Squelch#DCS

Digital Coded Squelch (DCS ) (English for "digital coded squelch ") is a selective call, the formation of user groups on a common carrier frequency in radiotelephony possible. The principle of operation is identical to that of CTCSS, but the technical implementation is fundamentally different.

DCS signals

The identifiers of the individual DCS codes are:

Simplified Functional Description

Example radio

The transmitted speech is highlighted in each transmission in the transmitter as one of 52 possible identifiers in the form of a digital data stream for the duration. Different from the conventional squelch circuit, which turns on the speaker of the receiver upon reception of each signal, it is in the use of DCS only be activated if the data stream with a pre-set of the 52 possible codes is detected. Thus, the recipient will receive only those messages that were provided by a transmitter with the expected of him identifier. Simultaneously transmitted with the speech data stream is rendered inaudible to the user in the receiver. Since the corresponding signal is outside the audio frequency band used for voice transmission, it may simply be how effectively filtered out.

In this way individual devices can in principle be addressed by simply using each of them the expected identifier for the transmission. In fact it is such that and for this purpose much simpler and more flexible systems with larger address spaces, so a larger amount of individually addressable wireless devices, such as 5 -tone call, DTMF and in Europe, less common techniques MODAT, MDC -1200 Quik Call are available.

Practical benefits

In practice, there are two things for DCS applications:

Avoidance of noise

The loudspeaker is activated by electrical equipment, only upon receipt of a desired transmission and not by noise as overreaching or radiation. If the speaker of the receiver once turned on by a current, desired or underlaid with the expected DCS code transmission, this is however of course not protected as such against further interruptions or disturbances.

Formation of user groups

Parts, for example, a road construction company and a taxi company in a big city necessarily the same radio frequency, because due to the citywide number of users simply is a separate frequency is available for any operation, they are unlikely to be interested in the news of the other. Now use both companies a different DCS code, the radios, the taxi driver remain silent if material is re-ordered on the site.

However, DCS is good only to mask out unwanted transmissions. Sends example, the taxi operators on the common frequency, so this is busy and there would be interference in a simultaneous transmission by another participant - regardless of whether it is a different taxi driver with the same or the contractor with a different DCS code concerns. However, since the radios of each other party remain silent, it can only recognize a visual indication of whether the common frequency is clear, so he would not disturb his co-users. Therefore, there is the possibility to prevent releases in already occupied frequency with technical means for professional radios.

Protection against unauthorized monitoring

An interception security is given in the use of Digital Coded Squelch in any way, since only the receiver is enabled DCS at all signals are suppressed. Disables the listeners to his radio this function, all transmissions are received at the specified frequency - regardless of whether they are provided with a DCS code, or not. The same effect is obtained if the receiving device does not support the DCS function simply. In the application, the fact that the transmit (PTT) button must be pressed a bit longer yields (about 0.5 seconds) until the group devices open the signal for the receiver. In contrast to the uncoded procedures can be spoken immediately where after key press.

Alternative names

The procedure was described in 1994 in the British telecommunications standard MPT 1381 under the name Digital Coded Squelch.

However, it is known by various manufacturers of radios under different names:

• Digital Private Line ( DPL) is a trademark of Motorola.
• Digital Channel Guard ( DCG ) is called the technology at General Electric.
• Digital Quiet Talk ( DQT ) for Kenwood