Processing for Part 15 AM Radio


One of the most asked-about topics by those new to broadcast engineering (as well as one of the most argued subjects among those who have been at it for awhile) is the subject of audio processing for radio.

Because the Rangemaster is an AM unit, the subject matter of this article will focus directly on processing for AM radio.

Large-market stations spend thousands of dollars for their processing gear, and often upgrade it when the "next big thing" hits the market. While the "big boys" may carry things a bit too far in this regard, this does show the importance of proper processing.

But for the operator of a part 15 station, spending top-dollar for the latest high-tech box just coming down the pike is probably more than somewhat cost-prohibitive.

Fortunately, with a little knowledge and careful spending, surprisingly good results are obtainable on quite a modest budget.

In order to go about this intelligently, before we get on to what affordable and effective equipment is available today, let's take a look at the history of audio processing for radio (which starts with AM anyway) to see how all this began:

First A little History:

In the early days of radio, a raw untreated signal was fed directly into the transmitter.

Therefore, everyone operating audio equipment in a radio station was a trained radio engineer, and was expected to manually control audio levels from moment to moment and from source to source.

This was a valued skill because too low an audio level would seriously impair the station's effective range (as it still will today), while too high a level would not only sound bad, but would also potentially damage the transmitter and/or cause interference with other stations.

The FCC could fine a station for overmodulation, but station managers wanted the station as loud as possible, so engineers who were more skilled at controlling levels were in high demand.

Before long, smart chief engineers began to install the first audio limiters (developed by the telephone industry) to "put a ceiling" on the maximum level and safeguard against a fine from the FCC. This allowed their engineering staffs to push levels up as high as possible to please the managers without incurring the wrath of FCC officers. Also, these engineers discovered that they could drive the limiter (within reason) a little harder, and pick up extra volume.

Over time, limiters (and other devices) were developed specifically for radio, and the broadcast processing industry was born, and has gone through a remarkable evolution ever since.

Probably the most significant single advance was made by CBS Laboratories in 1959 with the introduction of their Audimax, then later their Audimax/Volumax combination (affectionately known as the "Max Brothers" to engineers since the 1960s).

In fact, CBS was so successful with "The Max Brothers" combo that many of these boxes remain on the air on some commercial stations even today! (More on these classic units in our section on gear.)

The Airchain:

Let's take a look at the basic elements of processing in what is known within the industry as the "airchain":

The airchain is simply the chain of devices that the audio passes through on the way to the transmitter. These may be stand-alone components or "all-in-one" boxes (or a combination of the two). Although there are many things that may be done to the signal along the way, a basic airchain has two main components which MUST exist for truly professional results, known as "leveling" and "limiting."

Let's go through a basic airchain in the same order that our audio signal would do:


In a basic airchain, the first thing the raw signal hits is called a "leveler," or "AGC" (automatic gain control).

Remember the skilled engineer of old who manually controlled the levels? Well, this same "slow hand on the volume control" function is the job of our AGC. It simply slowly changes gain to keep things in the general ballpark level-wise for what is to come.

Of course, much like the engineers of old, the quality of work done by these devices varies tremendously. And also like the engineers of old, the better amongst them are more expensive. There are stand-alone levelers that cost thousands of dollars, but there are many available for much less.

One important thing to know about leveling is that most compressors sold for studio or home recording do NOT perform this function very well. After all, the designers of these compressors expect a skilled engineer to be setting the levels that are being fed into them!

Examples of stand-alone levelers include the Aphex Compellor (which also has other tricks built-in) and TransLanTech's Ariane (very expensive). But fairly decent performance can be had from something much less expensive, such as RDL's ST-GCA3 "Gain Control Amplifier."


Now that our signal is pre-conditioned to stay at roughly the right level, our signal now goes on to the limiter, which sets an absolute maximum possible level.

Again, remember the engineers of old who discovered they could push the limiter a little bit and pick up extra volume? Well, the idea stuck. And boy did it stick in a big way!

The trouble is that while pushing a limiter into gain reduction definitely gives us more volume, it also does other things.

On some program material, properly done, it can actually make things sound better (which is why both the motion picture and recording industries use limiters all the time). Overly used, or on some program material, over-limiting can sound very distorted, and can cause that which professionals call "listener fatigue."

The better limiters are capable of creating more volume with fewer "processing artifacts," and some programming, such as speech will often tolerate much more heavy limiting than will be tolerated by certain music. Also, some different music forms will tolerate more limiting than some others.

Of course, it's all a matter of opinion (and here's where the arguments begin):

We definitely want some of this limiting action to be going on (especially on AM), but if we set the limiter up to where it sounds pretty good on one thing, the next thing we put on the air might suffer. And (as we will find at every stage of our airchain) not enough processing, and we might lose some of our effective range, while too much processing can cause our listeners to tune out due to "listener fatigue."

Good luck figuring out the best settings for your station. The best advice we can give is to start with conservative settings which sound good on most of your stuff, and then when you make changes, make SMALL changes and wait a long, LONG time to make your mind up about them. If not, you will probably drive yourself crazy!

We'll take a look what limiters to consider purchasing for Part 15 AM a little later in this article.

Other Things in the Airchain:

Because engineers LOVE to change things, other manipulating (mutilating?) devices have come to be inserted into airchains: equalizers, compressors, "exciters," multi-band processors, etc. are all common additions to airchains.

While these other devices may often be useful and desirable, it is important to remember that the more knobs, switches, options and possible settings there are, and the more complicated our airchain becomes, the greater the chance for disaster.

While we may have more control with the addition of more gizmos, we also have more chances to get it wrong, more chances for equipment failure, and more signal degradation. Also, all of these things cost more money.

We in the business have often heard radio stations with basic, simple airchains who (hands down) beat the immortal bejeeziss out of some mega-buck airchains operated by their competing brethren!

"NRSC" (Good news / Bad news):


However, the truth is that on MOST receivers (most of the time), a louder, clearer signal will result from our compliance with NRSC.

Also, we are much less likely to cause interference by the operation of our transmitter, so our voluntary compliance with NRSC makes us a "good neighbor."

On the other hand, it is also true that with a pristine audio chain and a very high quality receiver, AM radio can sound stunningly good. Frequency response can extend well out to 20khz and beyond!

This is something that a licensed high-power station cannot do no matter how much money they spend, because they are bound by law to cut off their audio at 10khz.

Also, if nostalgia is your bag, you may wish to ditch the idea of NRSC and use vintage processing gear.

The choice is yours, but if you DO decide to opt out of NRSC, please be careful not to boost the high-frequency content of your audio to compensate for lack of high-end on cheap receivers. Doing that will cause what is known as "splatter" (a very irritating form of interference which will win you no friends).

We'll get into how to deal with NRSC as we progress. But first, let's take a look at what NRSC is, and how it came to be:

How FM Has Changed AM (and the creation of 'NRSC'):

FM radio is certainly a wonderful thing, and while there IS such thing as non-licensed (Part 15) operation of FM, we find that the rules are different for part 15 FM. Basically, with the rules for part 15 FM, you will legally get about as far as your property line (if you're lucky).

What this basically boils down to is that with an unlicensed operation, AM is the way to go if you want to actually reach an audience (unless you want to break the law).

Knowing that, we still need to look at what the emergence of FM did to the world of AM, because when FM came to prominence, it affected AM in some surprising ways:

Back in the late '70s, even though the quality of AM transmission equipment had continued to improve, listeners had started migrating over to FM, and the quality of the average AM receiver circuits had begun to be much less of a concern to the manufacturers of consumer radios.

The manufacturers began to spend less and less money on the AM sections of their receivers, and real high-quality AM receivers have consequently become very rare as a result. After all, if consumers mostly care about FM, and if two receivers are sitting side by side on the store shelf, and if the only perceivable difference to the consumers is that the one costing $100 more gets better reception and better quality sound on the AM band, it doesn't take them long to make a decision as to which one to buy!

The truth is that the average AM section of today's receiver is of VERY low quality, and ironically, the AM sections of car radios and "Shortwave" portables are usually of higher quality than those found in home "Hi-Fi" units. That is NOT to say that high-quality AM receiver s are not available; it's just that our average listener is not likely to have one.

There are three major shortcomings of these newer cheap-o receivers:

First (and of most concern to Rangemaster owners) is that the cheap detectors in these things are not very sensitive. In other words, a perfectly usable signal for a "real" detector circuit may be just too low for these pieces of junk to detect.

Secondly, the 'selectivity' of what passes for an AM detector in your average receiver these days is quite deplorable. In other words, two stations who are close proximity to each other on the band seem to "walk all over each other" when you try to tune them in.

Well, in this situation, if you are a flea-power station up against a powerhouse blowtorch of a station, you are going to lose (and you are going to lose BIG). Your best defense against this situation, by the way, is to carefully select your frequency in order to avoid such a situation.

Third, these cheesy little detectors are severely lacking in their ability to detect the higher audio frequencies in our program material. In fact, it could be said that for the average AM receiver, we might as well not waste our energy (modulation) even putting these higher frequencies on the air! Basically, there are very few radios that actually know that anything above about 7.5 Khz even exists. Therefore, for all practical purposes, anything we broadcast above the "cutoff frequency" is wasted energy for most radios.

Furthermore, for technical reasons beyond the scope of this article, higher frequencies broadcast on an AM signal cause a type of interference called "splatter."

Because of all of this, a set of standards was developed and implemented called "NRSC."

The short way to explain the NRSC Standard is to say that the higher frequencies in our audio signal are gradually boosted as we look higher up in the frequency range -- right up until our "cutoff point," beyond which anything higher is completely removed from the signal. (A graph of this frequency response sort of resembles a ski-jump.)

It should also be pointed out that this scheme ONLY works well if we somehow take the higher frequency stuff, and separately apply some limiting (as described above) to this slice of the audio spectrum. Otherwise, the boosting of this range of frequencies will eat up too much of our modulation to make it worth our while.

Gee, this is getting rather complicated, isn't it? Come to think of it, it truly is a wonder that we can do this much manipulation to an audio signal and not totally destroy it beyond all recognition!

(Click HERE if you want a more detailed explanation of this process.)

So What Gear Should We Purchase?

OK, so what equipment is available within our budget that provides the most "bang for the buck" to a Part 15 AM operator? Let's take a look (we'll ignore NRSC for the moment):

The Max Brothers:

Remember the CBS Labs' "Max Brothers" combo we spoke of previously?

Well, the good news is that there were so many of these processors sold back in their heyday that they are relatively easy to find today on the used market.

The bad news is that many of them are in very poor condition and/or have been modified (and/or bullocksed-up) to the point that we should be very careful when buying them.

But the other good news is that these boxes are almost bullet-proof. They are really hard to kill. In other words, if you happen to have an old Audimax or Volumax that powers up and passes audio, a competent engineer can probably restore it to proper working order with just a little time, a few off-the-shelf parts, and a copy of the original manual.

And if you have a properly working set of these old CBS processors (and you properly feed this airchain with a reasonably clean source), you will probably have a perfectly respectable signal on the air (especially when received by a truly high-quality radio).

If we're lucky, we might get a reasonably well-working Max Brothers setup together for $200-$400.

When looking around for these units, remember that the Audimax is the leveler (therefore goes FIRST in the chain) and the Volumax is the limiter. Also, when shopping around, go for the AM-specific model of the Volumax, if possible.

(Click HERE to take a more detailed look at the CBS Max Brothers.)

RDL Stick-Ons:

There is a line of inexpensive gizmos manufactured by a company called Radio Design Labs (RDL) that can come in very handy for us Part 15 operators. They are very tiny (though very well designed). They are no-frills boxes that do their respective jobs very well. They are also very small.

The two units we will focus on here are their AGC/leveler (ST-GCA3), and their Compressor/Limiter (ST-CL2). These are truly a great value for the money. Advantages include the fact that they are inexpensive, reliable, and readily available. They are also available brand new (unlike the old CBS stuff).

(Of course, if you run BOTH of these units, run the leveler FIRST in the chain.)

Also, if you want to run your station "off the grid" for some reason (say you're in a mobile situation away from easy access to AC power, or you're doing some environmental responsibility trip), as these units are DC powered -- as is the Rangemaster -- with the addition of a laptop computer and/or battery powered mixer, you could easily run the whole station on batteries and/or solar power. (Pretty cool, huh?)

Other Options:

If you are running all your programming from computer anyway, it might be wise to install software that does the leveling portion (or more) of the airchain "in the box," and then run that into an Inovonics 222. This option is only reasonable if ALL of your programming is fed from computer.

There are lots of software processors out there, and the quality of them varies tremendously. Some are quite good, and others sound atrocious (especially on the air). Things in the software world tend to move too fast to keep up with, so we are loath to make recommendations. However, HERE is a new one that claims to be able to do everything needed. The price seems right, so if you wish to give it a try, please let us know how it works out for you.

Let's Spend More Money!

If we are going to spend more money (even another dime), it makes a lot of sense to start looking at NRSC compliance.

For most Part 15 operators, there is really only one cost effective option to going with NRSC -- The Inovonics 222.

...And the best part is that there are also other advantages to having this box:

First, a limiter is included as the first stage of processing in this unit. This can save you a bit of money as you don't need a separate stand-alone limiter (though you may want one anyway).

Secondly, there is another feature to the 222 that, until now, we haven't gotten into, and that is the subject of "Asymmetrical Modulation" for AM radio. For those who are spooked by such terminology, suffice to say that doing this will usually make your station louder (a great help for us flea-powered broadcasters). For those who want to know more about this subject click HERE (and scroll down to the top of page 9).

Furthermore, the Inovonics 222 is built like a tank! It was designed to run for years and years in a non-climate-controlled environment like a transmitter shack. Very seldom does anything go wrong with a 222.

The original purpose of the 222 was to place at the end of a station's existing airchain in order to place the station in compliance with the NRSC standards, so you can add it to the end of pretty much any airchain you choose.

Although some operators can and do use a 222 as their ONLY processor, it is highly recommended that you at least put a leveler ahead of it (like an Audimax, Compellor, or the RDL ST-GCA3 . The only exception to this is if you do all your programming live, and watch your levels like a hawk.

All-in-one boxes:

There are many more options if you want to really want start spending some serious (stupid) money.

By their very nature, all-in-one boxes are specific to their purposes (in this case, AM radio processing). And they can get pretty pricey.

While you can shop around on the used market, here are a few examples of top-of-the-line all-in-one boxes made especially for AM radio. They do a fine job, and they SHOULD at these prices!

Maybe too rich for our blood (but we should know about them anyway):

•    The Omnia 3AM:

•    The Orban Optimod-AM 9300

•    The Vorsis AM10-HD:
     Got $11,776.15 to spend on processing, do ya?

Of course, you could luck out and find some of the older products from manufacturers like this, but the fact is that when most stations replace their processing, they keep the stuff they replace as a spare airchain, and the things they sell are usually the old spares. Still, at the right price, that could be exactly what you're looking for!

Examples of Affordable Airchains:

Ok, back to reality now (for most of us, anyway). Here a few possibilities for those of us who live in the real world:

   •  Example #1 (NO NRSC compliance):

       Leveler - RDL ST-GCA3 - $109.50
       Limiter/Compressor - RDL ST-CL2 $125.99

       Total: $235.49

   •  Example #2 (NO NRSC compliance):

       Leveler - CBS Audimax - $100?-$200? (used)
       Limiter - CBS Volumax - $100?-$200? (used)

       Total: $200?-$400? (Plus possible refurbishment)

   •  Example #3 (NRSC compliant):

       Leveler - RDL ST-GCA3 - $109.50
       Limiter - Inovonics 222 - $850.00 Factory List (shop around)

       Total: $959.50 - With 222 new (used will be cheaper)

   •  Example #4 (NRSC compliant):

       Leveler - CBS Audimax - $100?-$200? (used)
       Limiter - CBS Volumax - $100?-$200? (used)
       NRSC Compliance - Inovonics 222 (limiter off) - $850.00 Factory List (shop around)

       Total: $1150? - $1350? - With 222 new (used will be cheaper)

   •  Example #5 (NRSC compliant):

       Leveler - RDL ST-GCA3 - $109.50
       Limiter/Compressor - RDL ST-CL2 $125.99
       NRSC Compliance - Inovonics 222 (limiter off) - $850.00 Factory List (shop around)

       Total: $1085.49.50 - With 222 new (used will be cheaper)

As you can see, the numbers climb fastest by adding a brand new 222 to the chain. But we have heard that with some shopping around, used 222s can be had for as little as $200.

And don't forget that the 222 has a built-in limiter that is optionally switched off, although it can be left on and used more gently (as can the main limiter). Many find that this option results in more volume with a more "open" and natural sound.

One more limiter we should also mention: If you're springing for the 222, you may also wish to consider checking the used market for a limiter called the Aphex Dominator 720 to use as your main (loudness-generating) limiter. This is a high-quality three-band limiter (meaning that it divides its incoming signal into low, mid, and high frequency bands, and limits them separately).

The 720 can make a signal very loud without destroying the musicality of your program material. They can be found used for around $300 - $500, we hear.

But WARNING!: The 720 also requires a little more knowledge and experience to properly set up than most of the other gear mentioned here. With too heavy a hand, it is quite capable of smashing your precious signal into an unlistenable mess. YOU HAVE BEEN WARNED!

As far as other processors go, you might just happen on to an old Doroughs DAP in decent shape. This was intended as an all-in-one AM processor back in the day, and (run ahead of an Inovonics 222) can give you a very competitive air-sound. Be careful, though. A lot of these old boxes are in horrible shape, and can be more trouble than they're worth.

Wrapping it Up:

It should again be stressed that we should always remember that the more stuff we have placed in our airchain, the less active each one element should be.

Remember that each device we add (in addition do doing what we want done) also does a certain amount of violence to our signal. The negative results can be quite cumulative, and can even make a signal almost unlistenable if we are not careful.

We hope that this article has given you a more comprehensive ignorance of this complex and maddening subject.

We also invite you to share your stories and observations with us, and with other operators.

Call or E-mail us (see our contact information on the contact page) if you have further questions or to purchase a Rangemaster.