Correct 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 self-generated by the internal electronics or induced into the signal path by external sources. Unnecessarily low gain settings can result in signal levels that are significantly closer to the noise floor, potentially causing a sound system to seem noisy. Conversely, excessively high gain settings may cause the audio signal to overdrive the electronics, resulting in severe distortion due to clipping of the audio waveform. In addition to being audibly undesirable, a distorted waveform may 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 that rely on signal strength as part of their normal operation may not perform as expected if they receive signals that are lower, higher, or simply different from what is anticipated. Examples of such components within Vocia are: Duckers, Comp/Limiters, and the Ambient Noise Controller 1 (ANC-1).
Comp/Limiter and Ducker functions are triggered by input signals that exceed a specified threshold. For Comp/Limiters, signal levels below threshold are not considered unusual (they simply are not affected by the processing). However, Duckers will not automatically attenuate the program signal if the sensing input signal is below threshold. Furthermore, signals with a large amount of background noise can falsely trigger these components if the level is set too high or the threshold too low. Please note that real-time control of signal levels should not occur before these types of components.
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. Please note that real-time control of levels should not occur after this type of component.
Generally, correct gain structure is achieved by establishing and maintaining appropriate signal strength throughout the entire sound system. In most cases, this means that the relative volume of loudspeakers should be determined by adjusting the power amplifiers (after 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.
The primary element of correct gain structure is input gain. Each VI-6 input provides adjustable Gain In (trim) in the Vocia software, with an associated status LED indicator on the front of the device per channel (see Device Dialogs, VI-6). Signal meters are also provided in the software. For best performance, the user can increase gain on a given input until the status indicator flashes amber on normal signal content:
First, the status indicator will illuminate in green with 48 dB of headroom remaining (before clipping occurs).
Next, the indicator will change from green to amber (the desired level) when the signal reaches -18 dB below clip.
Gain should be set to achieve a level that regularly illuminates the status indicator in amber. Indicators on the VI-6 and meters in the Vocia software can be used to monitor system levels. The Vocia meters should register signal levels around the 0 dB mark. With gain settings as described above, the inputs will be capable of providing around 18 dB of headroom. This will provide good signal-to-noise performance with a safety margin of available headroom.
The steps for achieving correct gain structure described above can also be used when configuring a paging station. With the meters and gain settings within the Vocia software, the user can set the level around the 0 dB mark while the paging station is being used according to its recommended operational method (see Hardware, WS-4, WS-10, DS-4, DS-10). Please note that even though system headroom in Vocia is 18 dB, default compression is applied within the paging station DSP to deliver effective input headroom of 30 dB. The default compression curve on paging stations may be removed to deliver an uncompressed output; however, in this case the compressor output trim should be set to -12 dB (in preference to lowering input gain). Reducing the output trim will lower input headroom to 18 db, commensurate with all other Devices.
Vocia VO-4 and VO-4e Devices may be adjusted per output to deliver one of three standard levels: +4 dBu, 0 dBu, or -10 dBu. For correct gain structure, the user should choose an output level that most closely matches the requirements of the input that the VO-4 output is driving.
Throughout the Vocia software, various audio components include level adjustment capabilities (faders). These faders default to 0 dB (unity gain), an appropriate setting for most applications. However, any of these faders can be used for real-time level control (as described above). Faders can also be used to alter the relative level of sources (e.g., microphones at 0 dB and music at -10 dB), and they can also be used to compensate for gain reduction (Comp/Limiting), which may have occurred earlier in the signal path.
Please note: The Digital Signal Processors used in Vocia Devices allow this type of gain staging without the danger of clipping distortion or the loss of data bits. If signals do not exceed the maximum level at the inputs or outputs (A/D & D/A converters), high- and low-level signals can be tolerated without negative impact on quality. However, some system components do not function well without proper signal strength (as described previously).