Recording studios use compressors and limiters to limit the dynamic range of a signal and to prevent clipping. The dynamic range is thus compressed, the difference in volume between the quiet and loud passages is reduced. Compressors have been the absolute standard for music recordings in the recording studio for decades.
From an audiophile point of view, music lives from a sequence of loud and soft. All nuances of the artistic performance should be audible. Ideally, with the same volume differences as in a concert.
But in order to be able to enjoy dynamic music beyond concert volumes, compression is a completely legitimate means of sound engineering. Uncompressed, quiet passages would otherwise be drowned out by the driving noise on the car radio, for example. Even at room volume, it can be helpful if music playing in the background has been compressed. So compression is not generally a bad thing. Used in a well-dosed way, it can definitely enhance the enjoyment of listening to music. Modern multi-channel productions are no longer conceivable without compressors.
What are compressors used for in recordings?
With many sound sources, such as voices, instruments or noises, very large differences in volume occur. For this reason, sound engineers in the studio use so-called compressors when recording such very dynamic sound sources, which reduce the difference in level between the loudest and the quietest signals. A recording track with such smoothed dynamics makes quiet passages more intelligible (louder) without overdriving loud passages or making them sound unpleasantly loud.
In pop productions, compressors help, for example, to make the vocals stand out against the other tracks in a complex mix with many tracks. Even with other instruments such as drums or bass, modern recordings are no longer conceivable or sensible without the use of compression.
Compressors therefore help to compensate for level fluctuations and thus achieve a constantly high average level. Compressed signals are initially quieter, but can be controlled louder, as less headroom is needed for short-term peaks, and therefore have a more present effect.
How is a limiter used when mastering records?
When mixing the multi-track tapes or files to the master tape, mastering engineers also often use compression. This results in an overall smoother signal with fewer level jumps.
In addition, a limiter is usually used to attenuate or even cut off short-term level peaks. In this way, the sum signal (usually two-track stereo) can be amplified to a high level without overloading.
A fixed threshold is set that the level should never exceed. In this way, a kind of volume barrier is created, beyond which even the loudest passages do not exceed.
This procedure is necessary, for example, to prevent overmodulation in the form of audible distortion. On TV, the commercial breaks are always highly compressed and limited and thus appear significantly louder than the rest of the programme.
How does analogue overdrive differ from digital overdrive?
Before digital technology found its way into recording studios, analogue master tapes were the measure of all things. The art of recording consisted of preserving the signal to be recorded as loudly as possible on the tape so that the quiet but constant tape noise was drowned out. At the same time, it was important to prevent too much overdrive, which would be perceived as disturbing distortion. Slight overmodulation of the signal on the tape, on the other hand, was consciously accepted, sometimes even used as an artistic, deliberate means. Because with slight overmodulation on analogue tape, the so-called tape saturation occurs first. This is a natural-sounding harmonic distortion and compression that is perceived as warm.
Led Zeppelin’s first LPs are considered masterpieces of calculated band saturation. Jimmy Iovine’s productions, such as Easter by Patti Smith, also work intensively with tape saturation.
The tape saturation effect cannot be used on digital storage media. Here, overmodulation is perceived as disturbing digital clipping. The aim here is to generate a high level without clipping.
Modern mastering studios therefore use a number of methods to increase loudness to just below the maximum level of -0dB. With the help of digital brick-wall limiters, they are able to process an audio signal in advance and thus limit the level without delay.
In many cases, therefore, CD mixes of re-releases are much more aggressively compressed and limited than their vinyl predecessors. In very simplified terms, a limiter allows the input signal to be controlled louder and louder without having to fear that overloads will occur. This is because levels that are too high are simply cut off by the limiter. This can be seen well in the example of Peter Gabriel III.
Analogue limiters or compressors cannot normally work in a predictive way. They react to the signal, resulting in a minimal reaction time of a few milliseconds. So a residue of dynamics remained unintentionally.
The battle for loudness develops into the Loudness War
The battle for loudness (not volume) as we know it today probably began in the jukebox age. Motown singles had a reputation for sounding the loudest, and so were best able to hold their own in the high background noise of a bar. Motown used half-speed mastering for this purpose, in addition to compression and many other methods common today, as early as the 1960s. Paul McCartney is also known to have scrutinised Motown hits very closely and copied the recording and mastering techniques to make the Beatles’ singles sound louder.
At the beginning of the CD era, loudness became more important again. Vinyl records were still the predominant medium at the time, but increasingly CD samplers were being made from the existing analogue tapes. Artists and producers noticed that their pieces seemed quieter than those of other performers and demanded improvements so that they could “keep up” with the others.
With vinyl releases, overly aggressive loudness and compression still meant that LPs or maxis were no longer playable. In the CD age, these physical limits fell.
With the advent of MTV and more and more rock radio stations, it became increasingly important for artists and record companies to get on the “heavy rotation” list in order to make a song or album successful. Therefore, music ideally already had to be delivered “radio-friendly”, i.e. with great loudness.
A real competition arose among music producers to see who could produce the loudest recording. This excessive use of dynamic compressors, both in the mixing and the transmission of music, increased loudness at the expense of sound quality.
This trend is also called the Loudness War. But that is another story.
In order to cope with the Loudness War, streaming platforms have created guidelines for the average loudness of songs. For example, on YouTube and Spotify, songs may not exceed an average loudness of -14 LUFS (approximately -14 dB). On Itunes, the default is even -16 LUFS, so music is usually played back a little more dynamically here.