I'm glad this forum is still around. I really appreciate the help I've received here in the past.
I have a new issue. I'm currently using the stereo encoder on the FM transmitter I have. It sounds awesome listening on my car radio in the garage. But if I crank the power up and drive around the neighborhood, I noticed the stereo separation almost goes to a mono-like sound as I reach the end of the block; even though the signal is still clear. I've tried using the stereo encoding on the audio processor and doing either the right, left or mono mix of both sides. This sounds pretty good, but I can't get the same loudness that I can with using the transmitter's stereo encoder. I'm using the pre-emphasis on the audio processor and have it disabled at the transmitter. Just wondering what the issue might be. Thanks.
Your 19kHz pilot signal should be around 10% of the composite signal from your stereo encoder. If it is set lower than this then radios will switch to mono before they get on the fringe of the station signal so you might want to make sure that is setup properly.
Not knowing if your pilot signal is setup correctly can become a problem, especially if you don't have a spectrum analyzer which many of us don't because of expense. Luckily most computer soundcards work up to 96kHz with somewhat flat response so you can plug your MPX output into a spare computers sound card input and use some spectrum analyzer software to see what your composite signal looks like and measure the pilot level there. If your audio processor/encoder allows settings to be adjusted for this then you can get away with tinkering around and crossing fingers without using expensive test equipment.
Keep in mind a lot of car radios and even some portables have stereo blending where they start to mix the left and right together as the signal gets weaker to cut down on decoder static. Usually higher end radios have this alongside the feature of switching completely to mono reception when the station becomes too weak.
My only other guess would be the transmitter itself not passing the full composite signal through because of some low pass audio filtering somewhere.
Gents, thanks for taking the time to reply on my issue.
Kage, thanks for the clarification on the pilot signal. I have made sure it's around 10 percent when letting the audio processor do the stereo via L, R or Mix of both channels. That works great, but the loudness is lacking. I assume the built in stereo encoder in the transmitter would be up to the job, but perhaps that is not correct. My sound card is 192 kHz and I've hooked it up to an oscilloscope. Everything looked good there. Perhaps it's just the weak signal, as you say and the car radio blending the left and right. Also, it's very hard to find a truly free radio signal in today's metropolitan areas. Perhaps interference could be the problem?
tron24, I believe the sound card is AC coupled as most are. It's onboard realtek line out. Perhaps I'm wrong on this. If it's AC coupled then this is likely my issue as it seems I've had to work very hard to get even moderate sound, whereas I set up a station for a friend and it sounded 50 times better with little effort. Pre-emphasis is set up correctly - disabled at the transmitter via dip switch and enabled at 75 microseconds at the processor.
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