The sound of analogue tape

Introduction

What is in common between albums of different genres and years like Nirvana’s Nevermind, Pixies’ Surfer Rosa, Soft Machine’s Seven, Emerson Lake and Palmer’s Tarkus and Miles Davis’ Bitches Brew? All these albums have an exceptional sound, and all of them have been mixed on analogue tape.

In others word, the mastering engineer has received these albums in the form of an analogue tape; the digitalization has been done only in the mastering or remastering stage.
The albums mentioned above are only some of the examples of the extraordinary warm and rich sound that can be obtained with an analogue tape recording.
If we listen to these albums confronting their sounds, we can easily recognize how they all sound warm, smooth and sweet. I could listen to these tracks for hours without any effort. This doesn’t apply with many of the modern digital recordings, often characterized by a sound so rich of mid high and high frequencies that it results fatiguing after few minutes.
The analogue tape sound has an intrinsic warmness, and this is caused by technical properties.

Why analogue tape sounds so good

Audiophiles know well that an analogue tape recording has to be done in theory at a certain input level, above which saturation occurs, meaning that the waves become compressed generating odd harmonics (distortion).
Saturation occurs in the digital domain too, but in a different way. Indeed, in the digital domain the wave portion that exceeds the maximum recording level (0 db) becomes completely clipped, and this produces a great quantity of odd harmonics, a bad sounding form of distortion.
On the other hand, when the signal exceeds the standard recording level in an analogue tape (for example a reel to reel tape machine, or a cassette), it produces a soft compression of the waves, causing a smoother distortion, with few odd harmonics, typically the third, the fifth and, in part, the seventh.
This aspect contributes to create the warm sound associated with analogue tape…but there are other things to consider.
If it was so simple as described above, analogue tape saturation would produce a clearer, brighter sound, because of the generation of upper odd harmonics; but the analogue tape is well known for its distinctive warm and slightly dark sound, when the signal saturates. Why? To comprehend this fact we have to introduce two processes: pre-emphasis and de-emphasis.
The analogue tape is intrinsically noisy, on the contrary of CD and digital domain in general, and this of course is not good. To reduce the hiss, that is more annoying in the high frequencies, decades ago it has been invented the pre-emphasis and the de-emphasis circuitry.
The sound, immediately before it is recorded on the analogue tape, goes through the pre-emphasis circuitry, that consists simply by the amplification of the high frequencies with a certain high shelf equalization; then when the sound is reproduced (after it has been recorded), it goes through the de-emphasis circuitry, that reduces the high frequencies with exactly the opposite high shelf equalization applied in the pre-emphasis stage.
This process permits to obtain a reduction of the hiss on the high frequencies, because the hiss inherently present in the analogue tape, going through the de-emphasis circuitry during the reproduction, is reduced in the high frequencies that are the most annoying.
But what happens to the sound? If the recording level is very low, nothing happens: the highs are amplified during the recording stage and then are reduced in the reproduction stage exactly in the same amount, so the high frequencies are reproduced in a transparent way.
But with a standard or high recording level, a very interesting thing happens: the signal saturation during the recording stage occurs overall on the high frequencies, because these are the ones that are amplified before the recording by the pre emphasis circuitry. The final result will be a higher saturation / compression on the high frequencies, with respect to mid and bass frequencies. This will led to a warmer sound.

High frequencies saturation

High frequencies saturation compresses the high frequencies transients, like peaks from cymbals, voice sibilance, horns and so on; this kind of compression makes the sound smoother, sweeter, more musical…listening to a sound with compressed high frequencies peaks is pleasing and relaxing. This is why albums well recorded on analogue tape sound so good.
Listening to the albums mentioned at the beginning of this article having in mind these technical considerations, can make even more evident the magical effect that analogue tape can impart to the high register and to the sound in general.
This is why even today many artists deicide to mix on analogue tape instead that on digital mediums...and this is the reason why many digital plugins producers try to faithfully emulate old and famous models of reel to reel tape machines (see well known examples like Waves, Slate Digital and so on).
Analogue tape is not dead!