The Limiter module controls the output level of the audio.<\/p>\n
The number of audio channels it can process is configurable in the SFD and each channel has its own set of specifications.<\/p>\n
The Limiter has only one operating mode and has one additional variable – Maximum LookAhead – that can be configured at design time. The maximum lookahead value ranges from 0 to 0.15 seconds. Size of the lookahead delay buffer (in float Words) = round(sampleRate * Max Lookahead)<\/p>\n
For a sample rate of 48000 and maximum lookahead of 0.15 s, the required buffer size is 7200 float Words. Hence this variable is made configurable to enable the user to choose optimal maximum lookahead value based on the available memory.<\/p>\n\n
Attack Phase<\/strong>:<\/p>\n The Limiter operates in two modes during the attack phase when the input signal level is higher than the desired threshold:<\/p>\n Hence if the threshold is less than the input signal by less than or equal to 10 dB, then within the given attack time the Limiter output is restricted to the threshold level \u00b1 1 dB.<\/p>\n If the threshold is much lower than the input signal level (say 30 dB), the difference is first brought down to less than 20 dB within the attack time; the difference is further reduced to less than 10 dB within twice the attack time; finally the output matches the threshold \u00b1 5% within thrice the attack time.<\/p>\n Release Phase<\/strong>:<\/p>\n The release time is the time taken by the Limiter output to follow the input signal level from the time the input signal level drops below the threshold. During this phase, the Limiter output follows exponential (time constant) growth to reach the input level.<\/p>\n Accordingly the output level will be as given below during this phase (If the release time is denoted as R, and the difference between the output level at the start of release phase and the input signal level is P dB):<\/p>\n After R sec\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 : 63% of P dB<\/p>\n After 2R sec\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 : 86% of P dB<\/p>\n After 3R sec\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 : 95% of P dB<\/p>\n The AO supports in-place computation based on the core type.<\/p>\n In the GTT, the limiter exposes the following parameters for each channel of the limiter:<\/p>\n 0 \u2013 Bypass Disable<\/p>\n 1 \u2013 Bypass Enable<\/td>\n <\/p>\n\n
Tuning Parameters:<\/h2>\n
\n\n
\n Parameter Name<\/strong><\/td>\n Format<\/strong><\/td>\n Default value<\/strong><\/td>\n Range<\/strong><\/td>\n Notes<\/strong><\/td>\n<\/tr>\n \n Gain<\/td>\n Float<\/td>\n 0 dB<\/td>\n 0 to 30 dB<\/td>\n Controls the upper bound of gain that the limiter applies on weak signals<\/td>\n<\/tr>\n \n Threshold<\/td>\n Float<\/td>\n 0 dB<\/td>\n -30 to 0 dB<\/td>\n Controls where the limiter begins to activate<\/td>\n<\/tr>\n \n Attack Time<\/td>\n Float<\/td>\n 1ms<\/td>\n 0.1 to 20 ms<\/td>\n Controls the limiter\u2019s attack time for each channel<\/td>\n<\/tr>\n \n Hold Time<\/td>\n Float<\/td>\n 10 ms<\/td>\n 0 to 10 ms<\/td>\n Controls the hold counter<\/td>\n<\/tr>\n \n Hold Threshold<\/td>\n Float<\/td>\n 0 dB<\/td>\n -1 to 0 dB<\/td>\n Controls where the limiter enters hold or release<\/td>\n<\/tr>\n \n Release Time<\/td>\n Float<\/td>\n 150 ms<\/td>\n 1 to 2000 ms<\/td>\n Controls the limiter\u2019s release\/hold time for each channel<\/td>\n<\/tr>\n \n Look Ahead Time<\/td>\n Float<\/td>\n 0.0015 s<\/td>\n 0 to Max Lookahead s<\/td>\n Look Ahead buffer for controlling the overshoots.<\/td>\n<\/tr>\n \n Bypass<\/td>\n Unsigned Long<\/td>\n 0<\/td>\n \n Flag to bypass the Limiter operation for the particular channel<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n