Editing Dolby noise-reduction system (section)

== Process ==
When recording a signal on magnetic tape, there is a low level of noise in the background which sounds like hissing. One solution to this issue is to use low-noise tape, which records more signal, and less noise. Other solutions are to run the tape at a higher speed or use a wider tape. Cassette tapes were originally designed to trade off fidelity for the convenience of recording voice by using a very narrow tape running at a very slow speed of {{cvt|1+7/8|in/s}} housed in a simple plastic shell when {{cvt|15|in/s}} or {{cvt|7+1/2|in/s}} tape speeds were for high fidelity, and {{cvt|3+3/4|in/s}} was of lower fidelity. As a result of their narrow tracks and slow speed, cassettes make [[tape hiss]] a very severe problem.

[[File:Dolby B-C Encode Curves Res Lo.tif|thumb|400px|Encoding characteristics for Dolby B-type and C-type noise reduction systems for a &minus;60&nbsp;dB input signal level. The decoding curves used in playback are the exact inverse and thus the overall record/replay frequency response is flat]]

Dolby noise reduction is a form of dynamic [[preemphasis|pre-emphasis]] employed during recording, plus a form of dynamic [[deemphasis|de-emphasis]] used during playback, which work in tandem to improve the [[signal-to-noise ratio]]. The signal-to-noise ratio is simply how large the music signal is compared to the low level of tape noise with no signal. When the music is loud, the low background hiss level is not noticeable, but when the music is soft or in silence, most or all of what can be heard is the noise. If the recording level is adjusted so that the music is always loud, then the low-level noise would not be audible.

One cannot simply increase the volume of the recording to achieve this end; tapes have a maximum volume they can record, so already-loud sounds will become distorted. The idea is to increase the volume of the recording only when the original material is not already loud, and then reduce the volume by the same amount on playback so that the signal returns to the original volume levels. When the volume is reduced on playback, the noise level is reduced by the same amount. This basic concept, increasing the volume to overwhelm inherent noise, is known as pre-emphasis, and is found in a number of products.

On top of this basic 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&nbsp;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 evident 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&nbsp;B, a low-level signal will be boosted by 10&nbsp;[[decibel|dB]], while signals at the "Dolby Level", +3&nbsp;[[VU meter|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 (de-emphasis), based on the relative signal component above 1&nbsp;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.

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 [[tape bias|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 (noise reduction)|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&nbsp;VU on the VU meter(s). In consumer equipment, Dolby Level is defined as 200&nbsp;[[Weber (unit)|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.