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Fix in the README

wip/fm_mpx
Christophe Jacquet 2014-04-14 18:56:35 +02:00
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README.md
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@ -116,7 +116,7 @@ The shaped biphase symbol is generated once and for all by a Python program call
Internally, the program samples all signals at 228 kHz, four times the RDS subcarrier's 57 kHz. Internally, the program samples all signals at 228 kHz, four times the RDS subcarrier's 57 kHz.
The FM multiplex signal (baseband signal) is generated by `fm_mpx.c`. This file handles the upsampling of the input audio file to 228 kHz, and the generation of the multiplex: unmodulated left+right signal (limited to 15 kHz), possibly the stereo pilot at 19 kHz, possibly the left-right signal, amplitude-modulated on 38 kHz (suppressed carrier) and RDS signal from `rds.c`. Upsampling is performed using a zero-order hold followed by an FIR low-pass filter of order 60. I use a sampled sinc windowed by a Hamming window. The filter coefficients are generated at startup so that the filter cuts frequencies above the minimum of: The FM multiplex signal (baseband signal) is generated by `fm_mpx.c`. This file handles the upsampling of the input audio file to 228 kHz, and the generation of the multiplex: unmodulated left+right signal (limited to 15 kHz), possibly the stereo pilot at 19 kHz, possibly the left-right signal, amplitude-modulated on 38 kHz (suppressed carrier) and RDS signal from `rds.c`. Upsampling is performed using a zero-order hold followed by an FIR low-pass filter of order 60. The filter is a sampled sinc windowed by a Hamming window. The filter coefficients are generated at startup so that the filter cuts frequencies above the minimum of:
* the Nyquist frequency of the input audio file (half the sample rate) to avoid aliasing, * the Nyquist frequency of the input audio file (half the sample rate) to avoid aliasing,
* 15 kHz, the bandpass of the left+right and left-right channels, as per the FM broadcasting standards. * 15 kHz, the bandpass of the left+right and left-right channels, as per the FM broadcasting standards.