SECTION 4
THEORY OF OPERATION
Based on the Dolby SR spectral recording system, the Model 430 series design incorporates two separate noise reducing bands; one for low and the other for high frequencies. Unlike SR, however, the degree to which the signal is treated can be controlled by two slider controls and the level at which the expanders work is adjustable by a rotary control which has a nominal “cal” position.
In typical applications, the Model 430 is placed in the signal path of a sound mixing console where nominal line levels are present. It is intended to work at a level around +4 dBr (Ref: 0.775 Vrms = 0 VU = 0 dBr).
With maximum processing selected, the two noise reduction paths form level dependent shelves in their responses whose depth increases with decreasing input signal level. For the LF section where control of rumble, hum, generator noise, fan or wind noise is required, the response at low levels is a shelf which is flat for high frequencies and, starting at around 2 kHz, falls to a maximum attenuation of about 18 dB at
30 Hz. As the level of low frequencies increases the response flattens out so that around 10 dB either side of the nominal level there is little or no processing. See the figure below.
+10 VU |
(+4 dB)0 VU 
20 Hz | 100 Hz | 1 kHz | 10 kHz | 20 kHz |
Similarly for the HF band, where control of amplifier hiss and noise, tape noise, rain noise, script rustle or lighting “sing” is required, the response at low levels is a shelf which is flat at low frequencies and from about 200 Hz, starts to fall so that, at 8 kHz the response is around 16 dB down from flat. Again, as the level rises the curve flattens out so that around 10 dB below and above the nominal level there is little or no processing. See the figure below.
(+4 dB) 0 VU |
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20 Hz | 100 Hz | 1 kHz | 10 kHz | 20 kHz |
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