Digital Coded Squelch (DCS) codec part I

Looking for a Digital Coded Squelch package that I could include in one of my projects I found out that there is no such package released for open source.  Looking for more information I found no DCS detailed information related to physical levels, how it is modulated etcaetera. Therefore I decided trying to implement a DCS codec that could be included as a library in any project. I present below the steps I followed in making the DCS codec library and the tests I’ve done for it.

Introduction

The DCS stands for Digital Coded Squelch and is a method of controlling the squelch of a radio by sending a digital code over the audio signal. It is a further development of the CTCSS method (Continuous Tone Coded Squelch System) ans it is called sometimes CDCSS (Continuous Digital Coded Squelch System). Various manufacturers calls this method different, for example Motorola (TM) calls it DPL (Digital Private Line), General Electric (TM) calls it DCG (Digital Channel Guard).

The DCS not only opens the squelch for a radio but it is also used to close the squelch before the carrier goes off in order to eliminate the squelch tail. It was an unfortunate decision to use the word “Private” on the squelch technologies because no privacy is added to the system when these are used. A lot of discussions were held regarding the “Private” meaning of these technologies many times ethical reasons being the major argument.

One will find on internet a lot of information about DCS including history and its preceding technologies but for what I needed I have found only two resources with valuable information. They are presented below:

http://onfreq.com/syntorx/dcs.html

http://mmi-comm.tripod.com/dcs.html

For short, DCS sends digital signals with 134.4Hz rate using golay (23, 12) codes over the audio signal. The golay (23, 12) means DCS uses 23 bit words with 12 bit data included the 23 bits.  The spectrum of the DCS signal contains major components in the 0 to 300hz band and therefore a LPF (Low Pass Filter) could be used on the receive side to get rid of the DCS signal and remain only with the audio signal. That’s what most of the receivers do noadays. Because of the existence of LPF in the receiver, it is indicated to get the signal for the DCS receiver just after the discriminator so that the DCS signal will be available together with audio signal.

Having available the mix of audio signal with DCS signal it is not a good thing for DCS decoding just because voice audio will interfere with the digital signal and therefore the DCS decoder will need a good low pass filter in order to eliminate the unwanted audio component. Some radio equipment vendors are doing voice filtering under 300hz before sending it to transmitter so that the receiver’s DCS decoder does not need to do LPF. However more vendors forget to do that and therefore the receiver decoder has to do it. I present below some captures of the DCS signal in varius conditions:

The picture above captures the signal after discriminator before and after the PTT on the transmitter is released. One could see three main zone:

1st zone – normal DCS code – in this specific case the code is 205.

2nd zone – the PTT is released, the transmitter sends squelch off signal. In this case it is a sine wave with frequency 134.4hz.

3rd zone – the carrier is gone and the receiver outputs white nose.

Given the 134.4hz rate of the digital signal it could be calculated that sending one word (23bits) takes up less than 0.2 seconds.

The major problem I encountered in implementing the DCS decoder was the big level of voice audio signal compared with the level of the DCS signal. This made the need for a good LP filter and other signal processing techniques in order to make the signal usable and correctly decoded with the DCS decoder. I present below an example of the signal when the user whistle into the microphone:

One could see the level of audio signal is three times more than the level of the DCS signal.

One word about modulation and Golay codes

The DCS signal is obviously a digital modulated analog signal. Each pulse represents a transition from 0 to 1 or reverse. There is no start or stop bit in the signal, therefore DCS decoder needs to look each bit it receives form the stream and decide whether it matches or not a Golay code together with the bits it received up to now.

Golay (23, 12) words contains 12bit of data represented in 8th base (octal). From this 12bits only 9bits are actual data, the rest 3bits are fixed as “100”. A Golay word is represented like this:

P11-P10-P9-P8-P7-P6-P5-P4-P3-P2-P1-100-C9-C8-C7-C6-C5-C4-C3-C2-C1

where Px are check bits and Cx are data bits.

Golay words are having some interesting properties, one of them being the fact that if a Golay word is circularly shifted left or right, another valid Golay word is obtained. This property together with the fact that no start/stop bits are given results in multiple matches depending on where the start bit is considered.  This made the maximum number of so called “standard” DCS codes be reduced so that just one match is taken in consideration.

Another property of Golay codes is the fact that if “1” and “0” is swapped into the bitstream of a valid code, then another valid code is obtained. This is in connection with what is considersd “1” and what is “0” when looking at the signal. Therefore one could talk about positive polarity and negative polarity for a DCS signal. For example +023 code produces the same pattern as -047 code. The current implementation gives both positive and negative matching.

It worth mentioning another property of the Golay codes that are not used for DCS. It is possible to recover up to three bits using the check bits.

In the next article I will present in detail how the DCS decoding is achieved in software and what software blocks I used for successfully doing this. To be continued.

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