In the effort to understand early computers (systems revolving around the 6502 CPU). I have started studying their makeup, while the operation of the CPU & memory makes sense. I'm left stuck and confused when it comes to how all of this was interfaced and displayed on a TV.
Analog video as I've found out is a complex problem all on its own. So I've decided to start simple.
I want to design a simple Monochrome Composite Video Driver - Its only function will be to produce a white screen - However, to really understand the functionality I want to use technology only available during that time period.
My theoretical understanding (PAL):
- I need achieve composite sync. To do this I need to address:
- Horizontal Sync
- Vertical Sync
- Color Burst --- Can possibly leave this one out?
General PAL Timings:
- Line Period: 64 us (Micro Seconds)
- Line blanking 12.05 +- 0.25 us
- Line sync 4.7 +- 0.1 us
- Front porch: 1.65 +- 0.1 us
- Burst start 5.6 +- 0.1 us after sync start.
- Burst 10 +- 1 cycles
CCIR/PAL standard video signal has 625 lines/frame and it repeats @ 25 frames/sec.
Each frame is split into 2 fields; - each consisting of 312.5 lines, called odd and even fields. Thus field rate is 50. i.e. CCIR /PAL std has 50 fields/sec rate.
Interlacing: The lines of odd-even field lie alternately . This method of scanning is called interlacing. This interlaced scanning is used to reduce flicker while displaying the image on a monitor.
At the start of each Horizontal Line a sync pulse is fired.
There are 625 Hor Sync pulses per frame.
There are 50 vertical sync pulses per second.
To achieve a monochrome display:
I need to address intensity/luminance (Y Signal --- Don't have too much info on where this 'Y' signal comes from)
0 Volts = Black Level
0.7 Volts = Peak White Level
What I don't understand:
Lets assume I can put together the V/H sync pulses and the timings are perfect.
How do I assign a 'White' value to each and every pixel? Where does that 'information' come from?
Lets say the first H sync pulse is fired and the TV is now aligned and ready to draw the very first pixel on the first line (Hopefully I have the understanding right).
It is now time for the TV to see a voltage of +0.714 V to make that pixel turn white.
Does the electronic circuit now switch to a different sub-circuit to retrieve this picture information?
I have a digital mindset at the moment and see the operation in this fashion: The driver does the heavy lifting analog work to align the TV...and then the circuit looks elsewhere to see what color this pixel needs to be and thats my source of confusion where does that +0.714 V come from?
Disclaimer: Aside from what I've included here - Assume I know nothing more. I don't have any prior analog video experience.