Audio level control
Oct 1, 2008 12:00 PM, By Aldo Cugnini
Correct modulation requires a fine-tuned ear.
Electronically processed analog audio has been around since about 1876 when Alexander Graham Bell and Elisha Gray filed patents for “transmitting the human voice through a telegraphic circuit.” But while the evolution of digital technology some 100 years later added incredible sophistication to audio systems, the underlying signal processing principles remain fundamentally the same. This is due in part to the fact that the entire process is ultimately coupled to the human hearing mechanism — and that evolves much more slowly.
Manual level control at the beginning
In the early days of radio and TV broadcasting, all audio level control was done by a skilled operator at the transmission point. Because of FCC rules regulating the modulation of analog RF carriers, stations had to be vigilant to assure operation within legal limits, i.e., no overmodulation.
Soon we developed electronic systems that could do this automatically, resulting in automatic gain controls and peak limiters. But while these circuits maintained legal operating conditions, broadcasters (and advertisers) soon realized that there was an advantage to producing a certain sound quality, resulting in technology to automatically modify the sound. Among the tricks employed was the use of gain compression to make the program sound louder without exceeding regulatory limits.
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Volume compressors work by dynamically altering a gain element in the audio path. By raising the volume at low levels and lowering it at high levels, the dynamic range of the material is reduced, and the overall volume of the material can be increased.
Compressors can work in a myriad of ways, with the input-output relationship of the compressor having a linear or complex relationship. One of the challenges of automatic gain control is to avoid noticeable artifacts, especially when the processor must react to sudden changes in input dynamics.
Consider a situation where an aggressive amount of compression is taking place. If the input audio is riding at a low level, the compressor will work to increase the gain. Now, if a high-level passage suddenly appears, the compressor must quickly lower the gain so that clipping or overmodulation does not occur. These gain transitions must occur with an associated time constant so that the gain change is not noticeable. Too slow a time constant can result in signal overload; too fast will result in pumping or breathing, as the gain change is heard.
Loudness is not an exact science
The human hearing response is anything but flat, even for the perfect listener, having a roughly bell-shaped response that peaks around 3300Hz. In addition, the shape of the hearing response changes with the overall intensity of the sound.
Figure 1. Equal-loudness contours
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This gives rise to the notion of equal-loudness contours, which describe the sound pressures necessary at different frequencies to give an equal perceived sound level. Equal-loudness contours have been described by various researchers. (See Figure 1)
Because the loudness contours describe perceived sound levels, they have been developed empirically, by gathering statistics from large numbers of experimental listeners. The curves relate to each other by means of units called phons, which define the sound pressure level at 1kHz. The well-known A-weighting curve used to measure noise levels is an approximation of the inverse of the equal-loudness contour at the 40-phon level.
The perception of loudness is also a function of other factors. Loudness increases as the sound power is spread over critical bands, generally at a threshold of about one-third octave. Also, loudness is a function of the duration of the sound, with the human auditory system integrating the received power over a window of about 200ms to 1000ms.
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