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DPL / DCS Introduction: This section has background information that applies to a coded squelch system which is used by every major radio manufacturer. Because it is not just Motorola specific (i.e. only about Motorola products), I will use the generic Digital Coded Squelch (DCS) term instead of the Motorola trade mark Digital Private Line (DPL). DCS and DPL are functionally the same. DCS is a digital scheme which is sent by a transmitter to control the squelch opening of a receiver. DCS is designed to work in the "so called" sub-audible portion of the band below 300 Hz. DCS bit timing is based on 134.3 Hz timing. However, since it is a digital signal which can create audio harmonics well above 300 Hz (i.e. into the audible portion of the band from 300 to 3000 Hz), the receiver must have good audio filters to remove the unwanted DCS noise from the received signal. The entire DCS word is 23 bits long. The first 11 bits are all for parity. The next 3 bits are always a fixed "100" bit pattern. The next 9 bits are used for the familiar three digit number we use to program DCS codes (11 + 3 + 9 = 23 bits). These 9 DCS code bits are divided into 3 octets. An octet is 3 bits, so 9 bits divided by 3 bits per octet, makes 3 individual DCS code digits. Each of the 3 DCS code digits can have a value from 0 to 7 (the maximum number of different codes you can put into a 3 bit octet). There are 512 DCS codes total from 000 to 777. DCS words can be encoded or decoded in a normal polarity or in an inverted polarity. The DCS codes used for a normal polarity DCS word are typically prefixed with a plus sign (i.e. +000 to +777). The DCS codes used for an inverted polarity DCS word are typically prefixed with a minus sign (i.e. -000 to -777). 23 bit DCS word: PPPPPPPPPPP100DDDDDDDDD (P = parity bit, D = DCS code bit, 1 = fixed bit value of one and 0 = fixed bit bit value of zero ) When a DCS word is sent as a serial data stream, each of the 23 bits is supposed to have an exact 134.3 Hz per bit, bit rate timing. If reverse burst is enabled, a 134.3 Hz continuous tone is sent for about 200 milliseconds by the transmitting radio after its PTT is released. This reverse burst causes the receiver squelch to close before the Tx signal disappears, which eliminates the end of transmission squelch tail noise in the receiver. When receiving a radio signal, there is no way to tell exactly which bit is the first one in a DCS word. Each DCS code has an unchanging "100" pattern embedded in it, starting with the 12th bit. All you have to do is, take any 23 bit DCS word and start synchronizing the bit pattern with "100" as the 12th, 13th and 14th bits of the DCS word (i.e. barrel shift all 23 bits until a 100 appears in the correct bit position). However, there can be more than one of these "100" patterns in the 23 bit DCS word. This results in multiple DCS decodes for one single original encoded DCS code. Each 23 bit DCS word has its 9 bit DCS code embedded in it. Keep in mind, the whole reason for using DCS is so you can have a unique code to use in your particular radio system. Because of these multiple decode hits from one single transmitted code, it is simply not possible to use all 512 DCS codes and keep them unique. However, by reducing the number of DCS codes used, it is possible to come up with a usable set of DCS codes (i.e. DCS codes may be considered to be "unique" if you simply ignore and do not use the duplicate codes). Most manufactures only use 83 to 112 of the DCS codes. There is no enforced "standard" on which DCS codes should be used, so everyone uses however many codes they want, but everyone seems to use the exact same basic set of 83 DCS codes, with their own additions (if any). According to some checking I did, there appears to be 177 unique DCS code groups (synchronizing to the "100" bit pattern in each shifted DCS word mentioned before). So, each of the 177 groups has multiple unique individual DCS codes that belong to each group, but the extra DCS codes in each group are ignored. In review, out of 512 individual DCS codes, only 177 of them can be considered to be unique. The 512 individual DCS codes are actually sorted out into groups of related DCS code groups and there happens to be 177 of these related DCS code groups. Each one of the so called unique 177 DCS codes were picked from their own group of related DCS codes. There is no enforced standard for which DCS codes to use, but everyone seems to use the exact same set of 83 DCS codes, so I am calling this a "standard" in this document. Some manufactures use additional DCS codes beyond the standard 83 codes (112 unique DCS codes is the current limit I am aware of). Most two-way communications radios will let you select any one of the 512 DCS codes, but the DCS codes that go beyond the standard set of 83 may not work correctly in actual use (this depends on your individual radio and its DCS hardware/software). The actual 23 bit DCS word is sent in NRZ format. This means that when there are consecutive one values or consecutive zero values, the signal level does not change. There is lots of information on the web about NRZ format, so I will not rehash it here. The effect of NRZ coding on a 23 bit DCS word is that all DCS words end up having 8, 12 or 16 transitions from zero to one or one to zero. None of the DCS words with 16 transitions were used in the standard set of 83 DCS codes. Out of the 177 groups of DCS codes, 21 groups of DCS codes have 16 transitions in each individual DCS code. The standard set of 83 DCS codes contains both 8 and 12 transition DCS words. There is another problem with receiving DCS words. Because of the NRZ format some DCS words can have long strings of contiguous zeros or contiguous ones. This can cause relatively long time periods without being able to synchronize precisely to the 134.3 Hz bit rate timing at the receiving end. For example; ten ones or ten zeros in a row will multiply any individual bit rate timing errors by a factor of ten. This really gets bad when you consider the Tx signal might have a lower than expected bit rate and the Rx decoder end might have a slightly faster than normal bit rate, just as an example. The algorithm used by the receiving decoder can also cause difficulties, making some DCS words harder to decode reliably. The important lesson from all of this is some DCS word bit patterns do not decode well, so they are not used. The basic set of 83 codes should work in any radio DCS Tx encoder or Rx decoder. Below is an example of the DCS word for DCS code 023. It shows the duplicate DCS words that also match 023 when shifting the DCS word for a "100" bit pattern match. Then the inverted DCS word matches are also shown. The "logical inversion" of the 023 DCS word is not a match itself or even a valid DCS word, but if rotated to synchronize to the "100" bit pattern, it has 3 matching inverted DCS codes (some dashes and slashes were added to the DCS words below for visual formatting only). 11101100011-100-000/010/011 = 023 (DCS word, first "100" bit match) 01001111101-100-011/100/000 = 340 (023 word shifted to the second "100" bit match) 00111000000-100-111/110/110 = 766 (023 word shifted to the third "100" bit match) 00010011100-011-111/101/100 = logical inversion of 023 DCS word 11101100000-100-111/000/111 = 707 (inverted 023 word shifted to the first "100" bit match) 10000010011-100-011/111/101 = 375 (inverted 023 word shifted to the second "100" bit match) 00011111101-100-000/100/111 = 047 (inverted 023 word shifted to the third "100" bit match) Another way to look at this is, the waveform for DCS codes 023, 340 and 766 are identical, except they are shifted in time (i.e. they all create the same waveform starting at a different point). The waveform for the inverted 707, 375 and 047 DCS codes are identical, except they are shifted in time (i.e. they all create the same waveform starting at a different point) and their bits are inverted from the original positive 023 waveform. The (+) normal and (-) inverted polarity is simply how the DCS signal is transmitted and received. An inverted DCS signal is simply an inverted 23 bit DCS word (i.e. make all the one values a zero value and all the zero values a one value). When you tell the transmitter to send a + DCS code, it sends the normal positive DCS bit stream. When you tell the transmitter to send a - DCS code, it sends the inverted negative DCS bit stream. 11101100011-100-000/010/011 = +023 (Tx DCS positive word) 00010011100-011-111/101/100 = -023 (Tx DCS negative word) Inverting a 23 bit DCS word will always turn the magic 100 bit pattern into an invalid 011 bit pattern (see above). This is why the exact same normal and inverted code can never match itself. If you shift the inverted 23 bit DCS word with the 011 bit pattern until you get valid 100 bit patterns, you can find other valid DCS word or words buried inside it, as in the 023 example above. Any valid 9 bit DCS code really only has a normal positive polarity. An inverted minus polarity 9 bit DCS code is not really a valid 23 bit DCS word for that particular 9 bit DCS code, it is an inverted DCS word for that DCS code. An inverted minus polarity 9 bit DCS code must be re-inverted to restore the original 23 bit DCS word. If not re-inverted, an inverted minus polarity 23 bit DCS word can also be shifted until a different valid 9 bit DCS code is found (the new DCS code that originated from the inverted 23 bit DCS word will actually be a normal positive polarity 23 bit DCS word with the correct '100' magic bit pattern). The receiver can also Rx positive or negative polarity DCS codes. This means you can Tx a -023 and Rx a -023 DCS word. The transmitter will invert the DCS word and the receiver will re-invert it back into its original positive polarity (i.e. the magic 100 bit pattern will be in the correct place for the 023 DCS word). The Tx -023 and Rx -023 has the exact same result as Tx +023 and Rx +023. Here is what happens if you Tx a -023 and Rx a +023: 00010011100-011-111/101/100 = -023 (Tx DCS negative word) 11101100000-100-111/000/111 = +707 (Rx DCS word shifted to the first "100" bit match) 10000010011-100-011/111/101 = +375 (Rx DCS word shifted to the second "100" bit match) 00011111101-100-000/100/111 = +047 (Rx DCS word shifted to the third "100" bit match) You can see the receiver can not come up with a DCS code match because it is looking for +023, but it can only find +047, +375 or +707 in the received DCS word. Because the receiver is looking for positive DCS words, it did not invert the Rx signal that would have restored the 023 DCS word pattern. For example; a transmitter can Tx a +023 DCS code, then the receiver would Rx a +023, +340, +766, -047, -375 or -707 DCS code. If the transmitter sent a Tx -023 CDS code instead, then the receiver would have to have to Rx a -023, -340, -766, +047, +375 or +707 DCS code. So, the minus sign on a Tx DCS code simply inverts the 23 bit DCS word signal before sending it out. The minus sign on a Rx DCS code simply inverts the 23 bit DCS word signal before looking for any valid '100' magic bit pattern matches. Take note: DCS codes 023, 340 and 766 all have the same polarity. While DCS codes 707, 375 and 047 have the same polarity, but it happens to be the opposite polarity from DCS codes 023, 340 or 766. This means codes +023, +340, +766, -707, -375 and -047 all match each other. Codes -023, -340, -766, +707, +375 and +047 all match each other. However (as explained above), a -023 for example; does not match +023, +340, +766, -707, -375 or -047. Whenever the polarity is inverted (i.e. + to - or - to +) the matching DCS codes change as a result of the polarity shift. This means a normal + code will not match the same inverted - code (i.e. +023 will not match -023). The six above DCS codes, 023, 047, 340, 375, 707 and 766 can be transmitted as (+) normal polarity or as (-) polarity. What matters is a combination of how they are encoded (i.e. Tx) and decoded (i.e. Rx). The polarity can be inverted in either the Tx, Rx, both or neither one. For example, you can program a radio to Tx a normal polarity DCS code and and Rx an inverted polarity DCS code. In the +023 example above, it happens to have 3 normal polarity DCS codes (+023, +340 and +766) and 3 inverted polarity DCS codes (-707, -375 and -047). This means you can program +023 Tx and it can be received as an inverted polarity -707, -375 or -047. If you invert 023 and Tx -023, it can be received as a normal polarity +707, +375 or +047. This is why you need to know or figure out the polarity of the originating Tx DCS code, as well as the DCS code digits to successfully use DCS Rx to control the squelch. If you know the DCS code digits to use for your receiver, but do not know the originating Tx polarity, just try both Rx polarities for that DCS code until you find the one that works. It is possible that mixing different radios from different manufacturers in the same radio system might result in a built in inversion (i.e. their encoding and/or decoding polarities might just be different from each other). In this case it could be possible to Tx a +023 on one manufacturers radio and successfully Rx a -023 on a different manufacturers radio. Mathematically this not really possible, it would be an illusion created by the built in difference in signal inversions between the different brands of radios. As previously mentioned, there is a standard set of 83 DCS codes. Each set of standard DCS codes is paired down to just two matching codes with opposite polarity. DCS codes 023 and 047 happen to be two of these standard DCS codes. DCS codes 340, 375, 707 and 766 are simply ignored because they are just duplicates. Notice that because 023 and 047 codes are opposite polarity codes, +023 matches -047 and, -023 matches +047. Of course, do not forget about the same polarity matches (i.e. +023 matches +023, -023 matches -023, +047 matches +047 and, -047 matches -047). All six DCS codes +023, +340 +766, -047, -375 and -707 together are one group of the 177 total groups. For some variety, here is another example of DCS word 025. It has no same polarity matching DCS codes. However, it does have three inverted polarity matching DCS codes. 11010110111-100-000/010/101 = 025 (DCS word, there is only one "100" bit pattern) 00101001000-011-111/101/010 = logical inversion of 025 DCS word 11010100010-100-100/001/111 = 417 (inverted 025 word shifted to the first "100" bit match) 10100010100-100-001/111/110 = 176 (inverted 025 word shifted to the second "100" bit match) 00111111010-100-010/100/100 = 244 (inverted 025 word shifted to the third "100" bit match) Here is the inverted 025 DCS word with all of the matching "100" patterns shown in bold, before any shifting has taken place. Two of the "100" patterns are right next to each other and one is split on opposite sides of the DCS word. 00101001000-011-111/101/010 = logical inversion of 025 DCS word The amazing thing is the 11 bit parity is correct in each shifted or inverted and shifted DCS word. The math used to create this DCS coding scheme is really something else. You should also note that an inverted 23 bit DCS word is not same thing as an inverted 9 bit DCS code. For example; if you logically invert the 9 bit DCS code bits for 026, you will get DCS code 751. The 23 bit DCS word for DCS code 751 has nothing at all to do with the DCS group for DCS code 026, as you can see in the listing below.
The above timing chart shows the timing relationships. There is a vertical line at the beginning and at the end of the timing diagram. The 134.3 Hz square wave is what the bit timing is based on and also happens to be to the reverse burst signal. The 67.15 Hz square wave is what you would get sending a repeating 101010.... bit pattern in NRZ coding. This is not a valid DCS code and was only shown to illustrate basic NRZ coding of DCS 1 and 0 bits. If you count, you can find 12 one bits and 11 zero bits in the repeating pattern sample. The +023 waveform is the actual DCS word for the 023 code. Bit positions 1, 11, 15 and 23 are marked. The position of the 11 parity bits, 3 '100' bits and 9 DCS code bits are marked. The +023 waveform simply repeats constantly (i.e. after bit 23, just loop back to bit 1 and start all over again). You can count the eight 0 to 1 or 1 to 0 transitions in the 023 waveform. You can also see that there is a maximum of six consecutive 0 values and five consecutive 1 values, so the longest continuous interval without any signal transition to possibly re-synchronize timing on is six cycles at 134.3 Hz. The binary coding bit values for +023 is shown at the bottom of the chart. If a scanning receiver finds a frequency with an active DCS code it has no way to tell where it is in the DCS bit stream (i.e. it could jump in when bit 17 was being sent, or any other bit). There is no special start or stop synchronizing code, just a repeating bit stream. This is why synchronizing to the '100' bit patterns is the only way the receiver has to make sense of the bit pattern and pull out any valid DCS codes. If there was a special synchronizing code just to identify the starting or ending bits added to the DCS word, then all 512 DCS codes could be decoded as unique codes, but it was never done this way. This inability to synchronize to the exact start of each DCS word being sent is why there are so many codes that match each other at the receiving end. As mentioned before, there appear to be 177 groups of unique DCS groups. Each group consists of multiple individual DCS codes. Not all 177 groups are actually usable, which is why there is a standard set of 83 DCS codes. Phase modulated radios may have trouble sending DCS (i.e. Tx) because the 23 bit DCS code is a low speed digital signal. In general, phase modulators are not optimal for this type of modulation. All of the Motorola mobile radios on this web site use true FM modulation which is optimal. However, the Tx compensation control must be correctly adjusted to send good DCS signals. If you have worked on or replaced parts of your radio that affect the FM modulator hardware or its DCS input signal path, then you had better check/adjust the compensation alignment if you plan on using DCS. The timing of the entire 23 bit DCS code consists of 23 individual cycles of a 134.3 Hz signal. Just divide 1 by 134.3 cycles/second (this gives seconds/cycle) and multiply by 23 cycles to find out how many tenths of a second it takes to send a DCS code. You will get about .17125 seconds per 23 bit DCS word. This means you can almost, but not quite squeeze six 23 bit DCS words into each second. Below is a list of all 83 standard DCS code groups. In each group, the first line is a list of the matching normal polarity DCS codes. The second line below it is the matching inverted polarity DCS codes. The groups are separated by the dashed lines. This is a complete listing of all the matching DCS codes for each group, including the ones that are normally ignored and not used. The bold DCS codes show in blue are the DCS codes that normally get used from the standard set of 83 codes. Just a reminder, even though the ignored codes are shown, it does not mean your radio's DCS system can encode or decode them correctly. However, mathematically the waveforms produced by each individual group are all related through shifting for the "100" bit pattern decode and signal inversion. ------- The one anomaly in the standard set of codes, is the DCS group with +172 and -036 is used, but the inverted DCS group with -172 and +036 is not used in the standard set. Below are the remaining DCS code groups that are not part of the standard set of 83 code groups above. Some of them are used by different manufacturers (i.e. the encode and decode hardware they make will work with their selected extended DCS codes). The maximum number of groups, from the 177 total groups, that are being used at this time is 112 (this includes the 83 standard DCS codes). The remaining DCS codes appear to be unusable, even by the more adventurous manufacturers. The matching DCS codes +112, +250, +505 and +512 are unusual as they have no inverted polarity matches at all. If you invert any of these DCS codes, there is no "100" bit pattern produced. I suspect this may have something to do with why the group with these four particular codes were never used. I do not have any lists of extended DCS codes (i.e. beyond the standard 83 codes) that are used by different manufacturers (so they are not marked), however the DCS codes with 16 NRZ transitions are marked in bold red. ------- Both polarities of all 512 individual DCS codes are represented in the two above listings, except for codes -112, -250, -505 and -512 which are not valid/usable DCS codes in their inverted form. To keep things straight, remember that the actual DCS code is an octal number. These can easily be converted into binary by converting each of the octal digits into binary and putting all of them of them together. Most calculator programs will also convert from base 8 (octal) to base 10 (decimal). There are 512 (decimal) individual DCS codes total, but DCS code +512 (octal) converts to 330 (decimal). DCS code +777 (octal) converts to 511 (decimal). Since the first DCS code started at +000 (octal), that makes 512 (decimal) DCS codes total.
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