Development Update – May 21: The Goldilocks Principle

Not too hot, not too cold, but just right…  In broadcasting, this is a well-known principle when it comes to modulation levels.  It doesn’t matter whether it’s an AM or FM station, the modulation level, the level of how loud the sound is, is critical.  Too little modulation and noise becomes a problem.  Too much modulation and the signal becomes distorted.

Radio stations pay big bucks for equipment that makes the modulation levels “just right.”  It’s not uncommon to see thousands of dollars spent on the audio system.  Unfortunately, the average parking lot radio user doesn’t have that kind of budget!

As it turns out, this is not a simple problem to solve.  A lot of low-cost short range transmitters solve the issue with simple audio limiter circuits that, quite frankly, don’t sound very good.  In our initial design efforts, we found a low cost part that’s used in inexpensive digital music players to prevent overly loud music from creating damaging headphone levels.  We built up a test circuit, and while it worked well enough initially, more extensive testing revealed some serious shortcomings.  We decided that while it might be good enough for some applications, it wasn’t good enough for all of them.

Back to the drawing board!  We drew on our experience in the design of high-quality professional wireless intercom systems.  For a few cents more (literally) than the low cost part, we designed a circuit with a professional part that produces sound quality that rivals the sound produced by processors that radio stations pay thousands of dollars for.  No, we won’t be going into the business of making broadcast audio processors any time soon, but for the parking lot radio system, this circuit is definitely the right answer.

Audio Processor Test Circuit
The Goal: Excellent Audio!

Yes, it is taking us a bit longer to finish the work here, but we think that the extra time and effort is well spent.  Hang in there with us!  We think you’ll be pleased with the end results.

Development Update – May 1:  As we design and test circuitry, we are learning several important lessons.  For example, after extensive testing, we decided the audio processor we had selected was not up to the level of performance we needed.  It was originally designed to limit audio levels in consumer headphone applications, and while it worked we were not pleased with the overall sound it produced.

Back to the drawing board!  We redesigned the circuitry with a professional grade audio processing engine used in high quality commercial applications such as performance-grade wireless microphones.  Much better!

But in the process of all this testing we came to the realization that no matter how good the audio processing is, if the levels being fed into our transmitter system are not correctly adjusted the quality of the sound will suffer.  Too low, and the modulation will be faint, the signal will be prematurely noisy and range will be reduced.  On the other hand, if the level is too high, the audio will sound compressed, and quiet periods will have all kinds of background noise amplified and transmitted.  Neither of these situations is desirable.  Something needed to be done.

So…  We are adding an audio level metering system to the control unit, with a front panel gain adjustment so that the user can ensure that the sound he is transmitting is optimized.  The cost of these improvements is fairly minor, and when weighed against the improvement in performance, well worth the difference.

On a non-technical note:  We have a started a “Go Fund Me” page for those interested in helping us finance the development effort.  Since this is a “ground-up” development, we decided not to go with a Kick-Starter campaign, as we cannot promise anything in return for donations.  If you want to help us with expenses, here is the link: https://www.gofundme.com/f/radio-design-group.  We will also put this link in the website menu as well.

Thanks again to all for your interest in Parking Lot Radio.  More updates coming, so “stay tuned!”

Professional balanced input and audio processing circuitry.
Vintage Grundig radio makes a great audio testing system for AM.

Development Update:  The audio processing circuitry test board is complete, and has been tested successfully.  This is a critical part of the audio path that keeps the transmitter from overloading on loud audio peaks without creating distortion.  The result is very pleasing sound, while maintaining good high level modulation, which is critical to getting adequate range.  Listening on my vintage Grundig tabletop radio to a Mozart piano concerto is a convincing demonstration that, properly done, AM delivers excellent sound.

What’s left to do?  The digital synthesizer and control circuity, chassis and a variety of miscellaneous items are left, and are being worked on by the team here at Radio Design Group.  Stay tuned for updates!

AM Transmitter Test Kludge
Transmitter Output Test Results
Transmitter Output Demodulated Signal

Development Update:  Houston, we have AM!  And very good AM at that!  Good output power, excellent modulation levels, and low distortion.  Typical AM stations run around 3%, and the FCC limit is 5% harmonic distortion.  Our prototype is running just under 1%, which is a very good figure.  Now, on to tidying things up, and getting the control circuits, power supplies and audio processing finished.  Stay tuned for more updates!

AM modulator circuit test board.
Modulator output waveform: Perfect!

DEVELOPMENT UPDATE:  First test of the transmitter modulator has been completed and it works perfectly, right out of the chute!  The scope shows how good the audio is, and it is very good.  Again, great progress in a very short amount of time!  More updates coming!

DEVELOPMENT UPDATE:  First test of the transmitter amplifier has been completed, and fine tuning has begun.  At the 100 milliwatt input level (the maximum per FCC Part 15 rules) we are getting about 60 milliwatts output.  This is a great start, and we think we can improve the efficiency to a better level with some circuit refinements.  Nonetheless, this is great progress, and in a very short amount of time!  Stay tuned for updates!

PA Test Board.
PA Test Board.