Expander
Overview
The expander is the inverse of the compressor. The expander dynamically modulates the gain when the signal falls below the threshold, attenuating the signal. The expander is used to increase the dynamic range of an audio signal by reducing the level of low-level signals (signals below the threshold) while leaving high-level signals unchanged, in contrast to a compressor, which reduces the level of high-level signals while leaving low-level signals unchanged. A certain type of expander, called the downward expander, is implemented by a class named expander.
| The expander can be utilized as a noise gate by setting the threshold to allow only audio signals above the noise floor to pass through. When the audio level falls below the threshold, the expander effectively gates out any low-level noise by reducing the audio signal level down to zero. The result can range from a hard and sudden cutoff to a soft and gradual reduction of signals below the threshold, depending on the desired ratio. |
Envelope tracking is done externally using an external envelope follower to make it possible to use different types of envelope tracking schemes, the output of which is the supplied argument to the function call operator.
The expander is an envelope processor that operates on the envelope of the audio signal, performs computations in the logarithmic domain, and returns a processed envelope.
| See Dynamic for further details. |
expander is a downward expander. This type of expander gradually decreases the level of the audio signal below the threshold.
Declaration
struct expander
{
expander(decibel threshold, float ratio);
decibel operator()(decibel env) const;
void threshold(decibel val);
void ratio(float ratio);
decibel threshold() const;
float ratio() const;
};Expressions
Notation
exp,a,b-
Objects of type
expander t,env-
Objects of type
decibel r-
Floating point value for compression ratio.
Constructors and Assignment
| Expression | Semantics |
|---|---|
|
Construct a |
|
Copy construct from |
|
Assign |
| C++ brace initialization may also be used. |
The ratio parameter, r, specifies the amount of gain applied. With the typical "1:n" notation for expanders, the ratio parameter is n, thereby the ratio for expanders is normally from 0.0…inf. (e.g. 1:4 expansion is 4). A ratio of 1:inf is a hard gate where no signal passes below the threshold. For every dB below the threshold, the signal is attenuated by n dB. For example, with a ratio of 4:1 (4), 1dB below the threshold is attenuated by 4dB.
Function Call
| Expression | Semantics | Return Type |
|---|---|---|
|
Process the input envelope |
|
The output is the expanded gain, also in decibels. Simply multiply the signal by the result converted to float using as_float (or double using as_double). For example:
auto gain = as_float(exp(env)); (1)
auto left_out = left_signal * gain; (2)
auto right_out = right_signal * gain; (3)| 1 | env is the computed envelope (e.g.) using an envelope follower.
gain is obtained from the env processed by exp and converted to float. |
| 2 | Stereo left_signal multiplied by gain. |
| 3 | Stereo right_signal multiplied by gain. |