From 921d60b0de7f81334ca38d105445c22537a47d3a Mon Sep 17 00:00:00 2001 From: Christophe Jacquet Date: Mon, 28 Apr 2014 19:28:06 +0200 Subject: [PATCH] Fix the README --- README.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/README.md b/README.md index 43d6530..cdcb6c9 100644 --- a/README.md +++ b/README.md @@ -63,7 +63,7 @@ By default the PS changes back and forth between `Pi-FmRds` and a sequence numbe 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 stated in the RDS spec. +In practice, I found that Pi-FM-RDS works okay even without using the `-ppm` parameter. I suppose the receivers 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 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 (audio card) has no clock error...**