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Dolby Noise Reduction Systems Explained: What are the Differences Between Them?

Dolby Noise Reduction Systems

One major problem with recording audio to tape is the hissing noise that gets included.

To overcome the problem a noise reduction system is used.

One such system is the Dolby noise reduction system.

The Dolby noise reduction system performs two functions:

  1. Boosts the energy of audio signals
  2. Improves the signal-to-noise ratio of audio signals.

This applies to the high-frequency spectrum of audio signals.

The audio signal energy is primarily focused on the low-frequency area of the spectrum. In other words, frequency below 2kHz.

The higher frequency section of the audio signal that outputs quality and ‘feel’ usually has a lower energy level. Even the smallest volume of noise can affect its quality.

When a tape is played back, the signal-to-noise ratio is much lower in the higher-frequency parts than in the lower-frequency parts.

You can hear noise much more clearly in the higher-frequency parts. This is because the signal energy doesn’t do a good job of masking it.

How Does the Dolby Noise Reduction System Work?

The Dolby noise reduction system works by broadly focusing on and improving the low-energy sections of a high-frequency signal before the signal is even copied or recorded.

Then when the signal is recorded, it undergoes processing and encoding employing a pre-emphasis filter and dynamic range compression.

During playback, decoding of the audio signal takes place. This is done using both a de-emphasis filter and a decompression circuit.

To avoid any audio distortion, the encoder and decoder must work hand in hand.

Types of Dolby Noise Reduction Systems

Among the noise-reduction systems developed by Dolby are Dolby A, Dolby B and Dolby C.
How are they different from each other?

Their main difference is in the band number and pre-emphasis method,

Dolby A

The Dolby A noise reduction system was created for professional use.

With Dolby A the signal spectrum is split into 4 frequency bands.

Band 1 is called low-pass and handles 0 to 80 Hz.

Band 2 is called band-pass and covers 80 Hz to 3 kHz.

Band 3 is referred to as high-pass and covers frequencies above 3kHz.

Band 4 is also called high-pass and handles frequencies above 9 kHz.

Dolby A offers a 10 to 15 dB gain (maximum) for each band if the signal level drops 45 dB below the maximum recording level.

Dolby A is used with film prints with magnetic audio tracks. It is also employed in high-speed multi-track recording machines.

It was in popular use from 1975 to 1986. Dolby SR has taken over its place nowadays.

Dolby B

Dolby B is used for consumer audio systems. Two bands instead of four are used.

It offers a maximum boost of 10 dB if the signal level is 45 dB below the maximum recording level.

It is used in stereo decks.

Dolby C

Also used in consumer systems and two bands are employed.

A maximum boost of 20 dB is offered when the signal level is 45 dB below the maximum recording level.

Dolby C works well in non-Dolby machines compared to Dolby A and Dolby B.

Dolby SR

Dolby SR (spectral recording) is Dolby’s second professional noise reduction system after Dolby A.

The system is created to be used with multi-track equipment. It reduces noise up to 25 dB on the high-frequency side.

Dolby S

Dolby S is an improvement of Dolby C, aimed at consumers. It’s actually a simplified version of Dolby SR.

Dolby claims ordinary folks can’t differentiate between a Dolby S tape and an audio CD.


Advanced Digital Signal Processing and Noise Reduction
By Saeed V. Vaseghi

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