Removing Dot Crawl?

[Penthode]

My old split sound RCA TV provides a full 4MHz video bandwidth. The set was built before color broadcasting and the 3.58MHz dot crawl on color programming has always been especially noticable on these sets (when aligned to manufacturers spec.) because of the wide bandwidth.

It occurred to me that all DVD players and some ATSC DTV over the air set top boxes feature S-Video connectors. This means that a "pure" black and white (luma) only signal is available without the chroma contamination (the dots) to run old black and white wide bandwidth TV sets. Has anyone in this forum considered this?

It is interesting to note that the engineers in the early fifties working through the second NTSC, knew that the introduction of "compatible color" would mean that the interference may be a problem. It was their opinion that the benefits of color would more than offset the loss of a little picture resolution. This may explain why the bandwidth of B&W TV's after the introduction of color in 1953 rarely exceeded 3MHz.

[Eric H]

I use the Green "Y" output connector on a progressive scan DVD player for just this reason.
Hook it to the Yellow Composite input of an RF Modulator and it works fine with no color information.

[wa2ise]

I've modified my Channel Master CM7000 converter box to make it switchable for B&W or color. Inside this box is a luma and chroma filter chip that also merges these together to create the NTSC color signal, and this box also creates the group delay pre-distortion real TV stations used to do, for feeding it to the box's TV modulator.

This shows the use of a switch (I commandeered the channel select switch) to either pass the chroma signal, or to ground that input (B&W mode), on this cap that feeds this chip

[Penthode]

The DVD player of course has a Y output. That is the solution for that. Thanks.

But the solution for the DTV OTA box is way too cool! I suppose the key is finding the NTSC encoder. How did you locate the encoder?

I have a Zenith box and was thinking of simply Googling the devices on the board to find the NTSC encoder. I like the great idea of being able to switch the color on and off.

Thanks, Terry

[doogie812]

Pentode, my first solution would to be to limit the bandwidth of the video IF to 3.0 Mhz. This could be done by detuning or an RC modification to the coupling to the video amp. However; I really like Wa2ise solution of modifying a converter box. You will need one to view off the air broadcast, and use it for a conduit for other video devices. Wa2ise’s solution sounds like the perfect solution to keeping your equipment original and getting the best picture quality.
Supporting the Wa2ise solution; I have a Sony High def projector that only has RGBHV inputs. I use a Samsung HDTV tuner as a scalier to feed RF from a 1984 Sony Laser Disk player to feed the projector. I see the problem but everyone else thinks it is true HD. Doogie

[wa2ise]

Originally Posted by Penthode

But the solution for the DTV OTA box is way too cool! I suppose the key is finding the NTSC encoder. How did you locate the encoder?

I have a Zenith box

Problem with the Zenith and most other boxes is that the NTSC encoder likely resides buried deep inside a chip, where you can't get at the luma and chroma separately. The reason I could modify the Channel Master box was that it had an S video jack. That meant that the luma could be accessed separately from the chroma.

[Penthode]

Thanks for the replys. I do not want to curtail the bandwidth to 3MHz. I have visitors look at my late 40's or early 50's RCA and cannot believe how sharp the picture can be. A properly aligned wide bandwidth receiver shows the extra detail quite well.

Below is a photograph I took from my 8T243 about years ago. It was from an over-the-air analog broadcast directly displayed on the receiver.

I have an Apex DTV box and it too has the S-Video output. Instead of getting into the box, I thought of just separating the luma coax from an S-Video lead and attaching an RCA connector to feed a separate modulator. However, using WA2ISE's solution means that I can use the internal modulator.

[miniman82]

Holy cats, that IS a good picture! My Philco had a picture that sharp, but I don't think the video amp had reolution nearly that good. Then again, it's possible I had the same issues with color info...

[wa2ise]

Originally Posted by Penthode

I have an Apex DTV box and it too has the S-Video output. Instead of getting into the box, I thought of just separating the luma coax from an S-Video lead and attaching an RCA connector to feed a separate modulator. However, using WA2ISE's solution means that I can use the internal modulator.

Don't know how Apex does it, it may be similar to how Channel Master did it.

But if you do use an external modulator, you may be able to change the channel it uses, from the usual 3 or 4 to 2, 5 or 6 by removing the usually 4 legged crystal and installing a regular 3rd overtone crystal cut for 55.25MHz for channel 2, 77.25 for channel 5 and 83.25 for channel 6. I found that crystals for 18.432MHz will also usually work for channel 2, 25.75MHz for channel 5 and 27.75 for channel 6. Depending on how much fine tuning the set's tuner will allow, 27MHz crystals from CB radios may work well enough. This assumes such crystals will oscillate in the channel 5 frequency region.