Dial Home Device

Dial Home Devices (or DHDs) are what the Ancients used to activate Stargates. Similar to a telephone dial or touch pad (although much larger), the DHD is used to specify which other Stargate to connect to when opening a gate or wormhole to another location. The external symbols on the DHD represent star constellations, surrounded by the central activation switch. The DHDs are composed of control crystals, used to store memory and information. The Dial Home Device also contains an update command to account for stellar drift.

Overview
Most DHD's are pedestal-shaped, with a round inclined control panel on top consisting of two concentric circles of "keys" and a translucent hemisphere in the center. The keys represent the corresponding symbols, also called "glyphs", on the rim of the Stargate, with the central hemisphere serving to engage the Stargate. In the Milky Way, activating a Stargate using a DHD also serves to allow for "quick dialing", activating the gate without waiting for the inner ring to spin. The DHD also provides power for the Stargate and appears to have a complex programming interface, of which most is not normally needed by the operator.

Apparently, originally every Stargate had its own DHD, located directly in front of the Stargate. However, over time some DHDs have been damaged or lost. This frequently presents a difficulty for Stargate travellers, as it is still possible to dial in to a Stargate that lacks a DHD, but dialling out again is much more difficult. Several times, SG teams became stranded on worlds without functioning DHDs, having to improvise lightning rods or other such power sources, and manually dialing the Stargate's symbol ring. One of the primary functions of the MALP that is sent to new Stargates before any SG team is to confirm the presence of a functioning DHD.

According to Dr. Zelenka, dialing an address leaves a small imprint on the control crystals of the DHD, and about fifty addresses can be recovered from a DHD using the proper equipment. However, this gives no indication of the order in which the addresses were dialed, and no guarantee can be made as to the accuracy of the recovered addresses.

Milky Way DHDs
The Milky Way DHDs have sparked numerous discussions as the design and operation of the device has resulted in contradiction throughout the SG-1 series. In the Milky Way, the Dial-Home-Device contains 38 of the 39 symbols on the Stargate, meaning there is always a missing glyph on each DHD. This missing glyph, however, is not the point of origin for the planet. This means that only certain addresses can be reached from certain positions in the galaxy. The only way to intervene and reach all destinations in the Milky Way is to manually dial the gate and use an external power source or to use a dialing computer like the SGC does.

Earth's DHDs
Earth's Alpha Gate was missing its DHD when first discovered, requiring the United States Air Force to develop the dialing computer as a replacement. This resulted in Stargate misbehavior from time to time, since most of its detailed interfaces and safety features could not be reverse-engineered.

When SG-1 was forced to return to Earth under fire, a freak accident caused the wormhole to shift to the second Stargate on Earth in Antarctica. This "second" gate &mdash; the Beta Gate &mdash; was Earth's original Stargate, used by the Ancients before the Goa'uld had even discovered the planet. This Stargate's DHD was still with it when it was discovered, and was used briefly by National Intelligence Department operatives to conduct covert technology raids through the backup Stargate. However, it has since run out of power and ceased to function. The Antarctic gate and DHD are thought to have been the oldest known, possibly among the earliest that were constructed by the Ancients.

The Giza DHD had not been destroyed, however, merely lost. Nazi Germany removed it from Egypt during World War II, and the Soviet Union subsequently captured it at the end of the war. It passed into the possession of the Russian Federation after the Soviet Union's fall. After the Russians recovered the American primary Stargate from the Pacific Ocean when it was lost in the crash of Thor's starship Beliskner, they set up their own Stargate exploration program using the DHD to secretly "override" the backup Stargate the SGC was using whenever Russian SG teams were scheduled to dial back to Earth.

The American Stargate program eventually suffered a mishap that their makeshift dialing interface was unable to correct, with Teal'c becoming "trapped" in their Stargate's data buffer after the wormhole connection was severed when a ship destroyed the DHD which was supplying power to the gate that was connected to the Earth Stargate. The Americans eventually made a bargain to exchange some of the information their more-successful Stargate program had gained for the Russian DHD, which was able to retrieve Teal'c alive. The DHD was destroyed in the process, however, leaving Earth with no remaining known DHDs.

Pegasus DHDs
In the Pegasus galaxy, conventional DHD pedestals are found in most worlds with Stargates, with mainly cosmetic differences from DHDs in the Milky Way &mdash; present because the Pegasus DHDs are newer, and obviously feature a different coordinate system specific to that galaxy.

The primary differences are that both the keys and the central hemisphere in the Pegasus galaxy are blue, the sound produced by dialing a Pegasus DHD is more of a buzzing than a swishing, and that gate-dialing is much faster. In addition, the DHD keys are made of a reflective, glass-like material.

Atlantis DHD
The Atlantis DHD is more similar to Earth's dialing computer than an actual DHD, and looks more like a set of crystal panels. A similar DHD is also used on gateships. However, it appears to have extra features, like blocking out certain gate addresses.

Consequently, the Atlantis DHD also has an extra control crystal allowing the dialing of an eighth chevron during the dialing sequence and is the only DHD in the Pegasus galaxy capable of dialing other galaxies. This was probably a security measure set up by the Ancients.

Wraith DHD
The Wraith also travel through Stargates in small spacecraft called darts and have some means of remote-dialling them in a manner similar to Ancient ships.

Gateship DHD
The gateship DHDs are very similar to the Atlantis DHD, with the exception of the extra control-crystal. The ship's DHD is used for travel through an orbital Stargate, in conjunction with a Stargate power node since orbital Stargates have no constant DHD.

These DHDs seem to be tailored to their respective galaxies, since the gateships from Atlantis have the Pegasus set of glyphs, whereas the gateship found on Earth had the Milky Way set. However, it may be that the ships possess a sort of 'universal' DHD that tailors itself to the gate system in question, as a gateship from Atlantis has been seen activating a gate from the Milky Way at the McKay-Carter Intergalactic Gate Bridge.