Audio Processing

April 1, 2001

The on-air audio processor is the pallet, and the individual settings are your brushes.

As listeners scan through the radio dial, different things can catch their ears. It's the challenge of each radio station to get the listener to stop on their station for a while. The station's format will get the attention of the button pusher first. Once someone finds your station, it's up to you to keep them coming back for more.

The way the station's audio sounds is just as important as the format being played. Two stations can play the same song yet sound very different. The proper use and setup of the audio processor is just as important to remain competitive as programming the right mix of music. This is where the engineer can become more of an artist.

When selecting and setting up the sound of the radio station, you must be aware of the format and your competition. The wrong processing settings can cause the listener to switch to another station, and not really realize why. A radio station pushing its processor too heavily can cause listener fatigue and he will switch to another station looking for some peace. It's a subconscious thing, but in time might make the listener stop turning to your station.

Tailored to your format

The sound of your radio station can actually set the listener's mood. Put yourself in the shoes of that listener and think about what you would like to hear. A rock or classic rock listener usually expects the music to be loud, even if he doesn't turn up the volume. For this, you want to push the processor more, keeping the average level of the music high. You also want to reduce the amount of loud peaks, since a listener might want to have his radio turned up as loud as possible. Frequent peaks can cause the music to distort. If the competing station is “cranked up” louder, the listener will feel like that's the better station with more power, and not even realize why.

For the adult contemporary radio station, a smooth, clean sound is desired. You want the listener to tune in and stay for a while. You don't want that fatiguing effect, which will cause a station switch or turning off the radio altogether. If you want to attract the listener at work though, an overall consistency of volume is desired. Many office listeners have small tabletop radios on their desk, and keep the volume low.

Urban and hip-hop stations want to have a sound with a boom to it. It's important for the processor to have a clean, and very distinct bottom end. A muddy rumble won't do it. The low frequency energy must be well defined and strong. At the same time, the music must be loud, but have a good perceived dynamic range to keep that strong, in-your-face punch.

Classical music stations have a real challenge. Many of classical listeners are true audiophiles, and want the music to sound pure and clean. The problem is that many classical listeners, like the adult contemporary listeners, tune in while at work and can't hear the quiet passages. High-quality home sound systems will bring out all of the noises and reception problems present in FM broadcasts.

Processing's role

The first processors had a different primary purpose than the processors of today. Early models were used to control the audio levels to prevent over-modulation. Some were also used as automatic gain controllers to keep the volume more consistent on the air. For AM radio stations, higher modulation carried the signal farther.

Engineers and programmers took these processors and pushed them to do more than they were designed to do. The result was a “pumping” sound as the processor would rapidly turn the volume up and down to keep the level constant. This led to newer designs and new features. Controls and circuits were added to clamp down on the volume to prevent the over-modulation, but were slower to turn it back up to reduce the noticeable side effects. An audio gate was added to stop the processor from turning up the volume when the audio level was very quiet. This would prevent the compressor from turning up the volume on tape hiss or other recording noises.

As the quality of recordings improved, so did the demand for new processors. Bass drums were now coming through the recording better, and would cause the processor to turn down not only the drumbeat, but also all of the other areas of the audio, once again creating that pumping sound. The same would happen with trumpet sounds, and other strong instruments. Designers added multiple bands to their units to allow the drum beat to be controlled without effecting the guitars, pianos, and vocals. When one radio station jumped on the latest and greatest processor, the others had to do something or lose the battle. The rush for manufacturers to create the best processor on the market was born.

More sophisticated

The audio processors of today have become much more complex and help radio stations achieve audio quality that was not possible only a few years ago. There was a slow period when analog processors had reached their limits, but digital audio still had too many artifacts to really sound right. However, as digital signal processors (DSPs) improved through higher sampling rates, and now with true 24-bit resolution, the digital processors have taken the market by storm.

Digital processors have many advantages over their analog counterparts. First, you are dealing with data (1s and 0s). Second, you can make very accurate changes in one area without affecting another. Audio filters can be much more precise, allowing us to squeeze every ounce of energy possible while protecting the integrity of the radio carrier. Some digital FM processors also include composite clippers. In analog circuits, a composite clipper can produce overshoots and harmonics that cause noises in the subcarrier channels. Stations leasing these channels had to limit the amount of clipping used, thus reducing their loudness. New digital clippers eliminate these problems, and the newest designs have even eliminated aliasing. Even the old problem of modulation of the stereo pilot is cured. Digital stereo generators can produce a pilot frequency that remains rock-solid and stable.

Today, many stations have asked for more control over their sound. The designers of the new processors have been responsive and give complete control over the functions of their machines. For a novice, this can be dangerous, as you can now make adjustments to such extremes that the audio can sound terrible, but with a trained ear, the audio can be tuned to sound incredible. For those who don't want to experiment, the digital processors offer pre-set memories for different formats. A processor can literally go from the box to on air in minutes. If you do decide to make adjustments, you can save your changes as you go. When you think you're close to the sound you want, just press save. If subsequent changes degrade the audio, go back to where you were. For stations with multiple formats, the processors can be programmed with day parts. Now, a station running classical music in the morning and jazz in the evening can have the best setting for both, and even change the presets automatically.

On IBOC and Internet

With IBOC on the horizon, we need to be looking at what our needs will be for processing. Perhaps the most important thing to remember is that IBOC will be able to pass frequencies up to 20kHz. Conventional FM broadcasting is limited to 15kHz. Many of the digital audio products on the market, including computer storage devices, STLs and digital processors use a 32kb/s sampling rate, which is fine for FM quality, but will restrict the potential quality for IBOC. In response to this, some of the newer digital processors on the market have increased their sampling to 48kb/s, which make them ready to give the highest possible quality for an IBOC signal.

On the subject of new technologies, streaming audio on the Internet is becoming more and more popular. With this new trend comes an entirely new array of audio challenges. It doesn't take much surfing on the net to notice that some stations sound good, while others sound horrible. What's the reason for this? The main reason is the lack of, or misused, processing. One of the biggest mistakes made by the broadcaster is to use the output of an FM tuner as the audio source. Another mistake is to take a spare audio output off of your on-air processor to feed the Net. To pass quality audio online, it is necessary to incorporate very sophisticated digital data compression schemes. These compression algorithms look for inaudible parts of the music and eliminate them. (See How it Works in the February 2001 BE Radio.) When you use an on-air processor, you are feeding pre-emphasised and clipped audio, which the netcoder doesn't know how to handle. Heavy processing produces a very dense audio signal, and again, the netcoder can't find quiet sounds to discard. You can hear artifacts in the sound. Some of these include ringing, swishing or an underwater sound.

To get the best quality Internet audio, use an audio processor designed for this purpose. Setup the processor to eliminate anything that can't be passed over the Net with a low-pass filter. For example, if your streaming audio carrier sets up your station with a 20kb/s service, you will have a usable audio bandwidth of 7.5 to 10kHz. Anything higher than this just causes problems with the encoder and is the cause of the ringing. Also, unlike most radio stations on the air, don't overdo your Internet processing. Keep the frequency response as flat as possible. Don't use any clipping or make your audio too dense. Again, this doesn't give anything for the encoder to work with. Since it has to eliminate something, it is now likely to get rid of important audio. For radio stations in the early stages of Internet streaming that don't have the budget for a separate processor, try to at least find an old equalizer sitting on the shelf to roll off the high frequencies.

Processing is here to stay

As the listener is becoming more and more critical of audio quality, we can expect to see higher demands on our broadcast sound. As a result, the manufacturers of audio processors will continue to move forward to provide what we are looking for. There's no shortage of technical advances available for them either. The power available in CPUs is increasing every day. To keep up with the competition, it may be necessary to upgrade your on-air processing on a regular basis, or you'll get left behind.

Steve Fluker is the director of engineering of Cox Radio, Orlando.

Volumax photo by Chuck Leavens.