RepeaterPage » History » Version 4
max, 04/22/2017 04:04 PM
1 | 1 | max | |
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2 | 4 | max | = SUMMARY = |
3 | * Simultaneous multi-channel transmission/reception |
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4 | * Uses USRP and/or conventional receiver/transmitter hardware |
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5 | * Generic interface to Asterisk via app_rpt (full VOIP, radio control, and standard repeater functions) |
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6 | * Features: P25, analog NBFM with CTCSS |
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7 | * Expected to be fully "ROIP" [Radio Over IP] compatible |
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8 | |||
9 | 1 | max | = Multi-channel reception using the USRP = |
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11 | Assume we want to simultaneously receive the four signals shown below; |
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12 | one conventional (analog) FM voice channel plus three P25 |
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13 | signals. The two P25 voice channels are to be IMBE-decoded whereas the |
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14 | 4 | max | P25 data channel is to be sent to Wireshark after decoding. For all three voice |
15 | channels, we want to forward the received PTT and audio info to Asterisk app_rpt. |
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16 | The received PTT [Push To Talk, or "keyed"] signal is a bit that ideally tracks |
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17 | the state of the PTT key at transmitter, indicating the presence or absence of the |
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18 | received signal. |
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19 | 1 | max | |
20 | 3 | max | [[Image(sa.png)]] |
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22 | 1 | max | Fig. 1 - Spectrum of repeater input band (sample; diagram not to scale) |
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24 | For now, it's necessary to edit the source code file manually to specify |
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25 | the list of channels/modes to be received (file usrp_rx.py): |
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26 | |||
27 | {{{ |
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28 | channels = [ |
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29 | {'freq':435.200e6, 'mode':'c4fm', 'port':32001}, |
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30 | {'freq':435.350e6, 'mode':'fm', 'port':32002, 'ctcss':97.4}, |
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31 | {'freq':435.600e6, 'mode':'cqpsk', 'port':23456, 'wireshark':1}, |
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32 | {'freq':435.775e6, 'mode':'cqpsk', 'port':32003} |
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33 | ] |
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34 | }}} |
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35 | |||
36 | Individual channels are defined one per line; note that all definition lines |
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37 | except the last must end with a comma. |
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38 | |||
39 | We choose a frequency somewhere close to the center of this band. |
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40 | which will set the USRP's nominal receive frequency; this must also |
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41 | be manually set in the source file: |
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42 | |||
43 | {{{ |
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44 | center_freq = 435.500e6 |
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45 | }}} |
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46 | |||
47 | Before running the receiver app, we |
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48 | * measure the current calibration error value (I use kalibrate) |
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49 | * determine the optimum USRP receiver gain value |
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50 | The values used in this example are +1234 and 35, respectively. |
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51 | |||
52 | We're now ready to start the receiver: |
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53 | |||
54 | {{{ |
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55 | usrp_rx.py -RA -c 1234 -H 127.0.0.1 -g 35 -d 25 |
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56 | }}} |
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57 | |||
58 | The receiver continuously monitors all four channels. For each of the three |
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59 | voice channels, the audio and the PTT info ("key up" and "key down" events) |
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60 | are forwarded to asterisk app_rpt over separate UDP channels. |
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61 | |||
62 | = P25 and/or analog NBFM Reception using a discriminator-tapped receiver = |
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63 | |||
64 | One or more disc-tapped conventional receivers may be used at the same time, and can |
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65 | coexist with one or more USRP's. |
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66 | |||
67 | 4 | max | An audio cable is connected between the disc-tap point in the receiver and the PC soundcard. |
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69 | 1 | max | Single-channel reception is possible using disctap_rx.py. |
70 | 4 | max | The app dynmically auto-detects the modulation type (P25 or analog NBFM). |
71 | 1 | max | |
72 | Audio and PTT events are forwarded to asterisk app_rpt over two separate UDP |
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73 | 4 | max | channels, depending on modulation type: |
74 | * received P25 audio is sent to asterisk on UDP port 32004 |
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75 | * when analog NBFM is received with the proper CTCSS tone (97.4 Hz), port 32005 is used |
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76 | 1 | max | |
77 | {{{ |
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78 | disctap_rx.py -i -A 0.05 -c 97.4 -H 127.0.0.1 -p 32004 -g 35 -d 25 |
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79 | }}} |
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80 | |||
81 | The -g (gain) parameter is used to set the proper audio gain level. See |
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82 | the hardware page for further guidance - this value is important for achieving |
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83 | correct operation. |
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84 | 4 | max | |
85 | = Asterisk and app_rpt = |
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86 | |||
87 | For all voice modes (IMBE and analog FM) the audio is transmitted |
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88 | as frames over the UDP channel to and from Asterisk in the standard native audio format: |
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89 | * 50 frames per second |
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90 | * 160 audio samples per frame |
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91 | * 8000 samples / second |
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92 | * signed |
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93 | * 16-bit |
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94 | * linear |
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95 | |||
96 | TBC |
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97 | |||
98 | = Transmission (USRP and conventional) = |
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99 | |||
100 | TBC |