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In audio, summing refers to combining two or more signals together. This can be as simple as using a Y cable or as relatively complex as a multi-channel audio mixing board. In analog mixers, signals are usually summed together by sending them from their source – usually somewhere after the channel fader – to a common bus, which in turn feeds another gain stage. Consequently, the integrity of the bus, the devices sending signals to it, and the device it feeds are all an important part of the overall sound quality. It is assumed that signals should be summed in phase and with minimal distortion. In digital audio systems, summing is done mathematically. For each sample, the numeric values of the signals being combined are added together, and the resulting value represents the combined or summed signal. Prior to this, the individual signals undergo any necessary DSP processes that may be set up on their channels in the system. These could range from simple volume adjustments (analogous to fader position in analog systems) to complex compression, normalization, or reverberation routines.

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