Dynamic Range Compression The word "compression," as it relates to audio, is often misused, overused and even misunderstood. Likewise, compression itself is often misused, overused and misunderstood. The process of compressing an audio signal is used in the recording studio, during live performances and in the playback of recorded music. So then, what is compression? Simply stated, compression, or Dynamic Range Compression, is the process of reducing loud sounds and amplifying quiet sounds to bring them closer together. A simple analogy would be turning down the volume on your TV because the commercials are just too darn loud. The effect can be used to make recordings sound better at low and high listening levels, to make the quiet passages of music audible in a noisy concert venue or to protect the components in an audio system. For our purposes, we will focus on using Dynamic Range Compression as a way to protect the components in an audio playback system. Let's begin by breaking compression down into five parts: threshold, attack, release, ratio and output level. • • • • • Threshold: The level at which the incoming signal triggers the compressor. o The threshold is generally set close to the upper limit of the system's capabilities. This is the point at which we want to reduce the signal to protect speakers from being over driven. If the threshold is set too low, the compressor will kick in before the system reaches full potential. Attack Time: Once the threshold has been reached, attack time determines how quickly the compressor begins reducing the signal. o If the attack time is too fast, the transients are eliminated and the music sounds lifeless and dull. If the attack time is too slow, a crashing cymbal can toast a tweeter or a kick drum can launch a woofer's cone. Release Time: Once the signal has been compressed or reduced in level, release time is how long the compressor maintains this reduction before releasing the signal level to normal or uncompressed. o A release time that is too fast may allow the compressed signal to return to its speaker damaging level before the music naturally reduces in level. On the other had, a release time that is too slow can make a fast paced, articulate bass line sound like one, long, muddy note. Ratio: The amount the compressor reduces the original signal, like turning down the TV's volume when a commercial comes on. o A compression ratio of five to one would be written as 5:1. If an input signal exceeds the compressor's threshold by 10 Volts, the output will be reduced to 2 Volts above the threshold. A ratio of 10:1 or greater is generally referred to as a limiter and prevents the output from ever going above a set level. This can save the life of many a speaker. Output Level: A volume control to increase the output of the compressed channel to match the volume of the other channels. o A compressed signal may need an increased level to balance its output with the rest of the system.
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