A Bypass Switch for the Time Base Corrector

Video Conversion Schematic CroppedVHS Digitising Equipment

In Part 3 of the Digitising VHS Tapes series, I indicated that the Time Base Corrector (TBC) was only switched into service when noise on the tape was bad enough to cause video capture to abort. There were several reasons not to leave the TBC in service all the time.

  1. A side effect of cleaning up the noise on a tape was a slightly softer image and slight, but noticeable changes to image colour and brightness.
  2. With certain PAL tapes, ghosting would occur when the Canopus and TBC were used together. This would disappear if I swapped the Canopus for the KWoRLD DVD Maker 2.
  3. The top of the frame would generally curl when the TBC was switched into service.

This is what switching in and out the TBC looked like in practice.

Switching the TBC in…


Switching the TBC out…


I was doing this often enough for it to become a nuisance. I also thought that it can’t be very good pulling cables out and plugging them back in frequently. Something would eventually break.

The solution is to put in some sort of switch to swing the TBC in and out of service as required. I looked for something commercially, but nothing really suited. There were AV switches, but they are designed to enable a number of AV devices to use, one at as time, a single set of inputs. In the end I decided to roll my own bypass switch.

These are the bits I needed to build an AV bypass switch:

  1. A four pole double throw (4PDT) toggle switch;
  2. Some wire;
  3. A gang of 4 RCA connectors; and,
  4. A suitable enclosure.


Total cost of the various bits and pieces was under AUS$20. Here’s the design…

Sketch_ bypass design

Note that the grid of crosses reflects the posts on the back of the 4PDT switch.

With the design complete, it was time to construct the bypass switch.

Step 1: Prepare the enclosure.

The switch box had to be slotted to accommodate the gang of RCA connectors. A hole had to be drilled in the top for the 4PDT switch.


Step 2: Install the components

I used superglue to attach the gang of RCA connectors to the box. You can clearly see in this view how the paper design will facilitate wiring of the switch and RCA connectors.


Step 3: Wire up the components


Step 4: Test and label the bypass switch


The switch made it so easy to swing the TBC in and out of service as needed. It was well worth the AUS$20 I spent on parts as well as the investment in effort.


Edit: Check out these postings as well for other relevant topics.

  1. Digitising VHS Tapes: Part 1(3) – Digitising Process Considerations
  2. Digitising VHS Tapes: Part 2(3) – Samsung Worldwide VCR SV-4000W Repair
  3. Digitising VHS Tapes: Part 3(3) – Digitising Process Improvements
  4. A Bypass Switch for the Time Base Corrector

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