Audio Signal Compressing

Compressor 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.

Noise Gate Recording with microphones is difficult. Background noise can be a big problem and noise gates offer a solution. The idea is that the sounds of interest are louder than background noise -- you set the gate to block low level signals and open with louder signals. The noise floor threshold can be set around -10 to 30db and the opening rate of about 1 second "fades out" the audio signal as the gate come on -- this prevents the gate from chopping off the tails of meaningful sounds.

Reverb creates the illusion of space, but interferes with stereo localization. To maintain stereo placement, use a mono reverb panned to the same position as the original dry sound. Reverb can make vocals better but tends to push the vocals back in the mix when they should be up front. Pre-delay values of 60-100mS and early reflections reinforce the dry sound. Avoid reverb on bass sounds. Use a short ambience program or a gated reverb to add space to a kick drum.

Equalization is often used to fix sound that was not correct at the source. Reducing the frequencies that are too prominent is better than boosting the weaker ones. Sounds that are pleasing alone may not sit well in a mix. The fix is sometimes to use high- and/or low-pass filters to restrict spectral content. Low frequencies, 200-400HZ, are produced by many instruments and may selectively filtered to avoid mix mud.

Dry mix until you are pleased before you add any effects or signal processing. Effects are best used a finishing touches. Vocals with pitch problem can be fixed with pitch-correction processors. Enhancers are best used on specific sounds in a mix to make them stand out. Use the enhance in a separate bus and send only the selected sounds that need enhancing. Adapted from Paul White’s Tips, Sound On Sound, July 1999

Staying Dry: Effects (wet) are added to an original sound (dry) creating a dry/wet ratio. My goal is to create dry mixes that are very good without any compression, equalization or reverb. Recordings using electronic keyboards and synths have many options for sound crafting without using effects.

Careful mixing with faders set properly and volume envelops on each track acts as a deliberate, controlled form of compression. I 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.

FaceBookShare Follow @personadigital

Topics presented at Persona Digital Studio are from the book, The Sound of Music by Stephen Gislason. Click the Download button to download the eBook from Persona Digital Online.

Persona Digital Studio is located on the Sunshine Coast, Sechelt, British Columbia, Canada. Our Music catalogue includes recorded performances by the P2500 Band, Em4U, and the Persona Classical Consort.
Music downloads are available from companion website: Persona Digital Online.

www.personadigitalstudio.com email music@personadigitalstudio.com.

Persona Digital Studio Music