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

General Specifications:

  • 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.

  • \$\begingroup\$ Analog NTSC uses 1 volt PP voltage; the black level is 0.3 volts; the sync tips are 0.0 volts; white is 1.0 volts; the impedance is 75 ohms; cables are both source and load terminated; this requires 2.0 volts be generated. \$\endgroup\$ – analogsystemsrf Mar 30 '19 at 3:41
  • \$\begingroup\$ So you're saying if I can continuously inject 2V it will 'set' pixel values to 'High'= White? \$\endgroup\$ – SheerKahn Mar 30 '19 at 3:46
  • \$\begingroup\$ You cannot "continuously inject 2V", because the sync tips need to be at 0V. If PAL sync goes the same way as NTSC sync, you also need 0.3V for the horizontal sync pulses during the vertical blanking interval. \$\endgroup\$ – TimWescott Mar 30 '19 at 3:51
  • \$\begingroup\$ If PAL works the way NTSC does, you don't need the color subcarrier -- if you leave it out, you'll get black & white. But then, you don't say which "PAL" you want -- there's a whole bunch of them. I'm not sure what was the most commonly used for computers, because I'm from NTSC country. \$\endgroup\$ – TimWescott Mar 30 '19 at 3:52
  • \$\begingroup\$ @SheerKahn One place to start is Geoff's Monochrome Maximite. (Also see the Monochrome Maximite Schematic.) He references the book "Programming 32-bit Microcontrollers in C", where you should jump to page 333 and read forward from there. If you start by generating just black and white (no gray, no color), then the hardware is quite easy (three resistors, is all.) His Color Maximite might be your next step to examine. \$\endgroup\$ – jonk Mar 30 '19 at 4:23

There are hundreds of websites regarding analog video left over from the good old days, and it would be better for you to search for ones that explain things at your level and then come back with more focused questions.

To answer your two questions today:

  1. For a pure monochrome image (black, various shades of gray, white), color burst is not required. Depending on the country, it might be required in a broadcast signal if the monochrome segment is part of an overall color program, but that does not apply to a non-broadcast test pattern. Your receiver/monitor should not have a problem with missing burst.

  2. As you noted, a composit video signal is not continuous video information the same way an audio signal is continuous. Video is punctuated by the H and V sync pulses. The visual part of the signal is continuous only for the length of one line. During that time, a continuous analog voltage of 0.714 V (here in the US with NTSC rules) will be interpreted and displayed as pure white.

In its most basic (and slightly non-compliant) form, a white mono video signal needs three voltages:

0.0 V - blanking level (we'll skip the pedestal for now)

-0.286 V - sync tip level for H and V

+0.714 V - white level



  • \$\begingroup\$ I have edited the original question to clarify my confusion. \$\endgroup\$ – SheerKahn Mar 30 '19 at 4:06

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