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Analog Techniques
Published in Gillian M. Davis, Noise Reduction in Speech Applications, 2018
An important class of analog amplifiers is the voltage-controlled amplifier (VCA). The VCA is a two-quadrant analog multiplier (as opposed to a modulator, which is a four-quadrant multiplier**), where the signal input is bipolar, whereas the gain control input is constrained to be zero or positive. A VCA finds use in audio system applications such as programmable analog mixing desks where, e.g., gain or filter parameters require dynamic control from signals derived within a computer or remote controller. VCAs are also widely used in phase-lock loops as the phase-sensitive detector, although in this application a four-quadrant multiplier implementation is required. The basic specification requirements of a VCA are similar to other audio amplifiers in terms of noise and distortion. However, because the gain is programmable, a method is required to embody active devices that, although nonlinear, appear to be linear from the signal’s perspective. The core principle exploited by BJT-based VCAs is to use the logarithmic method of multiplication, where () x⋅y≡e{loge(x)+loge(y)}
Mixing
Published in Mike Collins, Pro Tools 9, 2012
To understand VCA Groups a little more thoroughly, you need to know something about how these were developed. Firstly, VCA is the abbreviation for a Voltage Controlled Amplifier. Mixing console designers realized that they could include a set of faders on an analogue console that could be used to send voltages to the amplifiers in selected audio channels so that one of these VCA Master Faders would control the audio levels for a group of mixer channels. A group of mixer channels being controlled in this way by a single VCA Master Fader is then referred to as a VCA Group. No audio passes directly through the VCA Master Fader. VCA Master Fader is only used to set the level of the DC control voltage that is sent to control the levels of the faders in the group.
Fundamental Mastering Tools and The Primary Colors of Mastering
Published in Evren Göknar, Major Label Mastering, 2020
VCA compressors utilize a voltage-controlled amplifier whose control voltage is derived from the audio input signal itself to effect the gain reduction. Classic examples are the dbx 160 (and permutations) (Figure 4.28) and dbx 165. These versatile compressor designs offer a great degree of user control for attack/release settings, making them excellent for stereo bus and mastering applications. Common VCA compressors suitable for mastering are the Alan Smart C2 (Figure 4.30), Neve 33609 (Figure 4.33), SSL G-Series (Figure 4.29), API 2500 (Figure 4.31), Overstayer 3722 SVC, and Vertigo VSC-3.
Design and Evaluation of Compliant Modular XY Positioning Stage
Published in Australian Journal of Mechanical Engineering, 2022
Santosh B. Jadhav, Kishor K Dhande, Suhas P. Deshmukh
Figure 6 shows the test set up for evaluating static and dynamic performance of the stage. To fulfil the requirement of large range of motion, VCA (32–30) with maximum force of 60 N is used for actuation. Linear current amplifier (LCAM 5/15 from Quanser) is used to provide the current to VCA. The output position of the motion stage is measured by the linear optical encoder from Reni-shaw with 50 nm resolution. A controller board, i.e. dSPACE DS1104 is used for precisely implementing the signals output. The sinusoidal positioning tests are carried out to study point to point positioning and error of the micro-positioning stage. Figure 7 shows the hysteresis loop and corresponding input voltage–displacement curve. The maximum permissible range of motion in X or Y direction is ± 5 mm and coupling motion of 10 which indicates that there exists in limited kinematic coupling.
Signal Converter with Three-Phase and Quadrature Outputs for Driving Synchros and Resolvers
Published in IETE Journal of Research, 2022
Darko Vyroubal, Vedran Vyroubal, Damjan Belavić, Zoran Halić
The novel controlled phase shifter (PS) circuit diagram is in Figure 4. The shifter consists of the standard, first-order all-pass filter realized with an operational amplifier (OPA) [10] and the feedback loop comparing the PS input and output signals by the voltage-controlled amplifier (VCA) realized with operational transconductance amplifier (OTA). The OTA output controls the charging rate of C in Figure 4, thus controlling the output phase. The OTA gain is variable, controlled by signal Vg. By inspection of the circuit in Figure 4, the following differential equation is derived: where and is the OTA time-variable transconductance modulated by Vg. For constant Vg, Equation (2) becomes linear differential equation with constant coefficients. Laplace transform can be applied to derive the transfer function where is the PS time-constant corresponding to Vg. For sinusoidal Vi(t) the PS transfer function is From Equation (6) follows that the PS gain is 1 independent of frequency and the phase changes with frequency from 0 to (−180°). The required phase shift (−90°) is just in the middle of the phase range when Vg adjusts . In Figure 5, there is an example of the numerical solution to Equation (2). It can be observed that the settling time is very short and the output phase varies around −90° following gm modulation. Small amplitude modulation can be observed too. In Figure 6, there is the spectrum of Vo(t). Due to shallow modulation, only the first-order sidebands are significant, although slightly imbalanced. The imbalance is caused by a combination of phase and amplitude modulation. The vector presentation of the modulation process is in Figure 7. It is apparent that the small phase error (Δϕ) is also present. This error vanishes in steady state when the phase feedback loop settles and Vg becomes constant producing no more modulation. The Hilbert demodulated PS output phase is shown in Figure 8. The distortion due to simultaneous amplitude modulation is noticeable what results in small phase error (Δϕ ∼1°-2°).