tags: dynamic-range-processing mus-407 compression expansion
Transfer Function
A transfer function converts a [signal] input to an output, usually applying some change. In [DRP], the signals are measured in [dB], and the shape of the transfer function determines the effect.
Example with downward [compression]:
- Standard amplifier: no compression, linear response
- Mild compression, peaks above -6 dBFS are slightly attenuated
- More severe compression, peaks above -6 dBFS are more strongly attenuated
Most DRP transfer functions only display one quadrant to avoid redundancy.
Typically, 0 dB input/output in upper right, axes labeled linearly with respect to dB.
Ratio
The severity of signal processing is determined by a ratio.
- Represents change in input signal (dB) compared to change in output signal (dB)
Example: [compression] ratio of 4:1
- above threshold, if input signal increases by 4 dB, output signal is attenuated so that the increase is only 1 dB
- ratios of 8:1 and above are generally considered "extreme" compression
- can result in a perceptual distortion of [transient]s, noticeable changes in [timbre], often described as a "squashed" sound
Knee
When signal level crosses a threshold, DRP ratio may be applied fully or gradually, depending on knee curve
Hard/soft knee refers to the smoothness of the transfer function graph across the threshold
- a hard knee has two straight lines, above and below threshold
- a soft knee curves gently between affected and unaffected [amplitude] regions
- differences between hard/soft knee tend to be very subtle, almost indistinguishable
Drawing Transfer Functions
- Mark threshold (threshold is always on the horizontal axis)
- Determine type of effect
- Which effect? Which direction?
- Which part of the graph is affected? Which part is attenuated?
- upward [expander]: draw the transfer function at threshold, then apply makeup gain downwards
- Based on input parameters, draw graph
