Hi, I just committed the operator "filter.rc" implemented by copying directly wikipedia[1]. The sound is not so great from first tests but in theory this is what you need (from you mail).
++ Sam. [1] https://en.wikipedia.org/wiki/Low-pass_filter On Thu, Aug 27, 2015 at 4:14 AM, Lars Kruse <[email protected]> wrote: > Hello, > > recently I stumbled upon the (probably well-known) FM-specific filtering > called > "Pre-Emphasis" (before broadcasting) and "De-Emphasis" (after receiving) which > is used for FM broadcasts worldwide (with slightly different filtering > parameters e.g. in US and Europe). > Here is the quick summary of this concept: > https://en.wikipedia.org/wiki/FM_broadcasting#Pre-emphasis_and_de-emphasis > > Here at the community radio where I am doing a bit of technical stuff (mostly > IT) we use a single sound processing server for the final signal preparation > (for FM broadcasting). > Until now we use the very same signal for a web stream. > > This is probably not a brilliant idea due to the above Pre-Emphasis (boosting > high frequencies). > Sadly I do not see a reasonable way to grab the processed signal before the > Pre-Emphasis filtering is applied. Thus my only option seems to be to apply a > De-Emphasis filter afterwards (similar to the one that is built into all FM > receivers). > > > I have to admit that I have a very limited amount of understanding with > regards > to signal processing. Thus the technical description of the "De-Emphasis" > filter > does not help me a lot: > "a simple RC filter circuit with a 50 µs time constant" (for Europe) > > I found a forum thread [1] where the audacity community discusses such a > De-Emphasis filter, but I failed to understand how I could apply a similar > filter with liquidsoap's processing functions ([2]). > > Additionally I found another source of information for a "De-Emphasis" filter > implementation in a 5-page design sheet [3]. It contained the following > four components for De-Emphasis: > > VCO: Output Amplitude is 2 V; Quiescent Frequency is 20 kHz; Input > Sensitivity > is 5 kHz/V; Initial Phase is 0. > > PLL Low-Pass Filter: Butterworth; Filter Order is 1; Passband Edge Frequency > is 2π × 10500 rad/s. > > Deemphasis Filter (First-Order Low-Pass Filter): > Time Constant is 1/(2π × 2100) s. > > Low-Pass Filter: Butterworth; Filter Order is 4; > Passband Edge Frequency is 2π × 4000 rad/s. > > Sadly again my understand of signal processing failed to turn this description > into a set of liquidsoap filters. > > > I would highly appreciate any partial hints pushing me into the right > direction > in order to solve this puzzle. > > Thank you for your time! > > Cheers, > Lars > > > [1] http://forum.audacityteam.org/viewtopic.php?f=13&t=9141&start=10 > [2] http://liquidsoap.fm/doc-svn/reference.html#Source___Sound_Processing > [3] http://www.ece.iastate.edu/~zluo/321f.pdf > > ------------------------------------------------------------------------------ > _______________________________________________ > Savonet-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/savonet-users ------------------------------------------------------------------------------ _______________________________________________ Savonet-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/savonet-users
