" in chapter 9.3. The chirp signal, called a GCR for Ghost Cancellation Reference, is transmitted on an otherwise unused TV line to train the echo canceller. There are at least three different GCR systems worldwide. One of the key things to notice is that there are "pre-ghosts," that is, the strongest signal may not be the direct path and can instead be a reflected path, so you have look for ghosts in both directions. This makes the problem harder. The Philips technology won several awards for innovation, and it is still used in some Philips professional gear, but cable TV market penetration rates made it less appealing for consumers. I am unsure of it status in consumer gear today. None of this really helps the original poster who said he has a digital CCIR-656 stream with ghosts in it. There is no possibility of a training waveform here and the advice to look for horizontal correlations across lines with similar phase offsets is pretty much the only hope. Perhaps some of the tricks from the video motion-estimation world could help here, but fundamentally without a training waveform you will remove certain visual-frequency objects from the picture with any approach you take. -- Brian K. Ogilvie bko-no-spam-please@ieee.org Please remove the no spam please to reply.
Eric Jacobsen wrote:

 > On Wed, 08 Oct 2003 12:25:45 -0400, Jerry Avins <jya@ieee.org> wrote:
 >
 >
 >>Guy Eschemann wrote:
 >>
 >>
 >>>Hi,
 >>>I'd like some advice on how to determine if a video signal has some
 >>>echos (ghost pictures) in it or not.
 >>>What I have is a digital video stream (ITU-656), and I'm looking for
 >>>some kind of DSP algorithm (line-based or frame-based) to solve this.
 >>>Any suggestions ?
 >>>
 >>>Many thanks,
 >>>Guy.
 >>
 >>Look for weak correlations in horizontal lines that have the same delay
 >>in all lines. (Easier said than done, I know.)
 >>
 >>Jerry
 >
 >
 > The best system I've seen used a chirp signal embedded in the vertical
 > blanking interval.  This allowed some processing in the receiver to
 > build a very good estimate of the channel response for ghost
 > cancellation.
 >
 > Since it changed the signal a bit from the standard transmission it's
 > a little bit of a cheat, but it worked quite well!
 >
 >
 > Eric Jacobsen
 > Minister of Algorithms, Intel Corp.
 > My opinions may not be Intel's opinions.
 > http://www.ericjacobsen.org

Nothing that works is cheating in this context, but it's not always 
possible to get help from the transmitter. I hadn't even considered that 
possibility when I answered Guy.

Eyes are pretty hard to beat with a program. I had a nasty negative TV 
ghost from a nearby water tower. I fixed it by hanging an open stub in 
parallel with the downlead, and trimming its length to move the positive 
ghost it makes onto it. Then the right resistor to terminate the stub 
pretty well cancels it altogether.

Jerry
-- 
"I view the progress of science as ... the slow erosion of the
  tendency to dichotomize."                    Barbara Smuts, U. Mich.
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Jerry Avins <jya@ieee.org> wrote in message news:<bm1tfn$n4j$1@bob.news.rcn.net>...
> Eric Jacobsen wrote: > > > > > Since it changed the signal a bit from the standard transmission it's > > a little bit of a cheat, but it worked quite well! > > > > Nothing that works is cheating in this context, but it's not always > possible to get help from the transmitter.
I agree this isn't cheating - it's just non-blind type. I recall a paper from maybe 20 years ago that described a blind equalizer (ghost canceller) based on reconstruction of the sync pulses. Don't remember the details but I musta found it interesting since it has nothing to do with anything I've ever worked on. I believe it was implemented with analog ccd. Matt
maboytim@yahoo.com (Matt Boytim) wrote in message news:<b90ff073.0310091953.588db0e8@posting.google.com>...
> Jerry Avins <jya@ieee.org> wrote in message news:<bm1tfn$n4j$1@bob.news.rcn.net>... > > Eric Jacobsen wrote: > > > > > > > > Since it changed the signal a bit from the standard transmission it's > > > a little bit of a cheat, but it worked quite well! > > > > > > > Nothing that works is cheating in this context, but it's not always > > possible to get help from the transmitter. > > I agree this isn't cheating - it's just non-blind type. I recall a > paper from maybe 20 years ago that described a blind equalizer (ghost > canceller) based on reconstruction of the sync pulses. Don't remember > the details but I musta found it interesting since it has nothing to > do with anything I've ever worked on. I believe it was implemented > with analog ccd.
Syunichi Ohnishi and Masaharu Obara, "Application of Charge-Coupled Device for Cancellation of TV Ghost Signals", NHK Laboratories Note, Serial No. 226, May 1978
On Wed, 08 Oct 2003 16:53:04 -0400, Jerry Avins <jya@ieee.org> wrote:

>Eric Jacobsen wrote:
...
> > The best system I've seen used a chirp signal embedded in the vertical > > blanking interval. This allowed some processing in the receiver to > > build a very good estimate of the channel response for ghost > > cancellation. > > > > Since it changed the signal a bit from the standard transmission it's > > a little bit of a cheat, but it worked quite well! > >
...
>Nothing that works is cheating in this context, but it's not always >possible to get help from the transmitter. I hadn't even considered that >possibility when I answered Guy.
It's "cheating", and I am taking liberties with the definition, since it only works with compliant transmitters. Requiring a cooperative transmitter rather than a generic one isn't even close to being a general solution, IMHO, so I consider it "cheating", especially if Philips or whoever claims it to be or markets it as a general solution. It won't work for stations or areas where the transmitters aren't compliant. As a specific solution for the cases where the transmit signal is compliant, it's great, and I've seen it demonstrated very effectively. Apparently all this is of little use to the OP, though... :( Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org

Guy Eschemann wrote:

> Hi, > I'd like some advice on how to determine if a video signal has some > echos (ghost pictures) in it or not. > What I have is a digital video stream (ITU-656), and I'm looking for > some kind of DSP algorithm (line-based or frame-based) to solve this. > Any suggestions ? > > Many thanks, > Guy.
Now here is an application of coherence - maybe? Tom

Guy Eschemann wrote:

> Hi, > I'd like some advice on how to determine if a video signal has some > echos (ghost pictures) in it or not. > What I have is a digital video stream (ITU-656), and I'm looking for > some kind of DSP algorithm (line-based or frame-based) to solve this. > Any suggestions ? > > Many thanks, > Guy.
With speech signals which have been reflected off a wall the coherence is less at certain frequencies than it would have been had it not been reflected (ie a direct path). I assume with images there is something similar. Tom