Bandpass Revisited
Posted on October 13, 2012
There is another blog on the subject of audio band pass filtering, but I don’t wish to review any of that, and prefer to start a fresh look at the subject.
There is another blog on the subject of audio band pass filtering, but I don’t wish to review any of that, and prefer to start a fresh look at the subject.
There are serious reasons, based in the laws of physics, why audio sometimes needs to be “confined” to the bandwidth of the medium toward which it is directed.
AM radio is a good starting point, knowing that the bandwidth of an AM channel is 10kHz (in the Americas). For my frequency of 1550kHz, for example, this means that my channel occupies 1545 to 1555kHz, centered on 1550kHz. This means that there are 5kHz negative and 5kHz positive, requiring an actual audio bandwidth of 5kHz to fit that channel like a glove.
The exact state of the FCC rules on AM bandwidth have been stretched by the application of IBOC (HD-digital), which spreads a station’s footprint significantly so that it blots out three channels instead of one.
I tried running a 20kHz audio stream on 1550kHz AM, and the splash-over was messing up 1570 AM, a station from 40 miles away that can be heard here albeit very weakly. I found that I could run a bandwidth of 15kHz, which splashed right up to the edge of the 1570 signal, which begins at 1565kHz.
On shortwave the spacing of radio channels is only 5kHz wide, requiring a bandwidth of 2.5khz audio.
My newly increased audio stream at 48kbps seemingly allows 24kHz audio bandwidth, although the sample rate of the stream is 44.1kHz, and I’m not sure how the two key numbers that define a digital streaming audio channel interplay with each other.
My low bit rate stream at 16kbps, sample rate 11.025kHz is a whole other concern.
The audio signal flowing from the Winamp Playlist is processed by Stereo-tools, a highly professional software with a very precise bandwidth control, but it only controls one audio stream, and right now four different band pass filters are needed in this radio station.
Time to declare a new project, the design of circuits that can be implanted at the input of part 15 transmitters to control their carriers as appropriate.
This still leaves the question of needing an extra software band-pass control for online streaming, but the part 15 half of the problem can be solved by outboard circuits.
