Post by ogrevorbis on Jun 25, 2015 13:51:13 GMT -6
So I have two AM transmitters. One is the AM88 and the other is a Greek one from Lesvos Electronics. They both produce this pulsing noise in the bass when I play music. It sounds like the bass is being switched on and off at around 10Hz. It never happens with talk or music without much bass. I've never heard this sound before listening to pro AM stations (but they usually don't play music, so maybe that isn't a good comparison.)
I found the problem is both the transmitters seem to distort frequencies below about 70Hz. I put an audio filter on with stereo tool and when I set the high pass filter to 70Hz, the problem is fixed.
Well the AM88 listed on northcountyradio suggests that its audio response is from 20Hz to 10kHz. No idea how flat that response is though.
Do you have a way to test the audio response yourself? At least use a software program to output a sine wave from 20Hz to 10kHz and measure the RF on a oscilloscope. Even a free online website like this works if you don't have the software.. onlinetonegenerator.com
Make sure to test on a few different radios including older analog tuned ones to narrow down the problem. Some cheap PLL radios do not like to demodulate low frequencies well. Also test the audio going into your transmitter right through an audio amplifier to make sure the sound going in isn't to blame.
To me this sounds more like an issue with something you are using for audio processing, or your sound source directly. AM transmitters should reproduce the bass audio range with no problem outside of much older transmitters that used cheap modulation transformers that had a narrow audio range.
This may be obvious but I need to ask.. You are not running both transmitters at the same time on the same frequency are you? If so that would cause some wacky phase errors with both signals fading in and out which would screw with any audio riding along their transmission.
I could try a sine wave generator just to see how it sounds, but I don't have an oscilloscope right now. I've tried two different receivers. Haha - no I'm not running both at the same time.
Anyway, it doesn't really bother me because I added the high pass filter and it fixed it.
Anyway, it doesn't really bother me because I added the high pass filter and it fixed it.
And you can live with that? Seriously though it wouldn't hurt to at least run a few tests to see where the issue is popping up. May save some hair pulling down the road.
You may have a power supply problem - low frequency oscillation in the audio is called "motorboating" (because it sounds like a motorboat) and is usually due to supply instability. It's often cured by adding extra electrolytic capacitors across the supply lines in the audio circuitry.
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