Hi again,
You would probably do better to simply use the VCR circuitry. It can take the input signal and make it into the right form for recording already.
It's not really that hard to create a frame for a TV set in black and white. The Sinclair computer did it with a single microprocessor, the Z80. And that generated a whole TV screen of characters.
You have to study the format for your equipment and go from there.
BTW, back in the 1980's i had built a TV generator using TTL chips. It's just a matter of generating the right timing and sync.
Logically using counters this means a string of counters that count up or down and then repeat, and state detection that detects every 15750^th clock and every 1/60 of the full count using exclusive or gates. Using counters though you can use 262 lines and that makes it easier. The first set of counters work at 15720Hz, and the vertical sync is generated with the second set of counters which get driven with the output of the first set.
Another thing you have to think about however is signal recovery. You have to be able to read the data back and that requires some fast analog circuitry. And instead of using a digital clock you simply record some approximate real time signals onto the tape after every N^th frame. You read the real time clock signal same as other data, and that shows your logic where you are on the tape. A few calculations (as well as control of FF) and you know how long to fast forward for to get near the data.
It's not the simplest project though
Oh yeah one more little detail...
At a data rate of 8 bits per line and 15720 lines per second which means about 15000 bytes per second, it's going to take a full MINUTE of time to read 1 MEGABYTE of data. A hard drive can read 20 MEGABYTES in ONE SECOND (and usually even faster).
This makes the hard drive 1200 times faster than the tape at least.