Other Parts Discussed in Thread: CC8530,
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Hello Steve,
Thanks for telling me about register 107. I have looked at all my configuration files and I do not program register 107. I will try this out and report the results to you.
Regards,
Andy
Spending time going over the AIC34 data sheet I now have a better idea of what it is capable of and how it is internally
connected. I have 4 slave microphones feeding 1 master. The 4 outputs from the master feed a mixer to give me a
single signal with all 4 microphones mixed. All 4 master outputs have 200mV p-p noise when the microphones are
active. This must be removed before mixing. I have been trying to remove the noise using the effects filters
configured as 20kHz low pass filters. I did program register 107, as you suggested, to say the default coefficients are
not being used. If a microphone channel is inactive, the DAC output for that channel is silent. When in playback
mode and the DACs are enabled, the effects filters are connected before the DACs. I think the noise I am seeing is
switching noise generated by the DAC. I need the low pass filter AFTER the DAC. Taking the 4 analogue outputs from
the DACs and passing them through a simple single pole R/C network (1k ohm/ 10nF 10kHz 3dB) the noise
disappears leaving a clean audio signal.
As I stated earlier the master output noise was only present on active microphones. This led me to consider the
noise could be originating in the slaves. This noise is very high frequency (1MHz+) and with a sample rate of 48kHz
should not be capable of transmission via RF. Even so, I added a 20kHz low pass filter on the slaves with the DACs
powered down in the record only mode. I wouldn't say it cured the problem but the noise from each of the Master's 4
analogue channels reduced from 200mV p-p to 150mV p-p.
The board is made on a 4 layer PCB with a dedicated ground plane, 2 signal planes and a shared power plane. There
are 3 DC-DC converters for 3.3V 1.8V and 1.2V. I would prefer to have no noise at all but with so much switching on
such a small board I think 4 extra resistors and capacitors is a small price to pay to give me good results.