NTSC had to reduce the frame and field rate to prevent abberations in the Luma. PAL and SECAM didn’t require this.

NTSC and PAL use a single subcarrier for color and SECAM uses two subcarriers.

Sound has it’s own carrier 4.5 MHz above the Luma carrier.

This applies mostly to NTSC

The video carriers is 1.25 MHz above the lower part of the channel. There is a 250 MHZ buffer at the lower part of the channel. The video signal starts at 500Mhz and ends at 5.45 MHz. It is AM and thus has two sidebands, although they didn’t truly use a dedicates Single Side band back in the 1950s so only 750 MHz is transmitted on thie 4.2 MHz wide Vestigal sideband while the entire signal is transmitted on the other side.

The color subcarrier goed at the tail of the LUMA at 3.579545 MHz above the Video carrier, it is quadratured (90 degree angle In Phase and Quardrature phase (IQ).

This subcarrier can also genearte audio and data tracks. Closed captioning can be transmitted on the color subcarrier.

Then near the top of the channel comes the FM carrier for audio.

Channel starts – Buffer – Luma start – Video carrier – Luma continues – Color carrier – Luma ends – Audio Carrier – FM audio signal – Channel ends

The frame rate changes was done to limit dots and lines in the luma for black and white sets and to limit them in color bars.