Weighting systems originally consisted of solid lead blocks attached to a belt around the diver's waist, but some diving weighting systems are incorporated into the BCD or harness. These systems may use small nylon bags of lead shot or small weights which are distributed around the BCD, allowing a diver to gain a better overall weight distribution leading to a more horizontal trim in the water. Tank weights can be attached to the cylinder or threaded on the cambands holding the cylinder into the BCD. Many closed circuit rebreathers use advanced electronics to monitor and regulate the composition of the breathing gas. Rebreather divers and some open-circuit scuba divers carry extra diving cylinders for bailout in case the main breathing gas supply is used up or malfunctions. If the bailout cylinder is small, they may be called "pony cylinders". They have their own demand regulators and mouthpieces, and are technically distinct extra scuba sets. In technical diving, the diver may carry different equipment for different phases of the dive. Some breathing gas mixes, such as trimix, may only be used at depth, and others, such as pure oxygen, may only be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported by a backplate while others are side slung from strong points on the harness. When the diver carries many diving cylinders, especially those made of steel, lack of buoyancy can be a problem. High-capacity BCs may be needed to allow the diver to effectively control buoyancy. An excess of tubes and connections passing through the water tend to decrease swimming performance by causing hydrodynamic drag. A diffuser is a component fitted over the exhaust outlet to break up the exhaled gas into bubbles small enough not to be seen above the surface the water, and make less noise (see acoustic signature). They are used in combat diving, to avoid detection by surface observers or by underwater hydrophones, Underwater mine disposal operations conducted by clearance divers, to make less noise, to reduce the risk of detonating acoustic mines, and in marine biology, to avoid disruption of fish behavior. Designing an adequate diffuser for a rebreather is much easier than for open-circuit scuba, as the gas flow rate is generally much lower. An open-circuit diffuser system called the "scuba muffler" was prototyped by Eddie Paul in the early 1990s for underwater photographers John McKenney and Marty Snyderman; the prototype had two large filter stones mounted on the back of the cylinder with a hose connected to the exhaust ports of the second-stage regulator.