Richard Dean Anderson Website - Stargate SG-1 Lexicon

DHD

The Dial Home Device, or DHD, functions as both a power source and as a means of controlling the stargate. It has a total of 38 symbols, and uses a combination of seven symbols to dial a stargate address. The first six symbols identify a point in space, and the seventh symbol represents the point of origin, sometimes identifiable as a unique symbol on the device. Dialing seven symbols chosen from a pool of 38 non-repeating candidates results in about 63 billion possible stargate addresses. The gate is dialed by pressing the series of seven symbols, or glyphs, on the DHD in sequence, followed by the red crystal in the center for activation.

The Egyptian DHD was discovered by the Germans in 1906, and therefore was missing from the original dig at Giza in 1928 when the stargate was discovered. It was later confiscated from the Germans by the Red Army after World War II, and it was used briefly by the Russian stargate program. However, when it was borrowed by the SGC to recover Teal'c from within the stargate memory buffer, the Giza DHD was destroyed. The DHD for the Antarctic gate was discovered intact and kept in storage at Area 51. Tests indicated that it was among the oldest in the entire system, perhaps as much as 50 million years old, however, shortly after its recovery, its energy source was depleted, suggesting that the power sources fueling the gate technology have a limited life span. There is currently no functioning DHD on Earth.

Like much of the technology of the Ancients, the DHD uses a crystal power source based on a main control crystal. It was designed to implement up to 400 specific safety protocols, including compensation for premature power loss, and the network undergoes periodic automatic correlative updates in order to compensate for stellar drift. Because the DHD was not recovered with the Giza gate, it took 15 years of research and three supercomputers to MacGyver a system to control the gate on Earth.

Although a stargate using a DHD will become dominant over a gate controlled by other power sources such as computers, the SGC has relied on computers to perform the function of the DHD in order to provide additional base security. However, without a DHD, many safety protocols are bypassed, and until the discovery of the cartouche on Abydos which allowed for proper drift calculations, it was only possible to dial planets within a radius of approximately 300 light years.

The SGC's attempt to use the Avenger computer virus to alter and disable the dialing protocol for certain off-world stargates failed to achieve the intended results when a modification of the virus was spread by the system of automatic correlative updates. The SGC was unaffected by the virus because of its reliance on a dialing computer, however the DHD is a far more sophisticated power source. In fact, off-world stargates using a DHD draw 10% less wattage than the Earth stargate, probably due to the way in which the superconductive crystals inside the gate absorb electrical energy, and to the DHD's ability to produce a purer, more efficient form of power.