In order to change the gain by a step amount to a signal in analogue circuitry without causing un-wanted artefacts, the point at which the gain change occurs needs to be when the input signal is at (or very near) zero voltage, otherwise a step occurs in the signal and artefacts are heard (usually a click). This isn't an issue when using a traditional analogue gain control (pot) because the transition is smooth rather than a step change. Consequently, any piece of circuitry that needs to apply step gain changes (1dB in the case of Liquid 4Pre) to a signal must be able to detect when the signal is at (or close to) 0. This part of circuitry is called a “zero-crossing detector”. In practice, because of laws of physics, zero-crossing detectors work by having a reference voltage (positive and negative) which is used to determine if the signal is within that range and, therefore, considered to be “zero”'. They must also have a form of timeout function so that, if the zero band isn't reached within this time period, the gain is switched regardless
The integrated circuit used for gain switching in Liquid 4Pre has a built in zero-crossing detector as described above, whose performance we have extensively analysed. When testing the microphone input with a sine wave signal at a level of -60dBu (774.60μVRMS (0.000774.6V), very low for a decent microphone) the zero-crossing works as expected and no artefacts are experienced. However, when the input signal level is reduced down to - 70dBu (244.95μVRMS) and the gain is swept then artefacts are heard. This is because of the practical limits mentioned in the previous paragraph with the zero crossing detector thresholds. We have found that, in practice, the Liquid 4Pre zero-crossing threshold is around -68dBu. As this corresponds to a threshold of 436.1μV (0.000436.1V), this is actually very good. To put this into context, to achieve a maximum level input signal the unit gain set to 80dB (maximum) the signal must be -58dBu. Therefore, there is a 10dB margin over this level, meaning that any signal that is applied that can be fully amplified by the unit will be correctly switched without any artefacts. As an example, look at Figure 1 below. This shows a section of a unit gain of sweep of 27dB to 80dB and back down to 27dB gain with a -60dBu input signal. If you examine it closely with a wave editor you will see that every gain step occurs at the zero crossing point resulting in no distortion of the sine wave.
Of course, it is very common for an Liquid 4Pre user vary the gain up and down without an input connected. In this case, the user will hear gain switching artefacts but, whenever the input signal is above -68dBu (way below ambient pick-up with a decent microphone) then the zero-crossing starts working and you do not see any artefacts. Figure 2 below shows a section of a recording of ramping the gain with no input from 40 to 80dB and back down again. The peak level during this ramp is -33dBFS and occurs because of the “timeout”
feature of the zero-crossing detector mentioned above. However, in practical use, the only time the gain is changed is when the gain is being adjusted to get the input level correct. In this situation, the zero crossing will be working and there won't be any artefacts generated.