I need to create a good low pass filter for recordings I have of a radio show I used to do. The recordings are on VHS hi-fi and were recorded over the air. The sound quality problems vary over the many tapes, which I'll tackle with various noise reductions as I go along archiving, but one thing is for sure... FM broadcasts are supposed to be limited to 15000 Hz. Anyone have experience or knowledge on this?
My best guess at this point is to use the scientific filters, low pass, cutoff at 15000. I'm not sure of the "order" or transition bandwidth to use, since I don't know how much usable signal there could be above 15000 and below the stereo carrier signal at 19000 Hz. Spectrum analysis shows the 19k carrier there at about -40db, so the ol' RCA VHS hi-fi recorder and Panasonic tuner I used back in the day were getting all they could from that transmission. I can see signal below and even above the 19k carrier, and live spectrum analysis shows it bouncing to the beat, so I am assuming much of that above 15k is unintended harmonics.
Any hints, ideas, or recommended settings for a filter?
Yes, analog FM & TV(audio) broadcasts are limited to 50hz - 15,000hz. You are correct about the 19,000hz pilot tone. This indicates to a stereo receiver that the station is in stereo. The audio you are seeing above 19k is the L-R stereo information.
The main audio 50-15k is in fact MONO. It is both the left and right channel. This makes all stations backward compatable to mono recievers. The "stero generator" also creates a left channel minus the right channel audio stream that is injected at a high frequency.
You radio uses both of these to recreate the orignal stereo information.