code, many receivers will decode a DCS signal when tuned to the CTCSS tone of 136.5 Hz (depending on receiver system tolerance). XTCSS XTCSS is the newest signaling technique and it provides 99 codes with the added advantage of 'silent operation'. XTCSS fitted radios are purposed to enjoy more privacy and flexibility of operation. XTCSS is implemented using a combination of CTCSS and in-band signaling. Tone burst or single tone Tone burst is an obsolete method of selective calling where the radio transmits a single 0.5- to 1.5-second audio tone at the beginning of each transmission. This scheme existed before circuitry for CTCSS had been developed. This method was in wide use in the United States from the 1950s through the 1980s. Human spaceflight operations made frequent use of this method. In the same way that a single CTCSS tone would be used on an entire group of radios, a single burst tone is used in a group of radios. The radio speaker turns on as soon as the tone is decoded and the speaker stays on until the carrier squelch detects that the carrier is no longer being received. At that point, the speaker mutes and the decoder resets. The receiver speaker turns off and remains muted until another valid burst tone is decoded. In some cases, burst tones were used to select repeaters. By changing tones, the mobile radio would actuate a different repeater site. A typical tone scheme might use the tones 1,800 Hz, 2,000 Hz, 2,200 Hz, 2,400 Hz, and 2,552 Hz. This was the scheme used by most State of California agencies during the era when tone burst was in use. Some systems have been observed to use tones as low as 800 Hz. The default or standard five Motorola tones used for single tone format as of the 1980s: 1,350 Hz, 1,500 Hz, 1,650 Hz, 1,800 Hz, 1,950 Hz. These were identified in system documentation for a number of remote control equipment models as well as sales brochures for Motorola Syntor and Micor mobile radio Systems 90 accessories. A common tone burst frequency used by many amateur radio systems in Europe is 1,750 Hz. In German public service radio networks the calltone 1,750 Hz (Tone I) and 2,135 Hz (Tone II) are used to activate different repeaters or call an operator. To double the calling features, tones are used in short call (1,000 ms)