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Target/Slew RAM is a hardware-optimized function that allows volume or other parameter level changes to ramp to subsequent levels without audible clicks/pops.
In audio systems, abrupt changes in volume position or other parameters produce unpredictable noises. See the sample schematic below showing possible real-time changes that can produce clicks/pops:
This noise arises from an audio signal getting scaled by a step function, which causes an unwanted response. The step function contains an impulse (Dirac delta function d(k)). It can be shown that the impulse d(k) is a frequency-limited white noise. This noise is unwanted in all audio applications. See below for a graphic representation of this signal noise caused by the step function:
There are a couple of possible solutions to consider for this noise problem:
SigmaStudio’s volume control and mux blocks both use the slew RAM to cleanly ramp volume from one level to another. Thus for the all the volume control cells, you have the option of using target/slew RAM as your algorithm to prevent the noise inherent in switching levels.
There are 4 types of slope that can be used with the target/slew RAM:
For details, see the AD1940/1941 Volume Control Algorithms page.
Although target/slew RAM is useful for eliminating unwanted noise, there are some drawbacks: