NTSC demystified - Cheats - Part 6

Throughout this series of articles we've seen a number of "cheats" or non-standard signals which simplify NTSC signal generation. This post summarizes them.

1. "fake" progressive scan instead of interlaced scan. Write 262 lines per frame. This makes it much easier to get a stable image on the TV. Only even lines are written / drawn. The odd lines in between remain as black lines. This technique has been used in NES, SNES, uzebox and a number of older game consoles. [1]

2. "simple v-sync". The NTSC spec states that half-scanlines should be used for v-sync. This includes 3 kinds of pulses pre-equalization, serration and post-equalization. All of these are unnecessary. One "long-sync" is sufficient to perform V-Sync. Of 262 lines in a frame, 261 lines have "short sync" lasting 4.7uS, one line (usually line number 248) has a "long sync" lasting about 58.8555uS. [2]

Lets see this is with a bit more detail.

Lines 10-234 are visible and will have active video. All other lines will have sync followed by black level.

Every line other than 248 has the usual 4.7uS "short sync". Line 248 has a long sync lasting 58.55uS

3. Color burst always ON. If color generation is also manually handled, this can simplify software a bit. Instead of giving out 9-12 cycles of color burst at the right time, the color burst can be left always ON. Yes, this includes h-sync period. In the active part of each line, the phase of the color sub-carrier is changed to generate different colors. The rest of the line including sync can have the color burst superposed with the rest of the signal( sync, front porch , back porch). TV's dont care since they remove the chroma signal anyway. 

References

1. NTSC demystified - B&W Video and Sync - Part 1

2. NTSC demystified - Color Encoding - Part 2