Dynamic range compression

With a compressor effects unit from the group of control amplifier is referred to in audio engineering. It belongs to the group of the dynamics processors and is used to restrict the dynamic range of a signal.

  • 3.1 Broad -band compressor
  • 3.2 Multiband Compressor
  • 3.3 Tube-modeled Compressor
  • 3.4 Opto Compressor


By means of an envelope curve demodulator is derived a control voltage from the level of an audio signal, which is supplied to an actuator. In most compressors, a voltage controlled amplifier affects the level of the signal to be processed. The dynamic sequence is thus compressed, the volume difference of the soft and loud passages is reduced.

Typical adjustable parameters of a compressor are

  • (Threshold )
  • Ratio ( compression ratio)
  • Attack ( Timer / settling time )
  • Release ( off / settling time )
  • Makeup gain

The threshold determines from which signal level processes the signal at the compressor. Ratio with the compression ratio and therefore the dynamic reduction of the input to output is set (for example 2:1 means a reduction of the signal above the threshold by a factor of 2). Attack is the on / settling time of the compressor and the time that the compressor needs to turn down when the set threshold ( Threshold), the output signal on the selected compression ratio ( Ratio). "Release" is the off / settling time of the compressor and the time that the compressor needs to be reduced can the output signal to the original ratio of 1:1 after falling below the set threshold ( Threshold). With makeup gain and eventually can be reduced by the set of characteristic level catch up, so that the peaks of the signal again reach the same control as before. The ratio is a selection function of a compressor. It determines with which ratio compresses the input signal (usually 2:1 or 4:1). Example Ratio 4:1: When the input signal rises by 4 dB, the output increases by only 1 dB. From Ratio 10:1 it is called a limiter.

Areas of application

Compressors can be used both for compressing a single signal of a musical recording or performance as well as the overall signal.

Compression of individual signals

Individual signals are compressed in order to smooth the dynamics of the course as a whole and thus quiet passages comprehensible (because louder) without making that loud passages have become too loud or uncomfortable. For example, has the human (singing ) voice naturally a high degree of dynamism that makes it problematic in its unedited form, should be replaced by the singing in a typical pop - mixing compared to the rest of the tracks in the foreground. Using a compressor can level fluctuations are smoothed, creating a constantly high average level and thus a significantly improved signal presence is achieved.

Also to comply with the technical limits at a music recording, a compressor can be used ( to avoid overloads especially when recording digitally ). Here, the original signal is limited before inclusion in the dynamics.

Individual signals percussive instruments such as drums are compressed for the targeted sound shaping. By setting a longer attack time the attack noise remains unprocessed and can thereby be adjusted independently of the decay phase by the latter is turned down by a suitably chosen compression ratio.

Compression of total signals

When compressing a finished piece of music, for example, can not be perceived short-term peaks are attenuated. Thereafter, the total signal can be amplified without causing a distortion. The louder overall signal provides technical and psychoacoustic benefits.

Often this technique is used with radio stations ( typical device: Optimod ), which compress the mostly already highly compressed original signal of a piece of music before sending it again to achieve maximum loudness and acoustic enforcement compared to other stations. In part, it comes here, however, in an audible negative change of sound.

Also in the radio compressors are used in order to disqualify the transmit power smoothly and as high as possible can without pushing the transmit amplifier in the nonlinear distorting area.


A distinction is made between broadband and multi-band compressors. If the level of the entire input signal processed equally, it is called a wide- band compressor. This type is often referred to as single-band or single-band compressor, but this is technically inaccurate, as a single-band compressor can work well only in a limited frequency range.

Broadband compressor

The full-band compressor circuit is in audio engineering by far the most common and occurs, for example, often used to give individual signals of a music mixing more assertiveness and presence. Broadband compressor circuits encounter but on principle to its limits as soon as the input signal more dynamic curves independently run simultaneously in different frequency ranges, as is the case in a mixture of several individual signals. Thus, for example, the use of a broadband compressor on a mix of music lead to a rise in the level in the bass range to mitigate the overall level of the mixture leads ( typical pump with the use of the bass drum ).

Multiband compressor

Specifically for the level of processing of such complex signals multiband compressors have been developed in which the input signal prior to the actual processing is split by means of a crossover network in a plurality of frequency bands, each of which is one of several independent compressor circuit through which the output signals are mixed together after compression. In this way it is possible to complex and wide-band signals for compressing the mixing homogeneously, without having to take unnatural interference of different frequency bands into account.

Since multiband compressors can interfere with the sound of music mixing basic and the complex parameterization requires a lot of experience with the operation and the operation of the equipment, there are attempts to automate the setting of the compressor. There are devices that are able to analyze the program material to be processed and on the basis of the spectral characteristics and dynamic attempt to compact the material as homogeneous as possible. This however, a certain aesthetic of sound the signal is asserted, which are not always in harmony with the musical character of the material.

Also for smaller studios affordable multi-band compressors have only been advent of digital technology. The large amount of circuitry with crossover and the one compressor per frequency band analog solutions can be very costly.

Tube compressor

Unlike compressors with semiconductor circuit comes in the tube compressor as a reinforcing component an electron tube used. Although both components have the same task, the tonal changes of the processed material can be very different, depending on the amplifier used as feed component and circuit-specific properties in the sound material.

Opto compressor

In this compressor type, the driving voltage of a light emitting diode is supplied, the brightness will change accordingly. In the signal path is a photo transistor or photo resistor which carries out the function of the control element. Especially for working with the photo resistor variant is a certain inertia in the control response characteristic that is often perceived as particularly musical. As the inventor of the opto- compressor, the British engineer Joe Meek is often mentioned in the literature.

Special shapes

If an alien signal is used instead of the original signal to the controller, it is called " sidechain " or " ducking ". Here, the original signal is down- regulated when the level of the control signal increases. Typical application is the automatic down-regulation of the music volume with announcements the moderator or DJs on the radio. Therefore, some DJ mixers have such a function built right in (Talk over). Use some styles of club music as a stylistic device a pumping in time with the bass drum volume within certain passages of a piece of music. To achieve this " ducking " effect, the signal of the bass drum (or, alternatively, a time clock controlled 4/4-Puls ) is the sidechain input of the specially integrated for this purpose compressor supplied.


Due to the excessive use of dynamic compressors both in the blend as well as the transmission of music will raise the volume is achieved at the expense of sound quality. This trend is referred to as loudness and was also is criticized for years. Among other things, the magazine " stereo" Compressing music tracks strongly criticized in its issue 9/2010. She compared an example of the original edition of a CD with a later issued Remastered Edition and noted that 'll often compressed remastering. The magazine interviewed Alan Parsons, who expressed himself as follows: "It is, however, become fashionable to highly compressed Under the aspect of high fidelity that is a disaster. "

In fact, it is such that the average volume level of today's CD productions is significantly higher than that which was recorded in the 1980s and 1990s. At the same time the dynamics of today's CD production has fallen sharply. One of the world's most famous mastering engineers, Bod Katz, who is also specifically speaks out against this trend were to mean that many customers this but so wished and engineers who do not fulfill the customers lose. Meanwhile, there is emerging, however, that a change in thinking has begun. Some see the " loudness war" on already as history.