We are trying to use the ADAU1761's MX3AUX and MX4AUX to control the level of the AUX audio that bypasses the DSP and we hear clicking when registers are changed. Is this the normal operation, or can we prevent the clicks?
The particular gain controls referred to in this case are analog PGAs that have a 3 dB per step gain control. They do not have slewing available between steps. This means that when the gain is changed, it will change instantaneously. Of course, when the gain applied to an audio waveform changes instantly, and that waveform is not at a zero crossing, there will be a disconnect in the waveform, meaning that the level of the audio "jumps" instantly. This causes high frequency noise to be output. Basically, these PGAs were meant to have a constant gain setting during operation of the device, for this reason.
There are other methods of changing the gain that will work-around this issue, depending on the particular audio path being used. For example, the RADVOL and LADVOL controls allow you to directly control the volume of the ADCs, and RDAVOL and LDAVOL controls allow you to directly control the volume of the DACs. The gain steps are much smaller (0.375 dB per step) and there is slewing enabled for gain transitions, meaning that there will be no audible artifacts when the level is changed.
Fine-step gain controls are also available for the individual output paths, with 1 dB per step. As long as the volume is changed in small increments, audible artifacts can be mostly avoided.
Is there really any Slew on the R31: Playback Line Output Left Volume Control, 16,421 (0x4025)
I have Default Slew analog enabled but I am hearing clicks when adjusting this control
The Datasheet says
The volume controls for these outputs range from −57 dB to +6 dB. Slew can be applied to all the playback volume controls using the ASLEW[1:0] bits in Register R34 (playback pop/click suppression register, Address 0x4028).
And On the digital Gain R20: Left Input Digital Volume, 16,410 (0x401A)
Is this gain after the DAC ? If I adjust it down will it attenuate the residual noise of the DAC?
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