Tape hiss When recording a signal on magnetic tape, there is a low level of noise in the background which sounds like hissing, known as
tape hiss. One way to make this less noticeable is to increase the volume of the recording, so that it is loud enough that the hissing is less noticeable. This does not solve the problem during quiet passages in the music, when the hiss will become noticeable again. Also, if the volume is boosted uniformly, the high-volume sections of the recording can overwhelm the limited capacity of the tape and lead to "clipping". This latter problem can be addressed by recording the sounds on a larger area of tape, either by using a wider tape or running it at a higher speed.
Reel-to-reel tape systems standardized on wide tape. To address hiss, as well as offer greater
dynamic range, they could be run at different speeds, or for higher fidelity recordings, or slowed to when long recordings were needed. Users could trade off total recording time capacity for quality, depending on the needs of a particular recording. The cassette format was designed from the start for ease-of-use and portability, using a thinner wide tape and running at the single slower speed of . This resulted in hiss being much more noticeable, but this was not a concern for its design use as a dictation recording format. With a small size and single speed, there was no simple way for the user to address the hiss when used for higher-fidelity recordings like music. Things improved through the use of "low noise" oxides, from the late 1960s, but hiss remained a major problem on cassettes and limited its use for music.
Companding Dolby noise reduction offers a solution to the problem of hiss by dynamically changing the volume of the recording. As low-volume passages need to be boosted to cover the sound of the hiss, while high-volume sections cannot be boosted too much, the underlying concept of Dolby noise reduction is to dynamically adjust the volume based on the current volume of the recording. Low-volume sections are boosted, and high-volume sections are not. The result is a "compressed" recording which is more even in volume than the original, boosting low-volume sections to cover up the sound of hiss. This general concept is known as dynamic
pre-emphasis. On playback, the volume range is moved back towards the original, a process known as
de-emphasis. The concept of using pre-emphasis and then de-emphasis is known as
companding. On top of this basic companding concept, Dolby noise reduction systems add another improvement. This takes into account the fact that tape noise is largely heard at frequencies above 1,000 Hz. It is the lower-frequency sounds that are often loud, like drum beats, so by only applying the companding to certain frequencies, the total amount of distortion of the original signal can be reduced and focused only on the problematic frequencies. The differences in the various Dolby products are largely in the precise set of frequencies that they use and the amount of modification of the original signal volume that is applied to each of the frequency bands. Within each band, the amount of pre-emphasis applied depends on the original signal volume. For instance, in Dolby B, a low-level signal will be boosted by 10
dB, while signals at the "Dolby Level", +3
VU, receive no signal modification at all. Between the two limits, a varying level of pre-emphasis is applied. On playback, the opposite process is applied, based on the relative signal component above 1 kHz. Thus, as this portion of the signal decreases in amplitude, the higher frequencies are progressively increasingly attenuated, which also reduces in level the constant background noise on the tape when and where it would be most noticeable. The two processes, pre- and de-emphasis, are intended to cancel each other out as far as the actual recorded program material is concerned. During playback, only de-emphasis is applied to the incoming off-tape signal and noise. After playback de-emphasis is complete, the apparent noise in the output signal is reduced, and this process should not produce any other effect noticeable to the listener other than reduced background noise. However, playback without noise reduction produces a noticeably brighter sound, as the higher-frequency sections have been boosted. The correct calibration of the recording and playback circuitry is critical in order to ensure faithful reproduction of the original program content. The calibration can easily be upset by poor-quality tape, dirty or misaligned recording/playback heads, or using inappropriate
bias levels/frequency for the tape formulation, as well as tape speed when recording or duplicating. This can manifest itself as muffled-sounding playback, or "
breathing" of the noise level as the volume level of the signal varies. On some high-end consumer equipment, a Dolby calibration control is included. For recording, a reference tone at Dolby Level may be recorded for accurate playback level calibration on another transport. At playback, the same recorded tone should produce the identical output, as indicated by a Dolby logo marking at approximately +3 VU on the VU meter(s). In consumer equipment, Dolby Level is defined as 200
nWb/m, and calibration tapes were available to assist with the task of correct level setting. For accurate off-the-tape monitoring during recording on 3-head tape decks, both processes must be employed at once, and circuitry provided to accomplish this is marketed under the "Double Dolby" label. == Dolby A ==