High quality limiters are an essential tool for the production sound mixer’s tool kit. As the number of sound sources increases it becomes increasingly difficult to anticipate sudden and excessive changes in the volume levels of multiple sound sources. To prevent input overload, the 788T, like all Sound Devices field production mixers and recorders, has input limiters.
When active, the 788T's limiters virtually eliminate the chance of digital signal overload. The 788T’s limiters have been enhanced with firmware revision 1.76. The ability to adjust limiter characteristics provides the sound mixer with advanced control more typical in studio gear than that found in location sound recorders. Key features of the 788T’s limiter system are:
- Two independent limiters per input channel - an analog pre-fader circuit and digital post-fader control
- Linkable limiters on L/R and Aux 1/2 tracks
- Variable limiter threshold and recovery time
- Hard knee and soft knee limiter curves
- 20:1 and INFINITY:1 limiter ratios (slopes)
Pre and Post-Fader Limiting
Each of the 788T’s eight inputs has a pre-fader limiter and post-fader limiter. The analog pre-fader limiter acts at the trim gain but before analog-to-digital conversion. This provides overload protection to any input recorded as an independent pre-fader ISO track. The post-fader limiters on the 788T act upon the signal after the mixer fader, preventing the elevated level from exceeding the limiter threshold. Pre- and post-fader limiters are activated by toggling the LIM soft switch in the input’s Input Settings Window.
Mix Track Limiting
When multiple inputs are mixed to a track, the summing of signals can increase the track level, causing a track level to approach digital overload (0 dBFS). To prevent overload, limiters are available on tracks L, R, X1 and X2. Track limiters can be set to act individually (for mono and dual-mono mixes) or can be link in pairs on L/R or X1/X2 to preserve imaging in stereo mix recordings. The track limiters are selected in the LEVELS screen in the Track Setup Menu accessed by pressing the INPUT button.
The main purpose of the 788T limiters is to prevent analog signals from overloading the analog-to-digital converter. They are not compressors. The limiters should be set so that only very high peak levels approaching 0 dBFS are limited. Setting the limiters to activate at too low a level reduces the huge dynamic range that the 788T offers. It is also good practice to use the 788T’s low-cut filters to prevent unwanted low–frequencies, such as wind noise and mechanical vibration, from triggering limiter action. Several parameters of the limiters can be controlled from the Setup Menu. Controls including limiter threshold, recovery time, ratio and knee-type are user-selected settings. Settings are global, applying to all limiters on the unit. The limiter threshold, the signal level where the limiter activates, can be set from -12 to -2 dBFS. The corner, or knee (the shape of the limiter curve), can be set to either soft knee or hard knee. When set to soft knee, limiter action is more gradual and subtle but starts at a lower level, about 6 dB below the threshold.When using soft knee, set the threshold higher. Hard knee limiters activate precisely at the threshold setting. Recovery time, or release time, is the time it takes for limiter gain reduction to recover. On the 788T it can be set from 50 to 2000 ms. For dialogue a value between 50 and 500 ms is recommended. There are two limiter ratio options, infinity:1 and 20:1. The ratio determines the slope of the limiter curve above the threshold. A 20:1 ratio means that for every 20 dB increase in level above the threshold, the limiter will restrict the level to a 1 dB increase. A 20:1 ratio offers a slightly less ‘squashed’ sound compared to an infinity:1 ratio, but very high peaks may briefly exceed the limiter threshold. Set the limiter threshold level a couple of decibels less when using the 20:1 ratio.
As a starting point, try the following gain and limiter settings for dialog recording:
- Set an inputs high-pass filter between 80 Hz and 150 Hz.
- Set input trims so that normal dialog levels peak at -12 dBFS. This provides a good amount of headroom without compromising noise performance.
- Set limiter threshold to -4 dBFS.
- Set recovery time to 200 ms.
- Set limiter knee to hard-knee with 20:1 slope.
Input Limiter Comparison of Sound Devices Products
The chart below shows the limiter specifications for each Sound Devices product equipped with an Input Limiter.
|variable -12 to -2 dBFS
|infinite:1 or 20:1
|variable 50 to 2000 ms
|hard or soft menu-selectable
|variable +6 to +18 dBu
How do I make the 788T limiter sound like the limiter on the 702/702T/722/744T?
The 702, 702T, 722, 744T, and 788T all incorporate analog limiters but the control and characteristics of the 788T limiters are slightly different. The 702, 702T, 722, and 744T have a fixed -4 dBFS threshold, 5 ms attack time, 20:1 ratio, and 200 ms release time. The 788T limiters have a variable threshold, release, and knee, and a fixed 2 ms attack time and an Infinite:1 ratio. To make the 788T Limiter sound similar to the 702/702T/722/744T Limiter, follow these steps.
- Set the Setup Menu option INPUT: LIMITER THRESHOLD to -4 dBFS.
- Set the Setup Menu option INPUT:LIMITER RECOVERY to 200 ms.
- Set the Setup Menu option INPUT: LIMITER KNEE to Hard-Knee with 20:1 ratio.
- Set the trim levels so that the normal level is about -20 and peaks around -12 dBFS.