a) Techniques for increasing apparent sharpness without increasing actual resolution. This usually takes the form of increasing the brightness change at edges. Since image enhancement has advanced continuously for nearly 50 years, ordinary NTSC pictures sometimes look better than the NTSC pictures derived from an HDEP source, particularly when these derived pictures are designed to be augmented by other signals in an ATV receiver. It is very difficult to enhance pictures for NTSC receivers and then unenhance them for receivers with augmentation.

b) Once the camera response has been made flat to 400 lines (by aperture correction), an additional correction is applied to increase the depth of modulation in the range of 250 to 300 lines (in an NTSC system), both vertically and horizontally. This additional correction, known as image enhancement, produces a correction signal with symmetrical overshoots around transitions in the picture. Image enhancement must be used very sparingly, if natural appearance is to be maintained.