synchro to digital converters, which provide a digital representation of the shaft angle. Synchro variants So-called brushless synchros use rotary transformers (that have no magnetic interaction with the usual rotor and stator) to feed power to the rotor. These transformers have stationary primaries, and rotating secondaries. The secondary is somewhat like a spool wound with magnet wire, the axis of the spool concentric with the rotor's axis. The "spool" is the secondary winding's core, its flanges are the poles, and its coupling does not vary significantly with rotor position. The primary winding is similar, surrounded by its magnetic core, and its end pieces are like thick washers. The holes in those end pieces align with the rotating secondary poles. For high accuracy in gun fire control and aerospace work, so called multi-speed synchro data links were used. For instance, a two-speed link had two transmitters, one rotating for one turn over the full range (such as a gun's bearing), while the other rotated one turn for every 10 degrees of bearing. The latter was called a 36-speed synchro. Of course, the gear trains were made accordingly. At the receiver, the magnitude of the 1X channel's error determined whether the "fast" channel was to be used instead. A small 1X error meant that the 36x channel's data was unambiguous. Once the receiver servo settled, the fine channel normally retained control. For very critical applications, three-speed synchro systems have been used. So called multispeed synchros have stators with many poles, so that their output voltages go through several cycles for one physical revolution. For two-speed systems, these do not require gearing between the shafts. Differential synchros are another category. They have three-lead rotors and stators like the stator described above, and can be transmitters or receivers. A differential transmitter is connected between a synchro transmitter and a receiver, and its shaft's position adds to (or subtracts from, depending upon definition) the angle defined by the transmitter. A differential receiver is connected between two transmitters, and shows the sum (or difference, again as defined) between the shaft positions of the two transmitters. There are synchro-like devices called transolvers, somewhat like differential synchros, but with three-lead rotors and four-lead stators. A resolver is similar to a synchro, but has a stator with four leads, the windings being 90 degrees apart physically instead of 120 degrees. Its rotor might be synchro-like, or have two sets