Development update, July 22:

Work has begun on debugging the control box circuitry.  This part of the system has the audio input connections, power supply and interface to the cable that connects to the remote transmitter. There is a display for frequency, audio levels and a transmitter on indicator, which you can see in the photo at the top (the display is on the vertical board).

Work continues on the rest of the system…  The remote transmitter unit design is complete, and circuit boards have been ordered.  Once everything is complete, and the unit has been debugged, we will submit it to the testing lab for FCC certification. Once that process is complete, we can offer the unit for sale.

Still moving forward…  Stay tuned!

Control Box PCB

Development update, June 12:  A balancing act.

This is not as much of an update as it is a bit of important information about our approach to the design of the Parking Lot Radio.  No picture this time, just a bit of “why we do what we do.”

Recently, I got an e-mail from one of the folks on our mailing list, and it’s clear from our exchange that she has a background in broadcasting.  She was mentioning that she has a Part 15 AM transmitter she bought previously, and that it has noticeable hum due to a ground loop issue.  I won’t try to explain ground loop problems here, but it’s one of those things that happens from time to time in audio installations.  For those of you who want all the gory details, here is a link to the Wikipedia page on ground loops:  https://en.wikipedia.org/wiki/Ground_loop_(electricity).

So she has a ground loop issue and was wondering if we had plans to address that problem in our product.

Turns out, we do!

One of the issues with most consumer audio systems is that they are “unbalanced.”  They have an audio connection that has a “hot” and a “ground.”  The problem is, when a ground loop happens, it implants hum on the ground lead, which then gets into the audio.

Professional audio systems avoid this by using a system known as “balanced audio.”  In a balanced system, there are two audio connections, neither of which is connected to ground, along with a ground connection.  This works very well, because no matter what is happening with the ground system, the audio connection is unaffected.  That’s why virtually all professional audio systems, especially in radio stations, use balanced audio.  If you want more information about balanced audio, here is a link to the Wikipedia page on the subject:  https://en.wikipedia.org/wiki/Balanced_audio

Since we already build other products for professional users, we are well aware of this issue, and so, right from the beginning, we designed the Parking Lot Radio with a balanced audio system.  Hum and other unbalanced problems are non-issues, and that’s exactly the way it should be.

Of course, someone will ask the question, “What if all I have is unbalanced audio?”  The answer is simple:  Connect one side of the balanced input to the ground connection, and our input will become unbalanced audio, just like other consumer devices.  If for some reason hum or other noise becomes an issue, you can buy an unbalanced to balanced converter.  They are relatively inexpensive and work well.  So either way, problem solved!

Just another part of our commitment to getting it right.  Thanks for following our progress!

Development Update, June 10:  Time for a tune up!

Most of the design is done, and we’re down to finishing up details and completing software.  The biggest design task left is also one of the most critical:  The antenna tuner.

You see, an AM broadcast antenna is typically several hundred feet long.  Really!  Commercial AM radio stations turn the whole tower into the antenna.  For lower frequencies, the antenna can get really long!  For example, KFI (640 kHz) in Los Angeles has a tower that is around 1000 feet tall.  Tall antennas are the normal requirement.  Back in my broadcast engineering days, I once found myself on top of an AM tower that was 360 feet tall, but that was when I was young and foolish.

But those big antenna lengths are a problem for the Parking Lot Radio.  The FCC limits the length of the antenna to three meters, or just under 10 feet.  Let me think…  10 feet, hundreds of feet.  Hmmm…  Something doesn’t seem quite right here.

And that’s the problem.

The FCC deliberately has restricted the length of the antenna to make sure that the signal strength of unlicensed AM transmitters is not excessive.  This works very well, but has a serious side effect:  The three meter long antenna is very hard to match to the output of the transmitter, and when you do match it, it’s only for a fairly specific, limited range of frequencies.

So for Parking Lot Radio, we are designing an automatic tuner that will allow operation over the entire AM broadcast band with a three meter long antenna.  We have the architecture completed, and are pleased with the test results (see the photo).  The final design is in the works, and we hope to be prototyping soon!  Stay tuned!

Antenna Match

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.