I’ve listened to music recorded on tape and the way the sound changes seems to be very subtle: the ‘warmth’ seems to be largely due to the soft-saturation characteristics of the tape and other non-linear components in the signal chain. This being the case, won’t a simpler valve emulation or mild overdrive plug-in achieve pretty much the same result, as both really just ‘squash’ the signal peaks? Why do tape emulation plug-ins cost so much more than overdrive plug-ins?
George Roque, via email
SOS Editor In Chief Paul White replies: You’re right in pointing out that the effects of recording to tape can be pretty subtle, but they become less so when you drive the tape harder, causing deliberate saturation. However, although it’s true that saturation is a big part of what makes tape sound the way it does, it’s far from the only factor.
To maximise signal-to-noise ratio, an equalisation curve such as NAB or CCIR is applied when a signal is recorded to tape, and the inverse EQ applied on playback. This means that the tape saturation is applied to the equalised signal, with all its attendant phase-shift implications, rather than to the flat signal. The outcome is that the spectrum of added ‘saturation’ harmonics will be a little different from those produced when a flat signal is fed via a simple non-linear saturation device. The equalisation means tape machines will, for example, distort at far lower levels for high-frequency sounds such as hi-hats than for lower-frequency sounds.
Other factors also come into play, such as the ‘head bump’ bass boost, which is a function of tape speed and head gap design, as well as subtle pitch modulation which we term wow and flutter. Even the best analogue tape transports are far less stable than a digital clock. All these elements add up to produce the analogue tape ‘sound’, but even that changes depending on the tape recorder model and its alignment, the tape speed and the formulation of the tape being used.
There are also other effects that may be considered more or less important such as modulation noise (a sine wave recorded to an analogue tape machine can sound alarmingly crunchy!) and the position of other non-linear devices such as transformers and valves in the signal path.
Tape machines also introduce significant phase shifts, as you’d see if you recorded a square wave and then checked the output on an oscilloscope. You might expect to see some integration or smoothing of the sharp rising and falling edges of the waveform because of the limited frequency response of the tape machine, but the reality is that the square wave often appears bent almost beyond recognition, thanks to different frequencies being shifted in phase relative to each other during the recording and playback processes.
In some cases, a simple valve emulation plug-in may well deliver the necessary warmth, but to design a plug-in that takes into account all the variables of tape machines including tape width, tape speeds, recording levels, tape hiss and tape formulation is a very complex task, which is why the best of these tape emulations (as in the most authentic recreations) don’t come cheap. However, most still cost rather less than an album’s worth of two-inch tape — and there are some surprisingly good freebies available too, such as Jeroen Breebaart’s Ferox and Variety Of Sound’s Ferric!