Technology 101: Still Remember Your Dolby Noise Reduced Songs?

Dolby NR TDK Cassette Tape

If one needs to listen to a particular song back in the 1980s, you will record them yourself either from the radio using blank TDK cassette tapes (encoded with Dolby Noise Reduction) or get it recorded in one of the many recorded cassettes selling shops out there. Image source: Wikipedia

What is Dolby Noise Reduction and how it works?

A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording.

The first was Dolby A, a professional broadband noise reduction system for recording studios in 1965, but the best-known is Dolby B (introduced in 1968), a sliding band system for the consumer market, which helped make high fidelity practical on cassette tapes, which used a relatively noisy tape size and speed. It is common on high fidelity stereo tape players and recorders to the present day.

Of the noise reduction systems, Dolby A and Dolby SR were developed for professional use. Dolby B, C, and S were designed for the consumer market. Aside from Dolby HX, all the Dolby variants work by companding, or compressing, the dynamic range of the sound during recording, and expanding it during playback.

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 1⅞ inches per second (ips) housed in a simple plastic shell, when 15 ips or 7½ ips tape speeds were for high fidelity and 3¾ ips was of lower fidelity. As a result of their narrow tracks and slow speed, cassettes make tape hiss a very severe problem.

The dominant “Dolby B” noise reduction scheme was widely accepted because if an inexpensive cassette player lacked the switch, they would just sound brighter, which often offset the dull sounds of cheap players.

The signal-to-noise ratio is simply how large the music signal is compared to the low level of the “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 it could in theory be turned down later, and the noise volume would also be turned down.

The idea is for electronics to automatically increase the recording volume when it is soft, but reduce the volume on playback. Some schemes like Dolby B concentrate only on the high frequencies so that the “hiss” sound of noise will be masked when the volume is turned down for playback.

Dolby noise reduction is a form of dynamic pre-emphasis employed during recording, plus a form of dynamic de-emphasis used during playback, that work in tandem to improve the signal-to-noise ratio.

While Dolby A operates across the whole spectrum, the other systems specifically emphasize the audible frequency range where background tape hiss, an artifact of the recording process that is similar to white noise, is most noticeable (usually above 1 kHz, or two octaves above Middle C).

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The older audio players that play cassette tapes (yes, way before the compact disc was popular) did not come with the all-important Dolby NR (noise reduction) button. In fact, some of them did not even have 2 speakers.

The later models had the NR button which effectively reduced the background hissing noise from the songs/music played on the cassette tapes. More importantly, players were bigger, had better speakers (some with dedicated treble and bass speakers) and auto-reverse function.

Even with the ones with Dolby NR button, this itself not a one-stop noise reduction solution because it depends on the quality of the original recording and also the quality of the cassette. There are times when we enabled the Dolby NR, the output gets worse – instead of just softening the hissing sound, the overall music simply gets muted considerably. The treble and bass go off and in the end, we just switch off the Dolby NR.

Cassette tape magnetic colour Dolby

But it is a totally different story when we use the expensive TDK cassette tape (the one where the magnetic strip is blueish in colour – see the image above – the worst is the one on top. Image source: Wikipedia) and we are doing duplication from an original cassette or the compact discs. The quality of recording improves considerably.

A bulk of maestro Iaiyaraja’s compositions until to the 1990s was all released in cassette tapes and the local sellers would maximise their profit by using the cheapest cassette tapes. Don’t expect high-quality TDK (or the next best Sony) cassette tapes on the shelves of these music shops. They make the matter even worse by porting over some of the analogue recordings were poorly ported over to compact discs later.

We had to wait till A R Rahman who came into the limelight using a proper digital encoding in compact discs. The quality of recording then becomes too apparent – you will know when it is recorded and played digitally.

By now where even compact disc has gotten out of fashion, there is no more a need to rely on noise reduction technology to hear music and songs in better quality. Everything is digitalised and everything is running on the internet.

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