jamming extensively and still does, plays a loop of traditional Chinese music while it is jamming channels (cf. Attempted jamming of numbers stations). The purpose of this type of jamming is to block reception of transmitted signals and to cause a nuisance to the receiving operator. One early Soviet attempt at jamming Western broadcasters used the noise from the diesel generator that was powering the jamming transmitter. Subtle jamming is jamming during which no sound is heard on the receiving equipment. The radio does not receive incoming signals; yet everything seems superficially normal to the operator. These are often technical attacks on modern equipment, such as "squelch capture". Thanks to the FM capture effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier. The receiver locks on to the larger carrier signal, and hence will ignore the FM signal that carries the information. Digital signals use complex modulation techniques, such as QPSK. These signals are very robust in the presence of interfering signals. But the signal relies on hand shaking between the transmitter and receiver to identify and determine security settings and method of high-level transmission. If the jamming device sends initiation data packets, the receiver will begin its state machine to establish two-way data transmission. A jammer will loop back to the beginning instead of completing the handshake. This method jams the receiver in an infinite loop where it keeps trying to initiate a connection but never completes it, which effectively blocks all legitimate communication. Bluetooth and other consumer radio protocols such as WiFi have built-in detectors, so that they transmit only when the channel is free. Simple continuous transmission on a given channel will continuously stop a transmitter transmitting, hence jamming the receiver from ever hearing from its intended transmitter. Other jammers work by analysing the packet headers and, depending on the source or destination, selectively transmitting over the end of the message, corrupting the packet. Types of jammers Portable jammers are phone-sized and low-powered devices. They can block data delivery at a distance up to 15 meters without barriers. Stationary jammers are more expensive and powerful. They usually have a larger jamming radius and wider frequency band. Strong jammers can require additional cooling as they can overheat. Stationary jammers usually have a range of 100 meters and require a power supply of 230 V. Self-made jammers are low-power devices that work over short ranges.