Noise - Noise reduction: Other Dolby systems

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16.8.1. Dolby B

Production year: 1969

Towards the end of the 60s it became necessary to employ Noise Reduction systems on home systems which had magnetic tapes with very slow rolling-speed and consequently made a lot of hiss noise. Since the Dolby A system was too expensive and only accessible in professional work-contexts, a simpler version was created called Dolby B.

In this case we have two filters: one low-pass filter with one fixed cut-frequency (1.5 KHz) and one pass-band filter with cut–frequencies that vary according to the input signal. The input signal's high-frequency content controls the filter's scroll: the more high frequencies there are, the more the filter moves towards the high frequencies. This is because if the original signal has an elevated high frequency content, the latter will hide the tape's hiss and therefore no NR operations will be needed. Vice versa, a signal with a poor presence of high frequencies will be far more prone to hiss noise and will therefore need to undergo a NR operation.



16.8.2. Dolby C

Production year: 1980

It was devised as an improvement to the Dolby B system, and it is indeed similar to the latter, except that Dolby C uses two variable frequency filters to which two 2:1 compressors are added. This system allows for up to 20 dB noise reduction, even though it must be said that it confers a slightly unnatural sound to the decoded signal, due to the amount of manipulation that takes place during the encoding phase.



16.8.3. Dolby SR

Production year: 1986

Dolby SR, where SR stands for Spectral Recording, makes use of various systems that each contribute to noise reduction.

  • Spectral Skewing : indicates the subdivision of the input signal into bands.

  • Antisaturation : an equalizing curve that has the same shape as an isophonic curve is applied to the signal. This allows high frequencies to be recorded at their highest possible values without saturating the tape. Applying the opposite equalization during the reproduction phase gives you back the original signal.

Reduces noise by up to 24 dB.



16.8.4. Dolby S

Production year: 1990

The next step up from Dolby C. It uses two separate circuits which are activated according to the input signal's amplitude. It reduces noise by up to 24 dB. This system is used at an amateur and semi-professional level, not in professional contexts.



16.8.5. Dolby HX

The main innovation (HX stands for Headroom eXtension) this system offers, lies in the bias current [Bias current ] . Seeing that when we emphasize the high frequencies to bring about compression we risk saturating the tape, Dolby HX uses a bias current whose amplitude varies depending on the input signal's high frequency content. The idea is that if there is an elevated high frequency content, a bias current isn't necessary to stimulate the tape's magnetic particles. Seeing that an ideal bias current varies depending on the type of magnetic tape in question, this system needs to be optimized depending on the tape's characteristics.



16.8.6. Dolby HX Pro

In analogue recording, the bias current is made up of two components, one is the bias current generated by an oscillator inside the recorder's circuitry, and the other consists in the natural actions of the input signal's high frequencies. So, this way the bias level is never constant. In HX pro the bias current's level is constantly analyzed by a specific circuit that controls the bias current's amplitude, readjusting it each time according to the input signal's high frequency content.



16.8.7. Other NR systems

Naturally there are noise reduction systems that are not made by Dolby. Some of them are: the dbx series (dbx I, dbx II, dbx 321, dbx III) and Telefunken's Telcom C4. Albeit with due differences, these systems also are based on the compression of the signal carried out during recording and its subsequent expansion during the reproduction phase.










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