This article describes establishing and maintaining good signal strength throughout the entire sound system.

Proper gain structure

Why is gain structure important? Proper gain structure is important because it affects signal-to-noise performance and available headroom within a sound system. Every sound system has some inherent noise, whether it be self-generated by the internal electronics or induced into the signal path by external sources. Therefore, unnecessarily low gain settings can result in signal levels which are significantly closer to the noise floor, potentially causing a sound system to appear noisy. Conversely, excessive gain settings may cause the audio signal to overdrive the electronics, resulting in severe distortion due to clipping of the audio waveform. Besides being audibly undesirable, a distorted waveform can also cause damage to some system components, such as loudspeakers. In addition to its influence on signal-to-noise and available headroom, gain structure can affect other aspects of sound system behavior. In particular, some audio components rely on signal strength as part of their normal operation. These components may not perform as expected if they receive signals that are lower, or higher, or even just different from what is anticipated. Examples of such components are: Auto Mixers, Duckers, Levelers, Comp/Limiters, Ambient Noise Compensators (ANC), and Acoustic Echo Cancellers (AEC). Auto Mixer, Ducker, Leveler, and Comp / Limiter functions are triggered by input signals that exceed a specified threshold. With Levelers and Comp/Limiters, signal levels below threshold are not considered unusual (they simply are not affected by the component). However, Auto Mixers will not pass signals that are below threshold, and Duckers will not automatically attenuate program signal if the sensing input signal is below threshold. Furthermore, signals containing a large amount of background noise can falsely trigger these components, if the level is set too high and/or the threshold is set too low. It should also be noted that any real-time control of signal levels should not occur before these types of components. For example, control of individual Auto Mixer channels should not take place ahead of the Auto Mixer. Instead, the Auto Mixer Input Level controls (which are post-threshold) may be used for this purpose. Ambient Noise Compensation (ANC) relies on a continuous and accurate model of the program signal level, to differentiate it from changes in the ambient noise level. So, real-time control of levels should not occur after this type of component. Acoustic Echo Cancellation (AEC) relies on a continuous and accurate model of the signal to be cancelled from the microphones. So, any real-time control of signal levels at the loudspeaker output should be duplicated for the AEC Reference. If these signals are different, a 2-channel ‘ganged’ Level control may be used.

What is proper gain structure? Generally speaking, proper gain structure refers to establishing and maintaining good signal strength throughout the entire sound system. In most cases, this means that the relative volume of loudspeakers should ultimately be determined by adjustment of the power amplifiers after all prior system gain settings have been established. Other system outputs (such as recording feeds) may require lower levels, which should be established by selecting an appropriate reference level at the output itself. Other than real-time level control (as described previously), signal attenuation within the system should be avoided.

How do I set proper input gain? To establish proper gain structure, the primary element and first concern is input gain. Each system input provides adjustable Gain In or trim level, with an associated Peak indicator. For best performance, increase the gain on a given input until the Peak indicator just begins to flash on normal signal content. The Peak indicator first comes on with 6dB of headroom remaining (before clipping occurs). To provide additional headroom (that is, allowing for occasional louder input signals), it is recommended that the gain then be reduced by 12dB (two 6dB decrements).

Using meters To monitor system levels, Peak Meters should be connected at strategic points in the signal path, including at the inputs and outputs. With gain settings as described above, input meters should indicate peak levels between 6dB and 12dB on normal signal content. This will provide a nominal level of approximately 0dBu, with good s ignal-to-noise performance and a safety margin of 12dB to 18dB of available headroom . You can click "Hold Enable" and adjust the Hold Time on a meter to make it easier to see the average levels that are being hit. A Hold Enable time of 1000 ms (1 second) is useful on Peak Meters. "Indefinite Hold" averages the level over an indefinite (as long as it is running) period. It is not an infinite Peak Hold, it does not store the maximum value ever seen. Peak Meters vs RMS Meters Peak Level is a moving target, sometimes high, sometimes low, representing the instantaneous value of the audio level. A Peak Meter displays the instantaneous level of the signal as fast as it can. This is useful for metering highly transient signals, or for gauging the highest level of an audio signal regardless of how brief the peak is. Peak meters are important in digital systems since they allow you to see if a signal is approaching full-scale in the system. An RMS meter has a slower response and displays a level that is averaged program level over time (300ms window). The result is that short-duration peak signals will not register as much on an RMS meter since there are corresponding lower levels aggregated in the averaging period, however the response of an RMS meter is generally considered to be closer to the response of the human ear. RMS stands for Root Mean Square and it is representative of the average energy of the material. Peak indicates instantaneous spikes typically lasting only a fraction of a second. The difference between the RMS and peak levels indicates the crest factor of the material, the ratio of how far the peaks exceed the average. Note that sending the same signal to a Peak Meter and an RMS Meter will not result in an identical decibel reading on both meters. For instance, sending Pink Noise at 0dBu to a Peak Meter with Indefinite Hold will show peaks of up to +15 or higher (the crest factor of the pink noise) while the RMS meters will show values closer to 0dBu for the same time interval. For a sine wave the crest factor is +3dB, this can be seen in the following capture.

Adjusting faders Many control components include internal level adjustment capabilities via level control (aka fader control). By default, these faders are at 0dB (unity gain) - meaning that the level passing through has neither been turned up or down. This is a very good setting for most applications, and does not necessarily need to be changed. Faders can be used for real-time level control. Faders can be used to ‘mix’ multiple signals at differing levels to create a more comfortable listening experience. Faders can also be used to compensate for gain reduction caused by other control components. This is called "make-up gain" and is frequently used to compensate for attenuation caused by Leveling, Comp/Limiting, etc. earlier in the signal path. Floating Point DSP's allow fairly extreme gain staging without the danger of clipping distortion or the loss of data bits. As long as signal levels do not exceed the digital clipping point (0dB Full Scale) at the inputs or outputs of a discrete floating point DSP chip extremely high and low level signals can be tolerated without negative impact on audio quality. This DSP chip "edge" includes passing over AVB, CobraNet, NexLink, or Dante links or going through D/A converters. However, remember that some threshold-dependent system components do not function well without proper signal strength (as described previously).