Analog Angle Article Blog

Saving my analog TVs, or why standard interfaces and a well-defined signal flow are such good things

I recently upgraded my over-the-air analog CRT TV to digital mode, using one of those basic converter boxes ($60 less $40 coupon = $20, a good deal!). [Read David Carey's teardown of a typical converter box here.] The hookup, via the baseband outputs (video, audio left, audio right), was not quite a trivial plug-and-play process, and the setup screens were somewhat confusing, but it's done and working well.

Then I used lessons learned from the process to set up another converter box for a 2-inch LCD unit we use upstairs, mostly as a TV-sound receiver. Let's be honest, for many TV shows, the sound is more than enough; the famous director Alfred Hitchcock supposedly said that TV is just radio with moving pictures.

Without the lessons learned and hardware of the larger TV, I would not have been able to connect and setup the smaller unit. Here's what I did:

  • Since the screen of the 2-inch unit was too small to read, I set up its converter box using the CRT.
  • I also tested the Channel 3 (non-baseband) output of the 2nd box, and set this 2nd converter for that output port.
  • I built a small UHF loop antenna, added a 300/75-ohm, balanced to single-ended transformer/balun and, using this setup, verified that the antenna and received signal strength was sufficient to drive the converter box
  • Then, I took the 2nd converter box and its loop antenna to the small TV, and connected the center conductor of the 75-ohm coax to the TV's external whip antenna, without using the coax's ground. The picture was barely there, many channels did not come through at all. But , since I had verified the box and the antenna independent of the small TV, I was pretty sure that the problem was inadequate RF signal strength coming in to the small TV, due to poor and ungrounded coupling.
  • I opened the small TV, found its internal ground, and connected the coax ground. And now, everything worked fine.

Best of all, the final arrangement is even better than the old tiny TV, since that 2-inch TV has crude, PLL-based, VHF/UHF scanning tuning. In contrast, the converter box as the tuning front-end provides direct entry of the desired channel, which is easier to use, always “spot on”, and allows random access. I'm a happy camper! I've got an improved mini-TV (or TV-audio receiver) with digital tuning, for an outlay of $20.

Next, I may even dispense with the small TV altogether, and instead connect an amplifier and speaker to the audio output of the converter box, so it becomes a real sound-only TV/radio with digital tuning. Back to the future, in a way. . . .

The point here is not that I solved a tough technical problem; actually, this was a very modest challenge. What this experience did was reinforce to me was the virtue of having standard, well-defined electrical interfaces, so that system elements can be substituted and tried in place of or in conjunction with other known, working elements.

It also made me grateful for situations where there is a clear direction of signal flow and a defined signal path, rather than the “back and forth and sometimes even sideways” situation that we have in many modern systems. I like it when the venerable concept of signal-path tracing is still viable, where each block has clear “goes into” and “comes out of” spigots. The nature of many system designs now is, unavoidably, one where almost nothing works until almost everything works, which makes debugging a real challenge!

Why did I do this retrofit? Not to save the planet and be “green”, but partially because I didn't feel like spending money yet on a new, digital TV setup. It's also because I am reluctant to trash the physical embodiment of the RS-170 (monochrome) and NTSC (color) analog TV standards, now over 50 years old.

Both have served us well over the years. And yes, it's easy for us to “trash talk” these standards due their limitations, in the light of our many technical advances, but they really are quite brilliant in how they accomplished what they did, within the bandwidth and component limitations of their time. The NTSC standard, in particular, is very clever in its implementation, and unlike so many of our standards today, it had to be both forward compatible (encompassing monochrome signals on color receivers) and backward compatible (the color signal had to render properly in gray scale on monochrome receivers.) How many standards these days are compatible in both directions?

-x x x-

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