Start • Simple Video Signal • Video with the LPC2138 • Video Algorithm • Tricks Used • Vertical Sync & Resolution • Video for other Apps • Oscilloscope • Implementation • Construction & Results • Generate Video • Sources and PDF

VERTICAL SYNC & RESOLUTION

According to the RS-170A and CCIR standards, the vertical sync pulse is a sequence of inverted horizontal sync pulses. In my program, they are obtained by inverting the PWM output. At the end of the TV frame, the content of the PWM match registers (defining the rising/failing edge positions) is temporarily swapped.

In my project, the number of TV lines per frame is even, so equalization pulses aren’t required. It seems all of the TV models admit such mockery.

The RS-170A standard provides 242 theoretical visible lines between the vertical sync pulses. The CCIR standard does 287. I decided to use the same vertical resolution (240 pixels) for both standards, although a value of 256 or higher is more preferable for CCIR.

Almost all of the TVs overextend the raster to use the all screen area. As a result, the upper and lower parts of a generated picture can be invisible in RS-170A because the only two blank lines (2 = 242 – 240) are reserved here. Try to adjust the vertical size if your TV has this option, or select PAL mode if you have a multisystem TV.