Compression and limiting
Figure 2 below shows an example of compression response curves for 2:1, 4:1, and infinity:1 (limiting). This demonstrates that, below the DRC threshold, the output vs. input has a fixed gain response. After the DRC threshold is crossed, the gain changes proportional to how far above the DRC threshold the original signal would have gone. Following that, it becomes clear what amount of compression will be applied for the given compression ratios as described above.
Figure 2: Generic compression ratio curves. (Wikipedia 2011)
The engineer can then increase the system gain to bring out the lower levels of the audio, while compressing the potentially distorting or over loading signals. You may hear the term "normalizing" the gain, where the engineer re-adjusts the system gain so that the highest levels (peaks) are synchronized to the same level as they were without the effect enabled, but now the lower amplitude signals below the DRC threshold will be boosted.
As compression ratios are further increased (above more than 10:1 for example), the effect starts to become what is called a 'limiter'. In this case, the automatic gain control will continue to decrease the gain such that the output amplitude will no longer be allowed to increase any further after the DRC threshold is crossed. In mobile devices it can be useful to implement both techniques together.
You could think of the compression region as a gradual approach to limiting, and if a higher final limiting level is reached, the limiter will kick in and prevent the signal from increasing any further at the output. Limiting can be useful in mobile devices to track the battery voltage. For example, if the limiting threshold tracks (is proportional to) the battery voltage, the limiter level will decrease along with the battery voltage to prevent distortion as the battery voltage naturally decays.
With Figure 3 you can see an example of the two effects working together for a combined response. In addition, with this setting, the initial gain is 24dB, and when the DRC threshold is crossed the circuit response moves to a 2:1 compression region. Finally, if the output signal would have peaks over 90% of the battery voltage, the output gain will be reduced further so that the output signal level cannot go above the limiter level. Then this limiter level would continue to move down as the battery voltage decreases since its value is always 0.9 * Vbatt.
Figure 3: Using compression and limiting together (using a Fairchild audio subsystem).
Compressors are also often supplied with attack and release times to control the speed at which the circuit operates on the signal. Usually it's desirable to attack down on the peaks rather quickly as not to overload and distort the speakers, and then allow the effect to release slowly as the envelope of the audio signal stays below the threshold level.
Figure 4 shows an example of attack and release times. The signal is shown as a pulse to clearly illustrate the attack and release times. Again for illustration purposes, the attack time is shown just slightly faster than the release time.
Figure 4: Attack and release times. (Wikipedia 2011)
These controls can be used to tailor the effect for the given application to optimize its usefulness. For example, a phone conversation may have different settings than playing music, or different instruments in a studio recording environment may have different settings to tailor the effect to that given instrument.
Since Fairchild audio subsystems have high-resolution digital volume controls, the gain can dynamically be controlled within the device to approximate the ideal compression ratio curves and limiting response when threshold levels are sensed internally to the device. These devices have on-board trimmed oscillators, so a very accurate time base can be applied to create programmable attack and release times for the DRC and limiting features.
New Fairchild mobile audio subsystem ICs on the market and soon-to-be-released products incorporate both fixed THD limiting as well as battery voltage scaled limiting, while others include fixed THD limiting, battery scaled limiting as well as dynamic range compression. Furthermore, boosted stand-alone class-D speaker amps from Fairchild incorporate limiting as well.
With the small low-profile speakers in today's mobile devices, these traditional Pro Audio techniques can be used to raise the lower level audio signals in a conversation to aid in communication (so you don't miss what the person is saying). Taking advantage of the multimedia entertainment features, compression and limiting can also be applied to maximize the audio level for listening pleasure (for a movie or video for example) while not allowing the louder dynamics to reach an unpleasant and potentially speaker damaging level.
Earl Schreyer has 20 years experience in designing integrated circuits. He began his career with Fairchild Semiconductor in 1999 joining the analog & mixed signal products group, building on analog and high speed ADC work in the application of video/graphics digitizers and continues time video filter drivers. He has a passion for audio design and has recently become a key member in the new mobile audio group.
Earl holds both a Bachelors and Masters Degree in Electronics Engineering from Arizona State University. In his spare time, he likes to design and build hand-wired, vacuum tube guitar amplifiers.
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