ADAU1701 Leaky Mute.
Hello,
I wish to use a GPIO controlled mute switch and have used the 'single slew ext volume' block to do so. This sort of works but allows a distorted (attenuated but audible) signal through when muted. Is there a attenuation figure for this block I can check my hardware against to see if it is working as expected?
The ADAU1701 is part of a larger item, with analog circuitry surrounding the ins and outs, on the DAC I have a fair amount of gain to bring up the 0.9 Vrms of the DAC to make use of the +/-15v full range analog output...I suspect this gain may be the problem, without which any leakage would be inaudible?
(On a slightly related note, any thoughts on if I am best off using a resistor value of around 100k for the ADC to make use of +/-15v rails or using a lower resistor value and attenuating my signal accordingly?)
Many thanks.
Audio Signal Processors
SigmaDSP Audio Processors
adau1701
HI Alex:
It is possible that the attenuation is not reaching 0 as it should. We can look into this if you send us the setting of your external slew volume control. In the mean time, there are ways to alleviate this problem.
1.- Slow down the slew rate to a higher value
2.- Connect 2 of these external volume slews in series triggered by the same GPIO and using a T connection to connect them to both external slew volumes connected in series.
Please let us know the results.
Miguel
Hello Alex,
Likely you're running a signal flow similar to this one:
I tried it on a stock Eval-ADAU1701Miniz board and could not duplicate your issue -- a logic Low at the MP2 pin completely muted the sound. This result makes sense because the DSP volume control, unlike a voltage-controlled amplifier or similar form of analog volume control, works perfectly. It multiplies its control input times its audio input, and zero times anything is zero. By contrast the -1701's ADCs and DACs are not perfect, but with THD+N better than -83 dB, and S/N of 100 dB. You can compare your measurements with these specs.
Here's a suggested test for your application: Make a copy of your SigmaStudio project and modify the copy as shown below:
Obviously, no input sound should reach the output in this program. Load the modified project and listen. If you hear the same weak, distorted sound as before, then its cause is not the volume controls letting audio through, but rather, another cause within the hardware. Possibly a ground loop.
Output gain in itself is not a problem (and would be present in any practical system driving speakers). The ADAU1701's inputs are fine with higher-value resistors to scale larger inputs. Where I work we protect the ADC inputs with back-biased diodes to the 3.3 V and ground rails, I've also seen this recommended on the forum.
Best regards,
Bob
Thanks to both.
Hadn't thought of Bob's method of testing my hardware, turns out it is a hardware issue! Now the fun begins...
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