Contents
History of the Compressor
The initial form of wireless transmission was wireless telegraphy. In the early days of voice transmission, the main purpose was often for government or military use, so sound quality was not a significant concern as long as the message could be accurately understood.
However, the situation changed when radio broadcasting began to be used for commercial purposes. Listeners did not want to hear radio broadcasts where the volume level changed drastically. Rapid increases in volume could even damage speakers. Therefore, equipment was needed to protect the receiver by keeping the levels of the signals similar.
The main purpose was to control the dynamic range of the audio signal, that is, the difference between loud and soft sounds.
The First Compressor
The world’s first dynamic processor was the Telefunken U3, designed in the early 1930s and first used at the 1936 Berlin Olympics.
U13 was the next generation of audio compressors. Introduced in the 1940s, it was developed based on the same components and technologies as the precursor to the V72. This device is considered the progenitor of all modern non-tube compressors.
Vari-MU Compressor
In the 1950s, RCA (Radio Corporation of America) introduced the tube-based BA6A compressor, making a significant impact on the audio industry. This was a Vari-MU compressor with a variable ratio, ranging from 2:1 to 20:1 depending on the gain reduction.
Known for its unique, smooth compressing characteristics, it was followed by the famous Fairchild 660 (mono) and 670 (stereo) developed by the Fairchild Recording Equipment Company in the late 1950s.
OPTO Compressor
The 1960s saw the advent of technologies like AGC (Automatic Gain Controller), which greatly standardized audio signal levels. OPTO compressors were also developed around this time, using light-sensitive materials like photoresistors to control the compression of the audio signal. When the signal is strong (i.e., when a high-level audio signal comes in), the light source shines brighter, causing the resistance value of the photoresistor to drop. This triggers the compression process. OPTO compressors are particularly suitable for vocals and acoustic sources due to their natural and smooth compression characteristics. The LA-2A is a notable example of an OPTO compressor.
Furthermore, optocell-based compressors have unique attack and release values compared to other types of electronic circuit compressors. These characteristics make them capable of providing a very distinctive tone, suitable for specific musical genres or sources.
FET Compressor
In the late 1960s, FET (Field-Effect Transistor) compressors emerged. Characteristically, FET compressors can have fast attack times, allowing them to react quickly to transients. Additionally, they provide clean amplification when operating in the linear range but start to exhibit overdrive or clipping effects once they exceed a certain amount of reduction. These non-linear characteristics are often responsible for creating a ‘warm’ or ‘thick’ sound. The UREI/Universal Audio 1176 is one of the most well-known FET compressors
Diode Bridge Compressor
Diode bridge compressors operate around a diode bridge circuit. Although the exact time of their introduction is unclear, they are known to have been commercialized between the mid and late 1960s. The first diode bridge compressor, the Neve 2252, was released in the mid to late 1960s, followed by its successor model, the 2254, in 1969. The most famous Neve 33609 compressor was developed in 1970, based on diode bridge technology and utilizing Class A/B amplification. It also uses a discrete transformer-balanced output, incorporating more modern manufacturing techniques and designs.
Diode bridge circuits are disadvantageous for handling large audio signals but are effective for small audio signals. The input signal first undergoes initial attenuation to make it suitable for the diode bridge circuit, and is then sent through that circuit. Subsequently, the signal undergoes a process to restore it to its original size. While very fast attack and release times are possible, the narrow operating range of the diodes means that it can be challenging to fine-tune the degree of attenuation depending on the size or changes in the audio signal. Most of the audio signal is processed in a balanced manner, which can increase manufacturing costs without necessarily guaranteeing commensurate performance.
VCA Compressor
In the 1970s, the VCA (Voltage-Controlled Amplifier) Compressor was introduced, operating by controlling the amplification of the audio signal through voltage. It is considered the most modern type of compressor.
The core of the VCA compressor is the VCA circuit, which controls the attenuation or amplification of the audio signal based on the input voltage. When the level of the audio signal exceeds a set threshold, the VCA reduces its amplification to compress the signal. It is characterized by its fast response time and precise control, making it one of the most commonly used types of compressors in studios. Well-known examples include the dbx 160 and the API 2500.
The development of such dynamic processors has greatly contributed to providing more refined and cleaner sound in the fields of audio broadcasting and recording.