I went back to 1986 and found a design I did. Back then I did this many times.
On the right side is the TV with the 75 ohm termination resistor and capacitor.
On the left is a digital IC. Fast CMOS works well but TTL will work. CMOS = 0/5V while TTL does not pull up well.
For sync: sync=0, V1,V2,V3=000 The output voltage is 0 volts.
For not sync, (black level): sync=1, V1,V2,V3=000. (wild guess, The resistor on "sync" might be about 750 ohms to get about 0.3V)
The resistors on V1,V2,V3 are sized to get about 1 Volt on the output when V1,V2,V3=111 and sync=1.
This design gives a sync level of 0V, black level of 0.3, and video has 8 levels from black to white.
View attachment 103597