Modify the Channel Master CM7000 DTV STB to generate B&W analog for vintage TVs
 

This is a way to modify a digital TV set top box converter to produce a modulated B&W RF analog TV signal to feed to the antenna terminals of your vintage B&W TV sets. Especially the pre-color days of TV (before 1953), which had wide bandwidth video amps wide enough that you'd see chroma checkerboard pattern (at 3.58Mhz, which comes out to 227.5 cycles per horiz line) crawling thru the picture of color TV broadcasts. Digital television broadcast the luma (Y) separate from the chroma (U and V), and most converter boxes downscale HD to 525i and create a composite NTSC luma and chroma to modulate onto channel 3 or 4. Here, in the CM7000 I found where you can intercept the chroma before it's merged with the luma, before it hits the modulator.
http://www.audiokarma.org/forums/att...6&d=1216149040
Looking at the picture, look for the cap arrowed. Near the STV6433 chip (this chip filters and merges luma and chroma to create the composite video output (yellow RCA jack, and also a special composite signal designed to be modulated (group delay pre-distortion and a notch to make room for the sound IF carrier at 4.5MHz) onto a TV channel, which is why I wanted to use this box's intermal TV modulator instead of an external one). What we are going to do is intercept the chroma before this chip, with a switch that will either pass it to this chip, or switch away from the chroma source and switch to ground. I placed this switch before the coupling cap, as the STV6433 has an internal video clamp that charges this cap during the horizontal interval (between scan lines) to give the internal amps inside the chips proper DC biasing. Switching away from the chroma source to ground still gives this charging circuit something to work against.

To do the mod: remove but keep the cap arrowed in the picture. Now stand it upright and solder it to the solder pad it was soldered to closest to the STV6433 chip (see drawing at the bottom of the picture. On the now top end of this cap solder a wire that will go to the center pole of a switch. Another wire soldered to the now unoccupied solder pad goes to one side of this switch, and the other side of the switch goes to ground. Label the switch "color" and "B&W" accordingly.

I'm not sure what this checkerboarding looks like, but I've got what can only be described as "checkerboarding" on this setup:
http://www.audiokarma.org/forums/att...2&d=1212892550

I thought it was an issue with the monitors being surplus and relatively well "used" and maybe developing some sort of problems with caps or whatnot. I would be very curious to see if this mod removes the pattern.

The CM7000 is a little pricey and harder to find than the usual suspects (Zenith, Insignia, DigitalStream, et. al.). Have you located this junction on other DTV boxes? Any plans to look?

The chroma "checkerboard" is a fine pitch pattern (at 3.58MHz) that seems to drift upwards on the screen. 227.5 cycles per horizontal line. The amplitude of this checkerboard is higher in highly color saturated areas, and low to nothing in gray, white or black areas of the video image. Your monitors are probably wide bandwidth enough to show this checkerboard.

Haven't had a chance to dig into other boxes to see if the chroma is easily intercepted or not. Some boxes may create the composite NTSC signal inside a big digital chip, in which case I can't get at it.

Just popped the lid off a Zenith box, and it looks like there is no separate luma and chroma signals, just composite video coming from the decoder chip. So the answer looks to be no.

You know, given that the circuitry, tuner modules, etc, are 'off the shelf', how hard would it be to actually engineer and build a converter box?

I tried to photograph the effect. Had to turn the brightness all the way down (the main screen shown is showing the same channel as the screen to the left of it).
http://www.audiokarma.org/forums/att...9&d=1216662584
The screen left was at comfortable viewing level, the main screen was nearly black. That's what the cheap digital camera needs I guess.

The checkerboard pattern in the light areas -- especially under the 'o' in 'Fort Wayne' -- is representative, I'm pretty sure:
http://www.audiokarma.org/forums/att...1&d=1216662732

unless it's just a jpeg artifact from the digital camera, that happens to look exactly like it. It was practically impossible to photograph. In real life, it's entirely noticeable and annoying. Drifts upwards as described. I'll want to defeat it with wa2ise's mod whenever I get around to my Frankenstein Project (after my current job assignment -- and associated stress level -- ends).

That's the chroma subcarrier you're seeing. Your monitors are in good working order, nothing wrong with them. And my mod to the CM7000 will remove it. Another approach is to just take an S-video cable, and cut it to attach to the luma line new connectors of the sort your monitors requires. S-video is a 75 ohm impedance system, just like composite video, only with the luma and chroma on separate lines. The luma has sync on it, and is exactly the same as black and white video.

Fry's Electronics had the CM7000 on sale for $60, if you live in a town with a Fry's nearby. Seattle, Portland, Silicon Valley, LA, and Austin TX has Fry's. Not sure if that sale is still on or not.

A less satisfactory approach is to insert in the composite video line a trap tuned to 3.58MHz. But that also will mess up the sharpness of the black and white.

nasadowsk, building a homebrew box would probably involve writing code to program the decoder chip, which makes it a rather big project. And that it's mostly surface mount chips makes for more fun...

You can get all the required parts for $60, or net $20 with a coupon. :D

Sorry, couldn't resist -- but I don't know of any place selling decoder chips retail. If available, your choices would involve an RF front end plus either a system-on-a chip (like the Insignia box) or several chips to do RF demodulation, MPEG video decoding, AC-3 audio decoding, scan conversion, and NTSC remodulation. Plus infrared sensor and control micro for remote control and channel selection, on-screen display, etc. If you're not selling it you could get away without captioning and parental control. (Of course, a hobby project might dispense with some of the on-box control functions and do them by connection to your computer). There is a heck of a lot of engineering in that little box!