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Music Studio Technology
Audio Processing Components
A compressor automatically controls the volume of an audio signal. Its main function is to avoid transients that exceed tolerances of the recording system and cause clipping. You set a threshold level and tell the compressor how quickly to react and it limits the volume. In a digital mixer 0dB is maximum before clipping occurs and recording peaks are best kept below -1dB. Compression can begin at -20dB.
The compressor's negative gain is determined as a ratio. A ratio of 5:1 will reduce the input signal (5) to an output of 1dB over the threshold. When the sound volume falls below the threshold level, the gain returns to normal. The attack time determines how long the compressor takes to reduce the gain. The release time determines how long the gain takes to return to normal after the input signal has fallen back below the threshold.
Compressors often have the option of the “knee.” A soft-knee compressor increases the compression ratio gradually as the volume approaches the threshold. A hard knee responds as quickly as the attack time specifies.
While a compressor may be the most useful of all audio processors, its proper use requires technical knowledge, experience and good judgment. Final decisions are made by listening and not technical preconceptions. Recording performances through a microphone may require some compression at the source. Vocal recordings tend to be the most demanding unless the singer is a real pro. In the studio, capacitor microphones are used for vocals, placing the mic about nine inches from the singer’s mouth. Use a pop shield. Vocal compression should be minimal - just avoid clipping- up front to avoid losing dynamics that cannot be restored later.
Careful mixing with faders set properly and volume envelops on each track acts as a deliberate, controlled form of compression. reserve compressors for the mastering stage when each composition must fit into the meta-composition of an album. I use a volume ceiling of -1.2dB for competed recordings and try to achieve that with minimal compression.