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@ -60,6 +60,10 @@ This helped to lower the channel allocation time.
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How it works
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</p>
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<p>
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Note that the following protocol description is based on my research. It may be incomplete or wrong on certain details.
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</p>
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<p>
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Two tones are used for signalling:
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</p>
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@ -397,6 +401,7 @@ The transceiver of the base station switches to channel 19 and sends a paging se
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Then the base station returns to the ordered channel and waits 2 seconds for the mobile station to send the call acknowledge signal ("Rufbestätigung").
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If it is not received, the base station repeats the paging sequence again.
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If there is still no call acknowledge signal, it returns to idle state and indicates announcenemnt to the calling party that the mobile station is (currently) not available.
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(German announcement sais: "Dieser Anschluß ist vorrübergehend nicht erreichbar!")
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</p>
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<p>
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@ -410,7 +415,7 @@ The base station receives the call acknowledge signal and sends the call hold si
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<p>
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When the mobile station receives the call hold signal, it indicates ringing tone to the mobile subscriber.
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Additionally the car's siren is turned on, if the mobile subscriber is outside the car.
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Additionally the car's siren is turned on (if connected to the phone), if the mobile subscriber is outside the car.
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</p>
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<p>
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@ -59,10 +59,25 @@ We need to calibrate the speed of our transmit signal (and receive signal as wel
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First of all, we need a reference clock.
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Since most systems have standard crystals, we cannot use our system clock as reference without any help.
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I decided to use the NTP daemon.
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After running the system for one day, the clock speed should be accurate enough for our needs.
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After running the system for several days, the clock speed should be accurate enough for our needs.
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Be sure to choose a place with constant temperature.
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</p>
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<p>
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In /etc/ntp.conf define one or more servers to geht the time from. I suggest to use the German atom clock of the PTB:
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</p>
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<pre>
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server ntp1.ptb.de
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server ntp2.ptb.de
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server ntp3.ptb.de
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</pre>
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<p>
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This clock provides time base for European radio clocks and most German clocks.
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It is perfect for this project, so please use this clock.
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</p>
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<pre>
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netbook ~ # ntptime
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@ -96,7 +111,7 @@ To minimize this effect, don't use extension cables.
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<p>
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Since we have precise system clock now, we can measure the speed of our sound adapter.
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Later we will use the measurement result to adjust our signal processor.
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Be sure to have your machine running for at least 24 hours (with your sound adapter attached).
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Be sure to have your machine running for at least 3-4 days (with your sound adapter attached, in case you have USB sound adapter).
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The machine must have network connection to the NTP server.
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I suggest to use 'ntp1.ptb.de' and 'ntp2.ptb.de' as reference.
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Use command line option '-M' or '--measure' to measure my sound adapter.
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@ -123,8 +138,7 @@ dsp.c:218 notice : Clock: RX=19.44 TX=19.49; Signal: RX=19.35 TX=19.43 ppm
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One can see that the measured clock from the sound card (sample rate) differs about +19.45 PPM.
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Because I did not do any correction to the signal processing, the same error applies to the signal.
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Use command line option '-S 19.45,19.45' or '--clock-speed 19.45,19.45' to compensate the clock error by adjusting the signal processor.
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Let it run again for one hour and you will see that signal is now below 1 PPM.
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(Or close to it.)
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Let it run again for one hour and you will see that clock deviation is below 1 PPM.
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</p>
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<p>
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@ -142,13 +156,13 @@ Let it run again for one hour and you will see that signal is now below 1 PPM.
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</p>
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<p>
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<font color="red">Importaint: We need to transmit correct polarity!</font>
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<font color="red">Importaint: We need to transmit correct polarity, use auto polarity detection always!</font>
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</p>
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<center><img src="c-netz-burst_part.png"/></center>
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<p>
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C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a transmitted tone.
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C-Netz uses FSK (Frequency Shift Keying) by directly modulating the carrier, rather than modulating a tone, which is then transmitted via FM.
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The advantage is much higher data rate.
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The base station's carrier frequency is raised by 2.5 KHz to transmit a logical "1" and lowered by 2.5 KHz to transmit a logial "0".
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(The mobile station transmits reversed polarity.)
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@ -158,7 +172,7 @@ In the picture above, the polarity is wrong.
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(The logical "1" points down and the logical "0" points up.)
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You must check, with what polarity your transmitter is doing a positive frequency deviation.
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Also you must check your sound card's polarity on positive value beeing sent.
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To change polarty, use command line option '-F' or '--flip-polarity'.
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To change polarty, use command line option '-F yes' or '--flip-polarity yes'.
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(Note: don't confuse with '-P'!)
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</p>
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@ -149,7 +149,7 @@ Some adapters have mono input, especially on microphone jack.
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In this case you will always receive audio, no matter if you cross the channels or not.
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If you cross the channels, the left channel of your sound adapter is used.
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I use mono cables with only one channel on the tip of the plug.
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My sound adapter uses left channel on the tip, so i need to swap channels.
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Usual sound adapters connect left channel on the tip of the plug, so we need to swap channels to use mono plugs.
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Use the command line option '-x' or '--cross':
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</p>
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@ -170,7 +170,7 @@ Emphasis
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Pre-emphasis is used on the transmitter and de-emphasis is used on the receiver side.
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The nature of FM causes noise amplitude to be proportional to the transmitted frequency.
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(double frequency = double noise volume)
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To compensate this, most FM radios, as well as all analog mobile networks emphasis.
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To compensate this, most FM radios, as well as all analog mobile networks use emphasis.
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</p>
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<center><img src="emphasis.gif"/></center>
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@ -266,7 +266,7 @@ bnetz.c:528 notice : Round trip delay is 0.001 seconds
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</pre>
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<p>
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If command line option '-L 1' or '--loopback 2' is given, an external loopback is performed.
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If command line option '-L 2' or '--loopback 2' is given, an external loopback is performed.
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The receiver needs to be tuned to transmitter, so the software receives its own signal.
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The received quality level is relevant. It should be not less than 90 for B-Netz.
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</p>
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@ -274,7 +274,7 @@ The received quality level is relevant. It should be not less than 90 for B-Netz
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<p>
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If command line option '-L 3' or '--loopback 3' is given, the audio from the input is sent back to the output.
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It can be used to check a headset.
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Everything that is spoken into the microphone should be appear in the headphone.
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Everything that is spoken into the microphone should appear on the headphone.
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</p>
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[<a href="index.html">Back to main page</a>]
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