Design Con 2015

The silence of the circuit

Harry Maddox, Senior Engineer -November 11, 2011

The silence of the circuit imageI was an engineering trouble-shooter for a large electronics company for many years. Long ago I had to solve some problems not too far from my old neighborhood. It was a great opportunity to also visit my mother since her birthday was coming up. She lived an old-fashioned life way out in nowhere. I decided to surprise her with her first VCR. She had never seen such a thing. I had to drive about 2.5 hours to my out of town meeting and had just enough time to make it. But before the trip, I stopped to buy a nice VCR. In a panic, I asked the young clerk if he could recommend a video my mom would like. He enthusiastically handed me one. I bought it without even looking. I solved the work problems then headed off to my mom’s house.

A lot of friends and neighbors gathered and everyone was ready to see this marvel VCR. Well, it didn't work. The picture was hopelessly garbled.

At that time I always carried my trusty “portable scope." That was a Swiss army knife to me; I used it for everything. I looked at the signals coming out of the VCR and noticed lots of strange activity in the blanking interval. I didn't know at the time but that turned out to be Macrovision encoding on the tape (prevents copying video tape). The new VCR was doing fine with it but Mom’s old RCA TV just couldn't cope. Now everyone was looking at me wondering why I couldn't get it working. I lost a lot of admiration points that day. Mom just kept right on knitting.

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This Tale is a runner-up in EDN’s Tales from the Cube: Tell Us Your Tale Contest, sponsored by Tektronix. Read the other finalists’ entries here.
I told everyone to come back in a couple of hours and I was sure I would have it going. I quickly designed a circuit on paper that would scrub the Macrovision signals but it required far more parts than I had. I decided to scrounge transistors and other parts from an old Hi-Fi. I also had a few parts and VFETs in my toolbox, but still not enough components or time to put it all together.

So back to the drawing board. I developed some clever ways to use parts for more than one function and came up with an incredibly tricky but simple circuit that I could build quickly. I used a 9V battery for power. My circuit looked like quite a kludge but seemed to work properly. Finally I had video just as everyone was filtering back. I was sweating bullets as I started the unknown video that I wasn't sure would even work. It worked great but the video turned out to be probably the worst choice in the world for this old-fashioned country audience.

Read more Tales from the CubeYikes! It was “The Silence of the Lambs.” Some people left, others stayed with confused looks on their faces. Mom just kept on knitting while watching occasionally over her glasses. I asked her if I should stop the movie. She said, "No, just change the channel." Well, Mom, a VCR has only one channel. If you want another one you have to buy another tape.

Mom, having no idea how clever I had to be to get this thing working at all, said, “Thanks for trying, son, but I like my programs. Why would I want to have only one?” As I hung my head in utter defeat someone showed up with a tape of “I Love Lucy.” That was a hit and saved the day. They must have played it 20 times. Mom loved the fact that they could stop, start and play it anytime she wanted, and even go backward.

I used a refined version of my Macrovision filter for years and I'm still amazed how I could solve that complicated problem in the time it took my mom to knit me a sweater. Actually that was a more difficult engineering problem than the one I was paid to solve.

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