NTSC and PAL frequency interleaving

This chapter focuses on the National Television System Committee (NTSC) and Phase Alternating Line (PAL) frequency interleaving. The frequency-interleaving scheme devised by the NTSC places chroma at frequencies that are used by luma. The NTSC scheme enabled transmission of 1.3 MHz of chroma signal interleaved or overlaid upon a 4.2 MFtz luma signal. In NTSC, frequency interleaving is exploited by a comb filter. The chapter also describes frequency interleaving in a moment, along with the discussion of some aspects of notch filtering. In studio video, the frequency of unmodulated NTSC color subcarrier is an odd multiple of half the line rate. This relationship causes subcarrier to invert phase line-to-line with respect to sync, as sketched and represented by a figure in this chapter. Frequency interleaving enables separation of luma and chroma using a comb filter. A simple (“2-1ine” or 1H) comb filter has storage for one line of composite video. Provided that luma and chroma are similar line-to-line, if vertically adjacent samples are summed, chroma tends to cancel and luma tends to average.