> Jerry Avins <jya@ieee.org> wrote in message news:<bm1tfn$n4j$1@bob.news.rcn.net>...
>
>
>>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, as I am an electronics neophyte, I am interested in more
> details about this TV ghost "fix" (this is the sort of thing that can
> make one a legend among friends and relatives)!
> 1. open stub = a wire? What does it connect to?
> 2. what is a downlead? (assume we are talking about TV 'rabbit ears'?)
> 3. where does this terminating resistor attach and what's a rough
> initial value?
>
> Thanks!
I'm talking about an antenna on the roof. The downlead is the wire from
the antenna to set, either 300-ohm twin lead, or in my case, coaxial
cable. A stub is a side branch. It can be connected anywhere, but one
end or the other os usually most convenient. An advantage of coax is
that it can be rolled up. Signals bounce of mismatched terminations
(cable ends.) If the end is open, the voltage bounces back in phase with
the arrival and unattenuated. If the end is shorted, the return is still
unattenuated, but the phase is flipped 180 degrees. When there's a
resistor at the end of the stub that matches its characteristic
impedance -- 300 ohms for twin lead, 75 ohms for coax -- there's no
reflection at all. So you can adjust the timing and amplitude of the
return signal by setting the right length and terminating impedance so
it just matches the negative of the ghost, canceling it. A mismatched
lead-in can create a ghost by itself; that's the easiest to fix.
The length of the stub should be half the time of the ghost. (Apples and
oranges, but lets convert.) The signal travels at about 80% of its
free-space speed on coax; faster on twinlead. One scan line is about
63.5 microseconds. The free-space speed is about 1 nanosecond per foot.
A ghost that's a quarter way down the screen needs about 8 microseconds
one-way delay down the stub, or more cable than you probably have lying
around. But it can be done. (Terminating with reactive components van
make the the stub appear to be longer than it really is.)
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by cb●October 20, 20032003-10-20
Jerry Avins <jya@ieee.org> wrote in message news:<bm1tfn$n4j$1@bob.news.rcn.net>...
> 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, as I am an electronics neophyte, I am interested in more
details about this TV ghost "fix" (this is the sort of thing that can
make one a legend among friends and relatives)!
1. open stub = a wire? What does it connect to?
2. what is a downlead? (assume we are talking about TV 'rabbit ears'?)
3. where does this terminating resistor attach and what's a rough
initial value?
Thanks!
Reply by Paavo Jumppanen●October 16, 20032003-10-16
geschemann@yahoo.fr (Guy Eschemann) wrote in message news:<b9f16a5b.0310151232.433296db@posting.google.com>...
> OK, thanks to everyone who answered my original post. There a two
> things I forgot to mention :
>
> - I don't need to cancel the echo(s), just knowing that the picture
> has one or more echos in it would be enough for now.
>
> - I don't want to rely on a training signal provided by the
> transmitter. All the processing has to be done on the digital video
> stream.
>
> To better illustrate my problem I put a sample picture affected by
> such an echo at the following address :
> http://www.guy-eschemann.privat.t-online.de/echo.jpg
>
> I already tried to compute the autocorrelation function for each line
> (720 samples) of the picture, but wasn't able to detect anything with
> it.
>
> Someone suggested the "mutual information" method. Any thoughts ?
>
> Regards,
> Guy.
Why not try cepstrum analysis. That method can pretty easily deal with
discrete echoes. In the example picture I would expect to see a pretty
large peak in the cepstral domain corresponding to the major delay.
For large delay echoes you could probably get a reliable indication of
ghosting. For shorter delays you'll most likely run into cepstral
overlap issues whereby you can't readily say whether a peak is due to
ghosting or is just an intrinsic part of the unadulterated image.
I guess you could get away with a 1d analysis but 2d cepstrum analysis
would probably give better reliablity.
Regards,
Paavo Jumppanen,
Author of AtSpec : A 2 channel PC based FFT spectrum analyzer
http://www.taquis.com
Reply by Jerry Avins●October 15, 20032003-10-15
Guy Eschemann wrote:
> OK, thanks to everyone who answered my original post. There a two
> things I forgot to mention :
>
> - I don't need to cancel the echo(s), just knowing that the picture
> has one or more echos in it would be enough for now.
>
> - I don't want to rely on a training signal provided by the
> transmitter. All the processing has to be done on the digital video
> stream.
>
> To better illustrate my problem I put a sample picture affected by
> such an echo at the following address :
> http://www.guy-eschemann.privat.t-online.de/echo.jpg
>
> I already tried to compute the autocorrelation function for each line
> (720 samples) of the picture, but wasn't able to detect anything with
> it.
>
> Someone suggested the "mutual information" method. Any thoughts ?
>
> Regards,
> Guy.
The usual correlation probably won't show much, as you found. The ghost
that's readily apparent to the eye is a correlation of waveforms at
different amplitudes, something that pixel-for-pixel correlation won't
reveal. This is essentially a pattern-matching problem. It may be easier
than most because size and orientation are known a priori.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Guy Eschemann●October 15, 20032003-10-15
OK, thanks to everyone who answered my original post. There a two
things I forgot to mention :
- I don't need to cancel the echo(s), just knowing that the picture
has one or more echos in it would be enough for now.
- I don't want to rely on a training signal provided by the
transmitter. All the processing has to be done on the digital video
stream.
To better illustrate my problem I put a sample picture affected by
such an echo at the following address :
http://www.guy-eschemann.privat.t-online.de/echo.jpg
I already tried to compute the autocorrelation function for each line
(720 samples) of the picture, but wasn't able to detect anything with
it.
Someone suggested the "mutual information" method. Any thoughts ?
Regards,
Guy.
Reply by Symon●October 15, 20032003-10-15
Does your format include the sync pulses? For composite video you can
look for correlation between the hsync and the rest of the line,
forwards and backwards. If the ghost delays are bigger than one line,
you need to use the vsync too! With bad ghosting on TV, you often see
a darker vertical stripe on the left hand side of the screen. This is
the ghost of hsync past!
cheers, Syms.
geschemann@yahoo.fr (Guy Eschemann) wrote in message news:<b9f16a5b.0310080818.74a45ed9@posting.google.com>...
> 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.
Reply by Tom●October 15, 20032003-10-15
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
Reply by Tom●October 15, 20032003-10-15
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
Reply by Eric Jacobsen●October 10, 20032003-10-10
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
Reply by Matt Boytim●October 10, 20032003-10-10
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