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7. Level issues

7.3 Gain compensation

In networked audio systems where multiple DSP processes and user interfaces (eg. mixing consoles) share the same inputs (eg. stageboxes), head amp control becomes an issue. When one user interface changes the analogue gain of a head amp to suit its related DSP process, the signal level to all other DSP processes changes as well. If the operators of the other processes are unaware of the analogue gain change, this can cause troubles - so in general, if multiple processes use the same inputs, the operators of the processes (in case of mixing consoles the sound engineers) have to clearly communicate with each other to adjust digital gains in the DSP process manually to compensate for the analogue gain change in the head amp. Two kinds of gain compensation schemes can be used to automate this process: console gain compensation and constant gain.

Console gain compensation
Console gain compensation can be used in systems with two mixing consoles that are capable of handling the same HA control protocol as the stagebox. In most cases, one console is dedicated as the HA master console, the other as HA slave console. The master console controls the head amp in the stage box through a HA control protocol that flows through the same network as the audio. If the stagebox receives a gain change command (or a gain change is made locally), it executes it and sends a HA status back to the master console to update its analog gain display, and to the slave console to compensate the gain change using the slave console’s digital gain. The advantage of console gain compensation is that it can be used with any digital audio format and network protocol. The disadvantage is that only stageboxes and mixing consoles that can handle the same HA control protocol can be used, and only mixers that have gain compensation implemented in the software.

Constant gain
Constant gain can be used if the stagebox offers dedicated DSP to compensate the analogue gain in the digital domain before sending the audio data to the network. The advantage of constant gain systems is that any DSP process (mixer) can be used in any quantity, as the compensation happens in the stagebox and not in the receiving DSP process. In a constant gain system, multiple operators can control the gain, provided they support the HA control protocol used in the stagebox. A disadvantage of constant gain is that it requires a distribution network with a higher bit rate than the audio signal to provide full resolution, eg. a 32-bit network to support constant gain for 24-bit signals.

Using a constant gain system requires a more strict understanding of the difference between digital and analogue gain functionality. In conventional digital mixing consoles, although a digital gain is always provided, the analogue gain is often used to align the analogue input level for optimal A/D conversion, as well as to set the incoming level to the suit the mixing process. In a constant gain system, the analogue gain can be used exclusively for optimization of A/D conversion (to optimise audio quality), leaving each console’s digital gain to set the incoming level. More details on constant gain implications on audio quality is presented in chapter 6.3: Constant gain A/D converters.

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