Figure 63 (a) bass; (b) treble
‘Tilt’ control, in which a single knob varies the relative levels of bass and treble (see Figures 65 and 66).
Figure 65 Circuit for ‘tilt’ control
Figure 66 ‘Tilt’ control frequency response
Compressors
Definitions:
1. A compressor is a device which reduces the dynamic range of an audio signal by a controllable amount with no significant waveform distortion.
2. Threshold. The output level of a compressor is the same as the input level up to the threshold.
3. Compression ratio. This is defined as the ratio:
4. Limiting. A very high (ideally infinite) compression ratio. In practice compression ratios higher than about 20:1 are regarded as limiting. On some units the threshold is raised by a fixed amount, typically 8 dB, when ‘limiting’ is selected.
Figure 67 illustrates the action of what is termed a ‘hard-knee’ device. Operators sometimes prefer a less abrupt onset of compression. ‘Soft-knee’ compression is shown in Figure 68.
Methods of Compressor Control
The control signal (‘side-chain’) may be used in a ‘backwards’ or a ‘feed-forwards’ mode – or possibly both, as illustrated in Figures 69 and 70.
Figure 69 (a)Feed-backwards operation; (b) Feed-forwards operation
Expanders
These increase the dynamic range. Figure 71 shows the basic block diagram and Figure 72 shows a typical input/output characteristic.
Figure 71 Basic diagram of an expander
Figure 72 Input/output characteristic of an expander
The expansion ratio is defined as
Noise Gates
This term is applied to expanders when the expansion ratio is high – 20:1 or more.