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wip/fm_mpx
Christophe Jacquet 2014-04-06 22:38:53 +02:00
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@ -57,13 +57,13 @@ All arguments are optional:
By default the PS changes back and forth between `Pi-FmRds` and a sequence number, starting at `00000000`. The PS changes around one time per second.
### Clock calibration (if experiencing difficulties)
### Clock calibration (only if experiencing difficulties)
The RDS standards states that the error for the 57 kHz subcarrier must be less than ± 6 Hz, i.e. less than 105 ppm (parts per million). The Raspberry Pi's oscillator error may be above this figure. That is where the `-ppm` parameter comes into play: you specify your Pi's error and Pi-FM-RDS adjusts the clock dividers accordingly.
In practice, I found that Pi-FM-RDS works okay even without using the `-ppm` parameter. I suppose the receiver are more tolerant than the RDS spec.
In practice, I found that Pi-FM-RDS works okay even without using the `-ppm` parameter. I suppose the receiver are more tolerant than stated in the RDS spec.
One way to measure the ppm error is to play the `pulses.wav` file: it will play a pulse for precisely 1 second, then play a 1-second silence, and so on. Record the audio output from a radio with a good audio card. Say you sample at 44.1 kHz. Measure 10 intervals. Using [Audacity](http://audacity.sourceforge.net/) for measure determine the number of samples of these 10 intervals: in the absence of clock error, it should be 441,000 samples. With my Pi, I found 441,132 samples. Therefore, my ppm error is (441132-441000)/441000 = 299 ppm, **assuming that my sampling device has no clock error...**
One way to measure the ppm error is to play the `pulses.wav` file: it will play a pulse for precisely 1 second, then play a 1-second silence, and so on. Record the audio output from a radio with a good audio card. Say you sample at 44.1 kHz. Measure 10 intervals. Using [Audacity](http://audacity.sourceforge.net/) for example determine the number of samples of these 10 intervals: in the absence of clock error, it should be 441,000 samples. With my Pi, I found 441,132 samples. Therefore, my ppm error is (441132-441000)/441000 * 1e6 = 299 ppm, **assuming that my sampling device has no clock error...**
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