'Tape hiss' originated as a term in the prehistoric times before the implementation of digital audio recording and playback. It describes the typical high-pitch noise inherent to analogue magnetic recording and playback. For this reason, tape hiss is typically heard on all kinds of magnetic tape-recordings (eg, cassette or open reel) and is particularly annoying in the quieter passages of musical pieces. In fact, tape hiss can be heard on any material that was originally recorded to tape — that is, the vast majority of all recordings until the late 1980s1. The amount of tape hiss increases linearly with every analogue copy. Hence a vinyl record produced from the master tape will have significantly less tape hiss than a fifth generation copy of a bootleg. This is also one reason why the archaic, mechanical vinyl records were not replaced by the more handy cassettes, when they became available, but replaced only late in the 1980s by the compact discs.
The Physics Behind Tape Hiss
Sound, as dynamic pressure oscillations, can be transformed into electric voltage oscillations (see, for example, How Guitar Pickups Work), which in turn can be used to produce magnetic field oscillations, which are finally transferred to a magnetic tape. This is the place where the tape hiss enters the scene: the texture of the magnetic tape that is being recorded on — the size and the thickness of the microscopic magnetic grains that are embedded in the plastic tape — produce fluctuations in the involved magnetic fields. These fluctuations are later heard as a characteristic noise, the tape hiss. As with toilet paper, there are many types and qualities of magnetic tapes, which will affect the amount of tape hiss produced. The point is that there is no such thing as a perfect tape that would entirely suppress tape hiss2. Even the highest quality tape, with the finest texture, small and homogeneous magnetic grains, produces tape hiss. Particularly annoying is the fact that this noise is 'inherited' by any copy made from the master tape, including vinyl records or CDs3.
Keeping Tape Hiss at Bay
The bad news is that it is not possible to eliminate tape hiss completely. Even so, around 1970, some noodlers (most prominently Ray M Dolby) figured out how to keep tape hiss, or noise in general, at a bearable minimum. Tape hiss suppression works, basically, by applying different sound-filters to the sound while it is being recorded and reversing the filtering process during playback, or copying.
Dolby A, for example, works by increasing the volume of the high frequency parts (which contain most of the noise) before the sound goes onto tape during recording. In this way the proportion between high frequency sound and high frequency noise (which is already on the tape) — ie, the signal-to-noise ratio — is increased. The filtering has to be reversed during playback, so that the high frequency part is brought back to the original level in comparison to the low frequency part. As a consequence the intensity of the tape hiss noise from the recording is also reduced. The price to pay for the noise reduction is a concomitant reduction of frequency dynamics. That is, the sound loses 'sharpness' or 'definition'. There are more complex and sophisticated processes and algorithms to reduce tape hiss, all with their own pros and cons. However, thanks to digital audio recording, tape hiss is not a significant problem these days.
Related h2g2 Links
Audiophiles may want to see what other Researchers have to say on related topics: