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Martin McCormick martin.m at suddenlink.netI have been playing with the rtl_fm program and most of it works amazingly well but I can not seem to get the squelch to stay open when receiving signals. I have not tried to analyze the source yet so I am asking what principle drives this squelch? In the analog world, the best squelches for FM receivers tend to be noise-driven and use a high-pass filter to filter out normal audio. When the noise drops below a preset threshold, the squelch opens. For AM receivers, a cheap and easy solution is to monitor the AGC and open the squelch when there is AGC voltage above a preset level. What I am noticing is that if I set the l value to a point just above where the noise stops, signals do open the squelch but even strong signals will not keep it from flickering on and off constantly. If I set the l value any lower, the squelch is always open so that is not the issue. I have tried signals that are absolutely full-quieting, with and without CTCSS and the squelch opens briefly, closes for a fraction of a second, opens for another fraction of a second and randomly flickers on and off for the whole transmission. On rare occasions, the squelch opens when the signal starts, stays open and then closes properly after the carrier leaves. Some of these signals are even ever so slightly noisy and I have heard this situation with and without PL tones or CTCSS so that doesn't seem to matter. Finally, I thought it might have something to do with too narrow a bandwidth so I increased the sampling rate to 24 K which made no difference at all. If the signal has voice on it, the flickering doesn't seem to be effected by the words. Basically, what is this squelch responding to to keep flapping? Martin McCormick WB5AGZ