When should GOP be less than 18?

[sky captain]

I'm working in NTSC, so from what I've read I should make my MPEG's with a GOP number of 18. What would happen if I made it a lower number, like 8?

Am I going to lose quality? Will it play better or worse?

I've read through most of the help files on GOP, but I can't find the answer.

[disturbed1]

In theory you would lose quality, unless the bitrate is high enough to compensate. Long GOP = better compression, so a lower bitrate can be used, short GOP = less compression, so a higher bitrate is needed. If you would actually notice the effects between 18 and 8 on a GOP with a decent bitrate (3500+) is another question.

I usually keep all of my encodes to a standard GOP length of 15 max. If you are using a good software encoder that can use varible GOPs and scene change detection, this will yield the highest quality.

[Sulik]

Short GOPs mean more I-frames, ie less compression, ie less quality at the same bitrate.

The DVD limit is 36 fields for NTSC, which is a GOP length of 18 frames, assuming that there are NO REPEATED FIELDS (ie: no inverse 3:2 pulldown). If you're doing with IVTC, then the max GOP length is 14:
14 frames = 28 fields, IVTC will add an extra field for every 2 frames:
14*2+7 = 35 fields.