The vertical blanking interval gets quite a rough treatment. You can’t rely on anything being there to make it to the tape, but it still has a decent chance. The Macrovision “DRM” for videotapes used alternating high and low intensity signal there to confuse the VCRs’ input amplifier into forcing a flicker into the recording; that could be stripped with “video stabilizer” gadgets. I almost built one for the fun of it when I was putzing with keying text into image with a microcontroller.
That assumes a good quality of the signal. With lousy signal on a degraded tape, more advanced signal processing methods are likely to be required.
These days, the wiring for the digitizing is much easier than the wiring for the signal decoding in hardware. Then you just have to sit down and fiddle with DSP in silico instead of having to putz around with the decoding hardware.
When I was a kid at high school I attempted to build a teletext receiver/decoder. Failed miserably but it almost worked.
