NTSC signal tollerence.

[marktbaldridge]

I am in the process of building a ZX80 (a microcomputer from the 1980's) clone. The Z80 uses a 6.5MHz TTL clock divided in half to make 3.25MHz The Z80 CPU also generates the video signal. So here's my question, Could I get away with using a 6.144MHz clock? How much tolerance do TVs have? If it was 3.024MHz instead of 3.25MHz will that screw up the TV?

ZX80 hardware page

~Mark T. B.

 

[Nigel Goodwin]

I wouldn't have thought so, a colour decoder phase-locks to the colour sub-carrier, and if it's not close enough it won't lock.

Just looked at the link, interesting site, and it reminded me that the ZX80 was only B&W anyway, so NTSC doesn't apply. Most TV's will probably work fine with the line and frame a little wrong, so you should be OK.

 

[crutschow]

I expect the horizontal and vertical frequencies will be about 8% low from using the lower clock frequency. Not sure if a standard TV can tolerate that much difference. If it's an old B&W TV with an adjustable horizontal and vertical hold control than it probably would work.

The NTSC standard applies to B&W as well as color. The first NTSC (B&W) standard was established in 1941. The color NTSC standard was adopted in 1953.

 

[audioguru]

NTSC uses a 3.58MHz colour burst, not 3.25MHz.
3.58MHz crystals are used in every NTSC TV so they are mass produced and are very inexpensive.