User manual BSS DPR901

[. . . ] DPR-901ii User's Manual This Manual is the COPYRIGHT of BSS Audio. All reproduction and copying, other than for the legal owner's personal use, or disclosure of part or whole to a third party, without prior written authorisation, is in violation of the European Copyright Convention. BSS Audio 1995 1 DPR-901ii Welcome V1. 1 9 August 1995 Welcome to the DPR-901ii Users Manual DPR-901ii ABOVE AND BELOW THRESHOLD DYNAMIC PARAMETRIC EQUALISATION. This manual is provided with the aim of assisting sound engineers, producers, musicians, system installers and consultants to fully understand the DPR-901ii, and to benefit from it's maximum capability. At the same time, the information is structured under a series of broad headings for easy access. [. . . ] The first green LED indicates a signal level 12dB below threshold. 14 Controls 14 BELOW Threshold When the switch is released, the processing operates above threshold, that is with compression or expansion being applied only to signal levels that exceed the threshold. When the switch is depressed to select Below, the green led beneath the switch lights. The threshold's sensitivity setting remains the same, but the processing is `turned on its head' as compression or expansion is now only applied to signals which are below the threshold. Below threshold compression now works like a frequencysensitive gate in that the further below the threshold the signal is, the more it is attenuated. There being progressively less attenuation as signal level increases to approach the threshold setting. Below threshold expansion acts to boost the selected frequency range up to (but not above) the threshold. The expansion meter 9 will light under these low signal conditions, slowly reducing as the input signal increases towards threshold. 15 FAST RELEASE When the switch is released, the compress/expand time constants are automatic. This is the normal setting if you're using the DPR-901ii as a leveller or otherwise processing a composite (mixed down) music signal. When depressed for FAST, the compress/expand release time is speeded up. This is likely to be the correct setting for less spectrally dense material, normally single instruments, especially dynamic sources, ie. vocals, percussion and any instruments played percussively. 15 DPR-901ii Principle of Operation 4. 0 Principle of Operation The DPR-901ii is fundamentally different to ordinary de-essers, Dolby systems and the like because it uses subtractive techniques to control the degree of compression or expansion. This enables the VCA (gain control element) to be kept out of the direct signal path. As a result, the throughgoing signal is not subjected to the noise and distortion that's intrinsic to even the best VCAs. Moreover, the resultant filtered output exhibits far less of the `time smearing' that accompanies most conventional units and has at times given equalisation a bad name. In fact, the subtractive VCA technique can only operate correctly if the `passing through' and `control' signals at the subtractor are correctly aligned. Figure 4. 1 shows a simple block diagram that illustrates the principle of operation of subtractive gain reduction and Fig 4. 2 that with the addition of filtering and invertor for gain addition or expansion. Referring to figure 4. 1, in operation the main signal path is simply through the single inverting op-amp as the VCA is held at maximum attenuation. When signal exceeds the set threshold level, the inverting VCA is gradually opened, allowing a small `subtracting' signal to pass to the main inverting op-amp. Furthermore, this signal is also conditioned by the filtering effect of the parametric filter. The resultant output is then either reduced in level, or increased in level dependent on the `compress/expand' switch, and it is only those frequencies passed by the filter, that are so processed. On the DPR-901ii, the threshold control is adjusting the voltage drive level to the VCA, in conjunction with a fixed reference point, whilst the `compress/expand' switch is replaced with a centre-off compress or Fig 4. 1 Subtractive Gain Reduction Fig 4. 2 Subtractive Gain Reduction or Expansion 16 Principle of Operation expand level control, whose function is similar to a ratio control on conventional dynamic processors. [. . . ] In modern audio, the dBm is loosely used to 25 DPR-901ii describe voltage levels, although strictly it only applies to 600 ohm circuits. dBu or dBv The addition of `u' or `v' after dB indicates an absolute scaling for the dB ratio. 0dBu (or 0dBv) = 778mV or 0. 778 volts, and it has no regard to power or impedance. dBu and dBv are widely used for expressing signal voltages in modern audio equipment, where output impedances are low, and input (load) impedances are much higher. [. . . ]