You may recall last month we discussed dynamic range and headroom, and the related term signal-to-noise ratio. In the studio we use audio compression to control dynamic range. Limiting the dynamic range of a signal allows us to boost the average audio level and produces a more consistent signal. Raising the signal, while maintaining a decent headroom, means we have improved the signal-to-noise ratio, which is a good thing. In past issues we have discussed how an audio compressor works, so in this issue we are going to look at the types of audio compressor.
Compressors were invented by the radio broadcasting industry. In the early days of radio, each station had an engineer on staff whose only function was to “ride the gain” of the broadcast signal. When the audio level was low, they would boost it and vice versa. The goal was have the signal as loud as possible without overmodulation. Overmodulation meant exceeding the ability of a carrier wave to carry a signal. Overmodulation not only created a very distorted signal, but was against FCC rules.
Then in the 1930s, someone invented the audio compressor, which reduced the peaks of the loudest sounds. This allowed the engineer to boost the level automatically, making it sound louder. Suddenly stations jumped from average modulation of about 30% to over 80%, and eventually greater than 90%. It wasn't long before recording studios realized the advantage of using compressors.
Since only vacuum tubes were available, early compressors were all designed using vacuum tubes. There are still many tube compressors in use today. Tube compressors tend to have a slower response than other types of compressors, and have a distinctive coloration that is often called a “vintage” sound. That sound is nearly impossible to achieve with other compressor types.
With the invention of transistors, the next type of compressor to come along was the FET compressor, which used a field-effect transistor (FET) to take the place of vacuum tubes. FET compressors tend to preserve more transients which adds more punch as compared to vacuum tube compressors. The result is a very warm and rich sound character.
Later optical compressors were developed. Optical compressors convert the sound to light, adjust the signal gain, and then convert the light back into sound. Optical compressors tends to produce a smoother sound than other compressor designss, because the response times of the light softens the attack and release.
Another type of compressor is the VCA compressor, which uses a voltage-controlled amplifier (VCA). A VCA compressor usually uses an integrated circuit to determine the input voltage and control the gain reduction, providing a clean and controllable compression. They are usually cheaper than tube compressors or optical compressors and tend to have less coloration.
The PWM compressor uses pulse width modulation (PWM). This process involves converting the audio signal into a series of pulses and adjusting the width of the pulses to control the average level over time to control the signal level. PWM compression is the fastest method of compression with the fewest number of distortion artifacts.
Of course, in today's recording studio, software is often used to emulate these various type of compressors. How well they accomplish this emulation is matter for debate. Some of the higher-priced digital compressors come very close. The emulations not only attempt to create the sound of a specific type of compressor, but many attempt to emulate the sound of some of the more expensive and vintage hardware compressors. That is why most studios have several compressor plug-ins to use to accomplish various goals and sounds.
• After hearing the Olympics' recording of “Good Lovin’” on the radio, the Young Rascals decided to record the song. However, organist Felix Cavaliere later said, “We weren't too pleased with our performance. It was a shock to us when it went to the top of the charts.” The song reached number 1 in April 1966.
• “Cindy Lou” was a song written by Buddy Holly, Norman Petty, and Jerry Allison about Holly’s niece, Cindy Lou Kaiter. However, Allison, the Crickets’s drummer prevailed upon the others to name it after his girlfriend, Peggy Sue Gerron, with whom he had recently broken up. “Peggy Sue” went to No. 3 on the charts in 1957, and Allison and Peggy Sue later got married. They were divorced in 1965.
• On August 16, 1977, Elvis Presley was found dead on the floor of his bathroom by his girlfriend, Ginger Alden. He had been seated on the toilet reading “The Scientific Search For Jesus.” The following day the number of orders for flowers that were delivered to Graceland exceeded the number for any other event in history according to FTD.
• The double-entendre title of the Bellamy Brother’s hit “If I Said You Had a Beautiful Body Would You Hold It Against Me” came from a quote by Groucho Marx.