While mobile phones continue to emerge as a key communications device and become more feature rich with multimedia applications, there still remains the challenge of achieving good sound quality from the small low profile speakers. Pro Audio techniques such as Dynamic Range Compression (DRC) and limiting can be designed in and applied to the speaker amplifying device to improve the user experience.
DRC and limiting are traditional recording and broadcasting techniques used to raise the volume of subtle audio (such as human voice), in order to bring out those nuances, but at the same time automatically attenuate the dynamic peaks as not to overload the level. This technique is also used on the final mix in recording to increase the average level of the entire track without overloading during louder passages.
In addition, the use of compressors to boost perceived volume is used by broadcasters who want their station to 'jump out' at the same volume as comparable stations on the dial. This can also be useful for "in-car" listening to compensate for natural vehicle and road background noise.
In common to all the above applications, if the lower level amplitudes can be raised while the higher level peaks are attenuated; the overall volume can be increased without distorting and over loading the electrical level or the speakers on the louder dynamics of the audio. This article describes how these techniques can be applied and how they can aid in improving communication as well as entertainment in modern mobile devices.
Compression and limiting have been around for a long time. In 1959 Sherman Fairchild licensed a compressor/limiter designed by Rein Narma giving life to the legendary Fairchild 670 compressor/limiter.
The Fairchild 670 has found its way onto many classic recordings over the years including famous Beatles recordings at Abbey Road Studios in the 60's. The unit is still revered to this day, can go for upwards of $30,000 in good condition, and many software plug-ins have been designed to emulate its tone and function. This unit was all hand wired, had over 20 vacuum tubes, 14 transformers and weighed in at a hefty 65lbs.
Mr. Fairchild couldn't have known that over 50 years later, Fairchild Semiconductor would be integrating dynamic range compression/limiting, as well as 1-W Class-D amplifiers, and stereo Class-G headphone amplifiers on to a single silicon die roughly 2mm on a side.
Figure 1. 1959 Fairchild 670 compressor.
Dynamic range compression and limiting can be useful in mobile devices with speaker amplifiers to raise the average level of the audio output (turn it up) while compressing or limiting the louder peaks of the signal to reduce distortion and speaker overload. A compressor is essentially an automatic gain control. Loud sounds above a certain threshold are automatically turned down while quiet signals are unaffected.
The dynamic range of a system is defined as the largest signal the system can handle linearly divided by the smallest signal it can process (or the noise floor lower limit). Therefore, the effect is technically compressing the dynamic range of the system because it is attenuating the higher peaks as compared to the lower level signals. However this is desirable in certain situations to raise the lower level signals.
Compression therefore has the potential to improve audibility in a mobile phone conversation. In addition, for entertainment such as audio tracks for video, it allows the smaller speakers to bring out the more subtle parts, but not overload on the louder dynamics for a better user experience.
A compressor reduces the level of an audio signal if its amplitude exceeds a certain threshold. The amount of gain reduction is determined by a ratio; a ratio of 4:1 for example means that if the input level is 4 dB over the threshold, the output signal level will be reduced to 1 dB over the threshold. The gain (level) has been reduced by 3dB.