Editing Dolby noise-reduction system (section)

=={{anchor|Play Trim}}Dolby C==
[[File:PlotDolbyCEncodeCurves.tif|400px|thumb|right|Encoding characteristics of Dolby C-type noise reduction for a range of input signal levels]]<!-- This section is linked from inside or outside this article. Do not rename without keeping in sync. -->

[[File:Dolby B-C Noise Analysis Res Lo.tif|thumb|400px|Comparison of noise and frequency response characteristics of Dolby B-type and Dolby C-type noise reduction systems, from a Nakamichi ZX-7 high-performance cassette deck]]

The Dolby&nbsp;C-type noise reduction system was developed in 1980.<ref name="DolbyAESConv1981"/><ref name="DolbyJAES1983"/><ref name="Hull1981"/> It provides about 15&nbsp;dB noise reduction ([[A-weighted]]) in the 2&nbsp;kHz to 8&nbsp;kHz region where the ear is highly sensitive and most tape hiss is concentrated. Its noise reduction effect results from the dual-level (consisting of a high-level stage and a low-level stage) staggered action arrangement of series-connected compressors and expanders, with an extension to lower frequencies than with Dolby&nbsp;B. As in Dolby&nbsp;B, a "sliding band" technique (operating frequency varies with signal level) helps to suppress undesirable ''breathing'', which is often a problem with other noise reduction techniques.

As a result of the extra signal processing, Dolby&nbsp;C-type recordings will sound distorted when played back on equipment that does not have the required Dolby&nbsp;C decoding circuitry. Some of this harshness can be mitigated by using Dolby B on playback, which serves to reduce the strength of the high frequencies.

With Dolby&nbsp;C-type processing, noise reduction begins two octaves lower in frequency in an attempt to maintain a psychoacoustically-uniform noise floor. In the region above 8&nbsp;kHz, where the ear is less sensitive to noise, special spectral-skewing and anti-saturation networks come into play. These circuits prevent cross modulation of low frequencies with high frequencies, suppress tape saturation when large signal transients are present, and increase the effective headroom of the cassette tape system. As a result, recordings are cleaner and crisper with a much improved high-frequency response that the cassette medium heretofore lacked. With a good quality tape, the Dolby&nbsp;C response could be flat to 20&nbsp;kHz at the 0&nbsp;dB recording level, a previously unattainable result. An A-weighted signal-to-noise ratio of 72&nbsp;dB (re 3%&nbsp;THD at 400&nbsp;Hz) with no unwanted "breathing" effects, even on difficult-to-record passages, was possible.

Dolby&nbsp;C first appeared on higher-end cassette decks in the 1980s. The first commercially available cassette deck with Dolby&nbsp;C was the [[NAD Electronics|NAD]] 6150C, which came onto the market around 1981. Dolby&nbsp;C was also used on professional video equipment for the audio tracks of the [[Betacam]] and [[Umatic SP]] videocassette formats. In Japan, the first cassette deck with Dolby&nbsp;C was the AD-FF5 from [[Aiwa]]. Cassette decks with Dolby&nbsp;C also included Dolby&nbsp;B for backward compatibility, and were usually labeled as having "Dolby B-C NR".

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