Interfering signals masked by tone squelch will produce apparently random missed messages. A radio with carrier squelch is unlocked and will let any signal in. A carrier squelch is unlocked and will let any signal in. It is believed Leung's lengthy sentence was intended to intimidate activists and squelch political protest. Tone squelch, or another form of selective calling, is sometimes used to solve interference problems. A carrier squelch or noise squelch is the most simple variant of all. The production features sound effects such as a bass squelch, synth echo, and feedback growl. Other whites attempt to squelch the rising violence with little success. It is sometimes referred to as tone squelch. Ground mobile radios used a similar system called CTCSS (Continuous Tone-Coded Squelch System). The crudest and oldest of these is called CTCSS, or Continuous Tone-Controlled Squelch System. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset. This can be used to activate the squelch on a receiver.