The measure of tape
Tape enjoyed a long history, and there are many types of tape deck: later models could achieve extremely high-fidelity, while earlier or cheaper models might colour the sound significantly before it even hit the tape at all.
Tapes came in many widths, from two inches down to an eighth-of-an-inch for compact cassettes; and tape speed could vary from 30 inches-per-second on a pro machine, down to just 1.875 ips for a compact cassette.
Different formulations of tape were available: even the humble compact cassette had three common types (ferric, chrome and metal), each of which sounded very different, and three possible different noise reduction systems (Dolby B, C, or S, although the last one was rare).
There are a few general characteristics that we associate with the tape sound, the most important of which is saturation. There’s a limit to how much you can magnetise a section of tape: once all the molecules are polarised the same way, there’s nowhere else to go.
But tape approaches this limit very gradually and gently. There’s no sudden hard ceiling like in an AD converter – instead, a smooth, gentle curve, which starts to gently round off and tame peaks in the signal well before they reach the limit. This means the added harmonics tail off very quickly as they get higher in frequency, more like a triangle wave than a square wave.
As tape saturation tends to be symmetrical, there will only be odd harmonics added, so you’ll likely end up with a healthy dose of third harmonic, maybe a bit of the fifth, and little or nothing higher than that: a recipe for a warm, relatively transparent type of saturation that can work well on complex signals like a full mix.
Tape exhibits hysteresis, which means, again, a type of ‘memory’, making saturation more complex than simple waveshaping. But in the same way very fast compression starts to add significant distortion, saturation with a memory effect starts to add a natural compression effect.
Tape saturation also typically reduces highfrequency transients, which helps tame things like cymbals or strumming. Many engineers also routinely boost treble while tracking to tape, knowing it can be cut back again at mixdown – this practice helps reduce hiss, but also pushes high frequencies further into tape saturation.