Back

---

By
Robert Orban and Greg Ogonowski
Orban/CRL

Authors’ Note
In 1999, we combined and revised two previous Orban papers on maintaining audioquality in the FM and AM plants, with a further revision occurring in 2003. In 2003,considerations for both AM and FM are essentially identical except at the transmitter because, with modern equipment, there is seldom reason to relax studio quality in AM plants. The text emphasizes FM (and, to a lesser extent, DAR) practice; differences applicable to AM have been edited into the FM text.

Introduction
Audio processors change certain characteristics of the original program material in the quest for positive benefits such as increased loudness, improved consistency and absolute peak control. The art of audio processing is based on the idea that such benefits can be achieved while giving the listener the illusion that nothing has been changed. Successful audio processing performs the desired electrical modifications while presenting a result to the listener that, subjectively, sounds natural and realistic. This sounds impossible, but it is not. Audio processing provides a few benefits that are often unappreciated by the radio or television listener. For example, the reduction of dynamic range caused by processing makes listening in noisy environments (particularly the car) much less difficult. In music having a wide dynamic range, soft passages are often lost completely in the presence of background noise. Few listeners listen in a perfectly quiet environment. If the volume is turned up, subsequent louder passages can be uncomfortably loud. In the automobile, dynamic range cannot exceed 20dB without causing these problems. Competent audio processing can reduce the dynamic range of the program without introducing objectionable side effects. Further, broadcast program material typically comes from a rapidly changing variety of sources, most of which were not produced with any regard for the spectral balances of any other. Multiband limiting, when used properly, can automatically make the segues between sources much more consistent. Multiband limiting and consistency are vital to the station that wants to develop a characteristic audio signature and strong positive personality, just as feature films are produced to maintain a consistent look. Ultimately, it is all about the listener experience. Each broadcaster also has special operational considerations. First, good broadcast operators are hard to find, making artful automatic gain control essential for the correction of errors caused by distractions or lack of skill. Second, the regulatory authorities in most countries have little tolerance for excessive modulation, making peak limiting mandatory for signals destined for the regulated public airwaves. OPTIMOD-FM, OPTIMOD-AM, OPTIMOD-DAB, OPTIMOD-TV, and OPTIMOD-PC have been conceived to meet the special problems and needs of broadcasters while delivering a quality product that most listeners consider highly pleasing. However, every electronic communication medium has technical limits that must be fully heeded if the most pleasing results are to be presented to the audience. For instance, the audio quality delivered by OPTIMOD is highly influenced by the quality of the audio presented to it. If the input audio is very clean, the signal after processing will probably sound excellent—even after heavy processing. Distortion of any kind in the input signal is likely to be exaggerated by processing and, if severe, can end up sounding offensive and unlistenable. AM is limited by poor signal-to-noise ratio and by limited receiver audio bandwidth (Typically 2-3 kHz). As delivered to the consumer, it can never be truly “high fidelity.” Consequently, multiband audio processing for AM compresses dynamic range more severely than in typical FM practice. In addition, pre-emphasis (whether NRSC or more extreme than NRSC) is required to ensure reasonably crisp, intelligible sound from typical AM radios. In AM, this is always provided in the audio processor and never in the transmitter. Audio quality in TV viewing is usually limited by small speakers in the receivers, although the increasing popularity of DTV, HDTV and home theatre is changing this, increasing consumer demand for high audio quality. In everyday television viewing, it is important to avoid listener irritation by maintaining consistent subjective loudness from source to source. A CBS Loudness Controller or multi-band processing, both included in OPTIMOD-TV, can achieve this. Netcasting, also known as webcasting, almost always requires low bit-rate codecs. Processing for such codecs should not use clipping limiting, and should instead use a look-ahead type limiter. OPTIMOD-DAB, OPTIMOD-HD FM, and OPTIMOD-PC provide the correct form of peak limiting for netcasting and other low bite rate services. Achieving consistent state-of-the-art audio quality in broadcast is a challenging task. It begins with a professional attitude, considerable skill, patience, and an unshakable belief that quality is well worth having. This supplement provides some technical insights and tips on how to achieve immaculate audio, and keep it that way. Remember, successful audio processing results all starts at the source.

 WWW.261.gr