VOX-APRS.txt INTEGRATING APRS ON VOICE CHANNELS APRS and VOICE REPEATERS PROPRIETARY Copyright 1993,4,5,6 WB4APR As mobile GPS/APRS continues to grow, we can eliminate the need for every mobile to have a TNC, digital radio, and second antenna by simply integrating the position report into a very brief tone burst at the end of a voice tramnsmission over any two-way radio. With this scheme, no additional hardware is required in the vehicle, other than a GPS unit. The system not only reports position and vehicle type, but also one of 7 canned messages and 4 analog telemetry values! By transmitting a position report at the end of a voice transmission, not only is this a period of dead time due to the almost universal courtesy beeps found on amateur repeaters, but the tone burst can be easily muted out at the repeater receiver, so that the other mobile users DO NOT HEAR it! If the tone burst is about 0.3 seconds, then it will be virtually transparent to voice repeater operation. The APRS MIC-ENCODER achieves a complete position report, course, speed, and digipeater information in about 30 bytes including header, vice the 90 or more bytes in a normal APRS position report. At the voice repeater receiver, a special APRS REPEATER NODE TNC picks off the position report and digipeats it out onto the dedicated APRS digital frequency for mobile position reporting using a path derived from the bits in the TO-SSID. In addition, it appends the repeater frequency onto the end of the position report so that digital users can see where the packet originated. If all voice repeaters digipeated onto the same digital position reporting channel (usually 145.79 if available) then anyone monitoring the APRS frequency will see ALL mobile position reports from ALL GPS mobiles on ALL frequencies! HARDWARE: To implement this APRS LOCATOR SYSTEM, there are two critical elements needed. First is an APRS MIC ENCODER to handle the integration of the mobile GPS data into the users microphone connector. Second is the specialized APRS REPEATER NODE that handles the digipeating of the compressed position reports heard on the voice repeater inputs over to the APRS digital channel. (in the interim, any TNC with true Data carrier Detect can be used at the repeater). Details follow. APRS MIC ENCODER: In order to make the APRS LOCATOR SYSTEM practical, the MIC-ENCODER had to meet several design constraints: * Must interface to UNMODIFIED radios via the MIC connector * Must use standard AX.25 for compatibility with existing TNC's * Must compress position report into about 0.3 seconds * Must Fail Safe so that MIC can always be used * Must be low enough in power to be powered from the MIC jack * Must have easily selectible user parameters * Must be very small for easy mounting and portability * Must accept the readily available NMEA output from GPS receivers * Optionally has 4 analog channels for telemetry The result is a 1200 baud position report compressed to 32 bytes including beginning and ending FLAGS. This equates to about 320 ms, including CALLSIGN, DIGIPEATER PATHS, and a minimum message capability. Plus, the packets are still receivable on ANY AX.25 TNC. PACKAGING: Although the electronics could be miniaturized into about 1 cubic inch, the requirement for user access to switches and the requirement for withstanding the pulling and tugging on the MIC cord results in a robust dash board box design. The cicruit is powered by the mic connector and the only external input is via a standard 1/8 inch phone jack to receive the NMEA data from the GPS unit. This makes the entire mobile vehicle position reporting system as portable as the microphone! Simply move the MIC from vehicle to vehicle, and as long as the radios are compatible at the MIC connector, then the vehicles are GPS ready! The suggested front panel for the MIC ENCODER is shown below. It connects to the radio with only a single 6 conductor pigtail back to either a back-to-back JACK/PLUG or soldered in parallel to the existing MIC jack. --------------------------------------------------------- | R A T E D I R P A T H M S G A U T O O N | | ----------------- | | /^\ | | | | | / \ O | | | O O | | | (O) | (O) | 7 | 7 | (O) (O) | | | \___/ | | | | | O M N I | | | O F F O F F | | ----------------- | --------------------------------------------------------- On the APRS MIC ENCODER, there are several configuration switches that give the operator real-time control over other dynamic MIC choices as follows: ON/OFF Used to enable or disable APRS beeps (power switch) AUTO/OFF Auto will transmit when repeater is silent more than N secs. DIR/OMNI Used to set Directional or OMNI digipeating paths 3 PATH BITS Used to set digi HOPS or North/South/East or West routes 3 MSG BITS Used to indicate up to 7 pre-defined messages Normally the MIC encoder will only send a POSIT if the POSIT timer has elapsed AND the user has been talking AND releases his PTT. In the AUTO mode, however, after a specified AUTO time period, AND after the repeater has been silent for the QUIET period, then a position packet will be initiated and transmitted. Most voice repeaters will never even key up on such a brief burst due to built in ker-chunk filters. The TNC on the repeater input, however, will hear it and digi it over to the digital channel. ROUTING PATH BITS: The OMNI/DIRECTIONAL switch and 3 routing bits are used to tell the repeater how to route the packet. There are actually two systems, one that can use standard TNC's at the repeater and the other that takes advantage of a new specialized APRS NODE TNC. We will call the standard mode the DIGI mode and the other, the SSID-NODE mode. In the DIGI mode, the path switches just select how many hops along one of two preset digi strings will be transmitted in the packet. The asvantage of this method is that it is compatible with ANY TNC and will work with existing systems. The disadvantage is that each digi hop takes 7 bytes, and just a 3 hop path almost doubles the length of the packet. The second mode uses only the 4 SSID bits for all routing information. This keeps the packet short, while also allowing for up to 7 hops in all directions! The following table shows how the routing is handled in both the DIGI mode and the SSID mode. The first path bit is the OMNI/DIR switch and the lower 3 bits are connected to the thumbwheel switch. In this esample, assume the MIC-ENCODER has been loaded with the digi string of RELAY,WIDE,WIDE,DIG4,DIG5,DIG6,DIG7 X PATH SSID DIGI VIA TNC APRS NODE DIGI's ONTO APRS CHANNEL - ---- ---- ------------------- ---------------------------------- 0 000 0 none direct 0 001 1 RELAY via WIDE-1 0 010 2 RELAY,WIDE via WIDE-2 0 011 3 RELAY,WIDE,WIDE via WIDE-3 0 100 4 DIG4 via WIDE-4 0 101 5 DIG4,DIG5 via WIDE-5 0 110 6 DIG4,DIG5,DIG6 via WIDE-6 0 111 7 DIG4,DIG5,DIG6,DIG7 via WIDE-7 1 000 8 none via NORTH UNPROTO path 1 001 9 RELAY via SOUTH UNPROTO path 1 010 10 RELAY,WIDE via EAST UNPROTO path 1 011 11 RELAY,WIDE,WIDE via WEST UNPROTO path 1 100 12 DIG4 via NORTH UNPROTO path + WIDE 1 101 13 DIG4,DIG5 via SOUTH UNPROTO path + WIDE 1 110 14 DIG4,DIG5,DIG6 via EAST UNPROTO path + WIDE 1 111 15 DIG4,DIG5,DIG6,DIG7 vai WEST UNPROTO path + WIDE CONVENTIONAL DIGIPEAT ROUTING: First notice that in the DIGI mode, the paths 0 through 3 simply select the number of digi hops in the original string to use. The paths 4 to 7 start over again at the 4th position. THis can be thought of as a completely independent second DIGI string. Usually you would load RELAY,WIDE,WIDE,WIDE,WIDE,WIDE,WIDE as the digi string. If you select 4 you get WIDE, if you select 5 you get WIDE,WIDE and so on. THis separation into two distinct strings gives you the chance to have a path beginning with RELAY or beginnning with WIDE. This is important for operating in areas which do not have the dual alias WIDE-RELAY digipeaters yet. Although the longest path is now limited to four hops, anything beyond 2 WIDES is frowned on anyway... APRS NODE ROUTING: The APRS NODE has two routing methods, the OMNI and the DIRECTIONAL method. In the OMNI mode, every APRS NODE repeats every WIDE-N packet it hears and then subtracts ONE from the SSID. They also keep copies of all such packets for 60 seconds and ignore all DUPES of the same packet. THis is a very effecitve OMNI reouting method that permits the packet to go out N hops in all directions. If directional routing is selected, the APRS NODE uses one of four unique directional paths stored at that site. If the 3rd SSID bit is set, then a WIDE is added to the end of that path. IMPLEMENTATION: The key to the success of the MIC-ENCODER is that it is very versatile and can operate in all required modes. This allows for growth and improvment in the APRS systems without obsolesence. There are five possible operational situations as follows: NO TNC AT REPEATER: MIC-E path is set to 0 and anyone monitoring the (OR SIMPLEX VOICE) the repeater output with APRS can track users. STD. TNC @ REPEATER: MIC-E path is set to 1, thru 7 in DIGI mode. The TNC with the alias of RELAY (or WIDE) repeats packets onto the APRS packet channel. APRS NODE @ REPEATER: NODE routes according to MIC-E SSID bits only. DIGITAL APRS CHANNEL: MIC-E path can be 1 thru 7 in DIGI mode. Since the APRS NODE always routes on SSID bits anyway, the only difference betweeen the MIC-E DIGI mode and the SSID mode is the length of the packet due to the DIGI fields. THis means for interim compatibility, MIC-E users can operate generally with a path of 1,2, or 3 on all possible channels without specific configuring. Of course, if they turn off the DIGI mode while on an APRS NODE repeater channel, their packets will be much shorter, while still being routed by the SSID alone. SETTING DIGI or NODE MODE: To allow the 4 PATH selection bits to have the necessary dual definitions, the user needs to be able to switch the MIC-E between the DIGI mode and the SSID-NODE mode. THis is accomplished on power up. THe highest order digital bit, the DIR-OMNI switch, is checked on power up. If it is a high (no switch closure) then the MIC-E will be in SSID-NODE mode, and no digipeaters will be transmitted; routing will be done based on the SSID alone. This is consistent, since this condition of that switch is defined to be OMNI, which is the nominal initial condition of that switch anyway in APRS-NODE systems. To initialize the MIC-E into the DIGI mode, (the nominal configuration until voice repeaters implement an APRS NODE), the switch should be left in the DIR/DIGI position so that the MIC-E will initialize to the DIGI mode. MIC-ENCODER ALGORITHM: The MIC-E monitors the PTT line and will send a brief POSIT compressed into about 0.3 seconds when the PTT is released and as long as the POS-PERIOD has elapsed. This prevents repetative reports when the PTT is used in rapid sequence. The MIC-E also has an AUTO mode which will key-up the transmitter on its own at least every AUTO-PERIOD if the radio channel has been quiet for a given QUIET period. This permits mobiles not actively talking, to be tracked without interference to other users. In addition, the MIC-E can append 4 channels of analog telemetry and/or a BEACON text onto the end of the POSIT. If the telemetry is added, then it will always be 5 bytes. If the BEACON TEXT is added, then APRS will display that on the LATEST page and no posit is transmitted. The MIC-E is based on the APRS Micro-Interface-Module (MIM) designed by Dr. Carl Wick (N3MIM) and produced by Dr. Will Clement. The MIC-E is configured via its serial port using a PC program called MIC100.exe. This program provides a nominal TNC type command mode for setting the MIC-ENCODER configuration. It provides the standard cmd: prompt. Once the MIC-E is configured, you use the PERM command to cause the MIC-E to save the configuration in EEPROM. The following items can be configured: MYCall Sets the MIC callsign MYSymbol Sets the APRS symbol character VIA digi1,, etc Sets the Unproto digipeater path TXDelay Sets the key up delay PERiod Sets the nominal MIC cycle period POSIT N Sets POSIT period as N * cycle period TELEMETRY N Sets TELEMETRY period as N * POS period BEACON N Sets BEACON period as N * POS period AUTO N Sets AUTO period as N * POS period QUIET N Sets the QUIET period as N * cycle period BText Sets the Beacon Text PTT (1:0) Sets sense of the PTT signal. For the MIC-E, this is 1 since an external PTT transistor is used. MIC-E HARDWARE INTERFACE: As noted, the initial MIC-E prototype is a standard MIM with a few external components to integrate it to the user configuration switches and to the microphone PTT circuit. The 8 digital BIT inputs on the MIM are used as follows: D8, D7, D6, D5 Routing BITS. These are placed in the TO SSID field D4, D3, D2 Three message bits. Active low. D1 PTT INPUT. Active LOW. A5 Multiples the PERIOD by N where N is between 1 and 15. HO The Hold OFF is active LOW PTT PTT output is ACTIVE HIGH since an external PTT transistor is required. NOTE: SInce all bits have internal pull up resistors, then the default values are "1" so we use ACTIVE LOW negative logic. A SWITCH to GROUND is considered to indicate the condition. GPS INPUT STRING: Currently, ONLY the $GPRMC is supported since it has both position, course and speed. $GPRMC,123456.xx,A,3859.11xx,N,07629.12xx,W,123xxx,321.x,..... HARDWARE ITEMS: THere are two subtle hardware items in the circuit. First, there is an LED on the PTT output line (diode isolated from the real PTT) that shows when the MIM is pulling the PTT low. This shows the user that a POSIT is pending and will be appended when he releases the PTT. Second, the HOLDoff is connected to the receiver AND to a switch labeled AUTO. If the switch is set to AUTO OFF, then the switch permanently GROUNDS the HOLDoff so that no matter what the receiver is doing, the MIC-E will never initiate an AUTOposit. If thw switch is in the AUTO position, then the receiver audio establishes the value of the HOLDoff, and this permits a long period of silence to eventually (via the QUIET TIMER) to allow a POSIT to be transmitted regardles of the state of the PTT. RADIO INTERFACING: THere are three ways to wire the MIC-E to your radio system depending on your preference: A) Put a MIC jack on the MIC-E and run a MIC cable from the MIC-E to the radio. B) Wire the MIC-E in parallel to your mic at its connector C) Wire the MIC-E internally to the radio or to an auxiliary input (But the MIC-E must be able to sense the MIC PTT independently) D) Wire the MIC-E to a back-to-back plug jack combination Option A will require you to hard mount the MIC-E so it doesnt get tugged all over the dash-board. Option B or C is suitable if you only have one radio and mic. I prefer the universal option D where you simply solder back to back connectors and then run a 6 conductor cable of any length to the MIC-E. Conveniently, a standard 8 pin plug and jack will fit very nicely in a standard 1/2 inch to 3/8 inch copper reducing coupling. Just saw it in half so you can place it over the connectors after they are soldered back-to-back. Drill a 1/4 inch hole in the side for the cable to the MIC-E. MIC JACK MIC PLUG diode >--------*------|<---------*------> * Note, this diode >--------|--*--------------|------> is part of the MIC-E >--------|--|--*-----------|------> schematic and can be >--------|--|--|--*--------|------> placed here or in the >--------|--|--|--|--*-----|------> MIC-E box >--------|--|--|--|--|--*--|------> >--------|--|--|--|--|--|--|------> | | | | | | | | | | | | | *---< PTT out to radio } | | | | | *------< Audio to radio } | | | | *---------< Audio, MIC gnd } | | | *------------* GROUND } | | *---------------> 8 volts DC } to MIC ENCODER | *------------------> Receiver audio } *---------------------> PTT input } MIC-ENCODER ASSEMBLY NOTES: The MIC-ENCODER is the original APRS TELEMETRY TNC transmitter on a chip (MIM Module) with different code for the MIC-ENCODER algorithms. To implement the MIC-ENCODER function use the following interface circuit. MICROPHONE RADIO ---------- -------- MIC >-----------------------------------------*----------------> AUDIO 62k | *--/\/\/--* *-------------> MIC gnd | | +8v <----------*--------------------------------------*--------< +8 V | | | \ 1k | D1 | | / PTT >---*---------------------|<----------------------\------*-> PTT | | | MIC audio | / | | |-|----------------------| | * | | D2 | * * | MIC GND | V LED | *-|<-|-* MIM MODULE *-|----------* --- | PTT in | | | D3 | | * * * | PTT OUT *--|<--* |-|---------------|----|-| 5.6K |/c | | *----/\/\/\-----| Q1 NPN | *----*-----* |\e | | | c\| Q2 NPN | | S1 * | |--------*-----||-------------< RCV AUDIO | \ | e/| | .2 uf | | * === | - | | | | 6uF | ^ D4 | | | | | | | GND >----------*----*----*-----*----------*-----------*--------> GROUND CIRCUIT DESCRIPTION: D1 isolates the microphone PTT from the radio PTT input so that the MIM can key the PTT line while also sensing the MIC PTT condition. D2 prevents the MIC-E from grounding the PTT lead when the MIC-E is turned off. Q1 is an open collector PTT transistor. The LED gives a visual indication that a PACKET is ready to go and that the MIM is holding the PTT waiting for the user to release the MIC PTT. D3 is necessary to isolate the LED from the PTT from the microphone. The 62k minimizes circuit loading and the isolated MIC ground is connected to the lower end of the MIM audio pot. Q2, D3 and the lower two caps are an audio rectifier to drive the HOLDOFF input to the MIM when the radio is in use. S1 is the AUTO/OFF switch. When closed, it asserts a permanent hold off, so that the MIC-ENCODER will never auto-initiate a posit on its own. The following additional switches connect to the MIM to provide the user input for the MESSAGE bits and the PERIOD timing. D8 D7 D6 D5 D4 D3 D2 A5 +5v * * * * * * * * * | | | | | | | | | | --------------------- | | | | | | | / O | 7 | 7 | *----->\ 5k (O) | | | / | --------------------- \ | | | | ----- ----- ----- ----- //// //// //// //// D8 is the OMNI/DIR switch. D7 through D5 in the first Octal switch select the DIGI path. D4 through D2 in the second Octal switch select the one-of-seven message bits. A5 is an analog input which selects the time period multipler from 1 to 16. YOu can use a POT or a switch with a 15K and 5K resistor. This forms a 0.25 voltage divider so that in the GND position the value is zero and in the up position the value is 0.25 of 5 volts or about 4. This will multiply your POSIT period by 4. (Zero is actually 1) CONSTRUCTION: Mount all components in a suitable enclosure that provides easy user access. Notice that all MIC-to-radio connections are straight through except for the series diode in the PTT line. You may either wire the 7 leads in parallel with your existing mic plug or build a back-to- back plug/jack as noted above: MESSAGE BITS: The one-of-seven message capability of the MIC ENCODER will use the following default messages unless custom definitions are provided. Regardless of the message, the color of the symbol will always correspond to the mesage number as follows: MSG COLOR DEFAULT DEFINITION --- ------- ------------------- 0 normal Off duty 1 normal Enroute 2 normal In Service 3 normal Returning 4 dim yel Committed 5 Brt yel Special 6 dim red PRIORITY Trips alarms & centers all maps to unit 7 Brt red EMERGENCY! " "... SERIAL PORT FOR GPS AND CONFIGURATION: All users should consider using the APRS standard 1/8 inch stereo phone plug/jack for their serial data port on the MIC ENCODER and other small stand-alone-trackers. The phone plug is small, readily available, and is compatible with the nominal mono 1/8 inch phone plug found on many GPS units: GPS UNIT or PC LAPTOP MIC-E or embedded TNC --------------------- ---------------------- mono or stereo PLUG 1/8th inch stereo JACK TXD (data out) --------------------> TIP RXD (data in ) <-------------------- RING GND *-------------------* SLEVE To help remember, just think of the DATA comming out of the male plug tip. This applies to the GPS by itself or to the laptop used to CONFIGURE the stand-alone tracker. By using a "closed-circuit" jack, an internal GPS can be normally connected to the internal MIC-E or TNC, but plugging in the LAPTOP opens that circuit and connects the laptop to the TNC... 9 PIN CONNECTOR OPTION: If you are going to use the 4 channels of analog telemetry in your MIC encoder, you may want to use a Male DB-9 connector instead. This connector will allow you to use your standard TNC cable to configure the Mic-E and also be used as an input connector as follows: Pin 1 Analog input #1 Pin 2 RXD Data from the PC or GPS to the Mic-E Pin 3 TXD Data to the PC Pin 4 n/c May be used for speaker audio if not avail on mic Pin 5 Ground Pin 6 Analog input #2 Pin 7 n/c Pin 8 Analog input #3 Pin 9 Analog input #4 Once you make the analog inputs available outside the Mic-E, you should jumper them to ground with 5.1 volt zeners for protection. USE OF MIC-ENCODER BEACON TEXT: The Mic-E's BText is included on the end of a posit report. But due to APRS processing on receive, only the BText will get through and the posit will be ignored. This is why you should always set your BText rate at a lower rate than your POSIT rate. For NON-GPS equipped MIC-Encoders, a null posit 000000/000000 will normally be transmitted. For fixed station use, however, you can put your full LAT/LONG in the BText, and APRS will get the posit from that. Use the format of BT 3859.11N/07629.11W$000/000 Where $ is the usual symbol character. If you choose to do this, then you can put no other text in the BText, or the MESSAGE bits will end up not being properly parsed. A good example for this, is as a burgler alarm at a fixed location. Put the location in the BText as above, and connect the MESSAGE bits and RATE bit to contact closures in your alarm system. Have AUTO ON. When the bits get tripped, the message rate increases by a factor of 16 and the message changes to EMERGENCY or PRIORITY... DETAIL CONSTRUCTION NOTES: 1) First you must determine how to power the MIC-E. Measure the voltage at your MIC jack with a 330 Ohm load. If it is between 5 and 7 volts connect directly to the MIM 5 volt input WITH A 5 VOLT ZENER and a large 100 uF or so capacitor to ground. This gives reasonably good power to the MIC-E since the regulator is bypassed. If 7.5 or more, connect to the Vin pin on the MIC-E. Check all your radios with compatible connectors and plan to work with all of them... A good place to put the Zener and Cap is on the end near Q1 between the 4.7k resistors and the corner screw pad. Even if you have 8 volts, put the 47 uF cap across the 8 volt source. 2) Interfacing ANYTHING to your microphone circuit is not a trivial exercise. Any ground loop will add noise to the MIC audio (remember the alternator noise problems...) Drawing 15 ma from the MIC circuit adds to this problem too. Separately powering the GPS from the 12 volt system and then connecting that data ground to the MIC encoder is also a potential noise source. Be sure to use the isolated MIC ground as shown in the circuit. To do this, you must cut the trace on the bottom of the board that connects the bottom of the audio output level pot to the ground on pin 8 of U2. Then connect MIC Ground to the pot. The large 47 uF electrolytic capacitor across the entire circuit also helps to fiilter the power bus. Without these, the circulating ground currents will degrade the packet audio. If your packets do not sound clean, you may have to power your MIC-E with a 9v battery to get clean power and audio... 3) Do not solder to the Mic-E directly. Cut apart some plastic DIP sockets and solder to them. The pins on the Mic-E are larger than a DIP chip, and will not fit sockets with machined pins. Use a cheap plastic socket. You will want a 7 pin for pins 1 through 7, then an 8 pin for 8 through 16, and then a 7 pin for the remaining 6 pins. These sizes match perfectly the divisions in the functionality of the pins. Solder all of the front panel switches and the serial port jack to the appropriate DIP socket pins while still on the work bench. All of the chassis parts can mount from the inside of the chassis later. TIN BOTH surfaces before tack-soldering to the pins. The switches MELT easily, so pre-form the leads and solder QUICKLY with NO stress or the switch pins will deform. 4) YES the OCTAL switches can be mounted from the back after soldering. Just snap apart the two switches, remove the side plates, push them separately through the square hole, re-assemble on the outside and snap back into the panel. APRS REPEATER NODE: This special TNC NODE is designed to be intgrated into typical amateur voice repeaters. The TNC performs a number of special functions to fully implement the APRS LOCATOR SYSTEM: * It has true DCD to destinguish between voice and data * It uses this DCD signal to mute the repeater audio during packets * It digipeats all position reports from the repeater receiver to the dedicated APRS digital channel * It implements the APRS Directional Digipeating algorithm * It implements the APRS FLOOD-N digipeater algorithm for OMNI packets * It appends ADDText (usually the rptr freq) to the end of all packets ("Via 146.940") * It uses the external carrier detect for the APRS packet channel for true CSMA effeciency, but this need only be an audio COR detector. Notice that although the APRS REPEATER NODE function only listens on the voice repeater input and only transmits on the digital APRS packet frequency, it must also have a secondary carrier detect on the APRS packet channel to avoid collisions. This special APRS node function is NOT involved in any further routing on the APRS digital channel (I mean that it does NOT serve as a general purpose APRS digipeater on the digital channel). All it does is to insert the appropriate directional or OMNI digipeater path and digipeat the packet. This distinction, of course, is only a functional distinction, since APRS digipeater functions can be co-located, or even built into the same NODE box as long as dual digital receiver channels are maintained. PERFECT DCD OR MUTE CONSIDERATIONS: Since the ultimate acceptance of the POSIT-PACKET on voice repeaters will be determined by the minimization of the BRAAAAAAP sound on the repeater output, the DCD and subsequent muting of the repeater transmitter are very important. The APRS NODE must therefore provide a separate MUTE signal that is 99.99% percent accurate. Since most voice repeaters have simple analog delay lines of up to 50 ms to eliminate the squelch tail, the actual MUTE decision can be made as late as 50 ms after the initial DCD, and still be able to mute the packet from the repeater output. NOTES: Notice that the APRS REPEATER NODE will also work on the digital channel! In other words, the APRS REPEATER NODE algorithms can also be running simiultaneously in all of the APRS DIGI's so that the APRS compressed format will be picked up directly on the digital channel. These original packets are distinguishable because they DO NOT have an original DIGI field. Once a NODE processes them and adds the DIRECTIONAL or WIDE routing, they will be forwarded as usual. Notice that the NODE hardware can actually do both functions as long as dual digital receive channels are provided. APRS PROPRIETARY APRS PROPRIETARY