Cassette Deck - Noise Reduction and Fidelity

Noise Reduction and Fidelity

A variety of noise reduction and other schemes are used to increase fidelity, with Dolby B being almost universal for both prerecorded tapes and home recording. Dolby B was designed to address the high-frequency noise inherent in cassette tapes, and along with improvements in tape formulation it helped the cassette win acceptances as a high-fidelity medium. At the same time, Dolby B provided acceptable performance when played back on decks that lacked Dolby circuitry, meaning there was little reason not to use it if it was available.

The main alternative to Dolby was the dbx noise reduction system, which achieved a high signal-to-noise ratio, but was essentially unlistenable when played back on decks that lacked the dbx decoding circuitry.

Philips developed an alternative noise reduction system known as Dynamic Noise Limiter (DNL) which did not require the tapes to be processed during recording; this was also the basis of the later DNR noise reduction.

Dolby later introduced Dolby C and Dolby S noise reduction, which achieved higher levels of noise reduction; Dolby C became common on high-fidelity decks, but Dolby S, released when cassette sales had begun to decline, never achieved widespread use. It was only licensed for use on higher end tape decks that included dual motors, triple heads, and other refinements.

Dolby HX Pro headroom extension provided better high-frequency response by adjusting the inaudible tape bias during the recording of strong high-frequency sounds, which had a bias effect of their own. Developed by Bang & Olufsen, it did not require a decoder to play back. Since B&O held patent rights and required paying license fees, many other manufacturers refrained from using it too.

Other refinements to improve cassette performance included Tandberg's DYNEQ, Toshiba's adres (ja) and Telefunken's High Com, and on some high-end decks, automatic recording bias, fine pitch adjustment and (sometimes) head azimuth adjustment like the Tandberg TCD 320.

By the late 1980s, thanks to such improvements in the electronics, the tape material and manufacturing techniques, as well as dramatic improvements to the precision of the cassette shell, tape heads and transport mechanics, sound fidelity on equipment from the top manufacturers far surpassed the levels originally expected of the medium. On suitable audio equipment, cassettes could produce a very pleasant listening experience. High-end cassette decks could achieve 15 Hz-22 kHz±3 dB frequency response with wow and flutter below 0.022%, and a signal-to-noise ratio of up to 61 dB (for Type IV tape, without noise-reduction) . With noise reduction typical signal-to-noise figures of 70-76 dB with Dolby C, 80-86 dB with Dolby S, and 85 - 90 dB with dbx could be achieved. Many casual listeners could not tell the difference between compact cassette and compact disc.

From the early 1980s, the fidelity of prerecorded cassettes began to improve dramatically. Whereas Dolby B was already in widespread use in the 1970s, prerecorded cassettes were duplicated onto rather poor quality tape stock at (often) high speed and did not compare in fidelity to high-grade LPs. However, systems such as XDR, along with the adoption of higher-grade tape (such as chromium dioxide, but typically recorded in such a way as to play back at the normal 120 μs position), and the frequent use of Dolby HX Pro, meant that cassettes became a viable high-fidelity option, one that was more portable and required less maintenance than records. In addition, cover art, which had generally previously been restricted to a single image of the LP cover along with a minimum of text, began to be tailored to cassettes as well, with fold-out lyric sheets or librettos and fold-out sleeves becoming commonplace.

Some companies, such as Mobile Fidelity, produced audiophile cassettes in the 1980s, which were recorded on high-grade tape and duplicated on premium equipment in real time from a digital master. Unlike audiophile LPs, which continue to attract a following, these became moot after the Compact Disc became widespread.

Some cassette decks have a MPX filter to improve the sound quality and the tracking of the noise reduction system when recording from a FM stereo broadcast.