Analog high definition carries high-definition video over coaxial cable using a modulation that older analog formats do not use. It lets a site with existing coax reach 720p or 1080p without re-cabling, provided the cameras and the recorder support the same generation of the standard.
AHD is a migration path rather than a destination. It keeps the cabling and the recorder location a site already has while replacing the cameras and the device that records them.
What AHD changed
Standard analog video was limited to roughly 960 lines of horizontal resolution. AHD sends a digital high-definition signal over the same coax, so familiar cable can carry a much larger frame. The recorder has to decode it, and an older recorder will not simply display it. Some units accept both formats on separate channels, which is why the channel setting matters as much as the camera.
Format compatibility traps
Several HD-over-coax formats exist, and cameras and recorders must agree on which one is in use. Mixing AHD with legacy analog cameras on one recorder usually needs a hybrid device or a per-channel setting. Cable condition and connector quality still limit usable distance.