Narrow bandwidth television
Overview
Narrow band television (or NBTV) is an analog mode that uses 30 and the NTSC color standard in an SSTV like format.
NBSC format
NBSC is NBTV+NTSC. It uses YUV color.
Modulation
NBTV is separated YUV or IQ (color components), it has three signals that are compatible with black and white receivers. The Y signal is the same original B+W signal. Y in this case is an admixture of the rgb signals. U is blue - Y, and V is red - Y. Putting this back together we add all three and get an image that can be viewed with color, but is still viewable on B+W receivers.
Color difference signals
The signal is separated into YUV. This signal can still fit in the normal signal space. Y is transmitted normally along with the normal sync pulses. The signal is also Quadrature Amplitude modulated, at a phase difference of 90 degrees. It is modulated so when the UV is zero, the output is zero, but when the UV is negative, the wave is modulated upside down. Because they are 90 degrees out of sync, when one sine wave is at its peak or trough, the other is at zero. The original signal can be retrieved by sampling when each carrier is at its maximum or minimum.
https://www.sigidwiki.com/wiki/Narrow-Bandwidth_Television_(NBTV)