Muting DSP Outputs using GPIO input
Hello,
We have recently developed an amplifier with the ADAU1701 DSP, and one of our requirements is that once you turn it off, the output must MUTE completely extremely fast, under 100 uS (micro seconds). One idea I had is to operate a "mute switch" in the DSP program by an external DSP pin (one of the MP0 to MP11 pins would do). I have a few questions that I'm hoping someone could help:
1 - Would that be fast enough, under 100uS?
2 - How can I flip the switch in RED by using a GPIO input? It would deviate from "audio" to "zero dc level", thus muting.
3 - It doesn't have to necessarily be a "switch", it could be anything that would result in a mute, such as a gain block from 1x to 0x. Any ideas?
Thank you very much!
Best wishes,
Fernando P.
Audio Signal Processors
SigmaDSP Audio Processors
adau1701
Hello Fernando,
I haven't seen you here in a while, welcome back! Possibly the largest contribution to your switch-OFF time is the latency of the digital reconstruction filters built into the ADAU1701's DACs. These oversampling filters "shape the DAC's output noise" to a higher frequency range, allowing use of a simple one or two-pole, 50 KHz filter at the DAC output (otherwise you would need a 20 KHz multistage brick-wall analog filter similar to that found in the very first CD players). After a sudden blanking of their inputs, output signals may remain as they clear these filters during a time of about 0.5 millisecond at a 48 KHz sample rate. This time is significantly better at 96 KHz for two reasons: The filters need do less work on faster samples, thus needing fewer of them for the job -- and besides, those samples come along twice as fast. My tests show an overall (ADC + DAC) latency of 0.32 mS at 96 KHz -- if half of this is DAC latency, you'll still have somewhat over 100 uS for the DAC alone at 96 KHz.
I have not measured the speed of the ADAU1701's digital GPIO inputs. You may need to choose the no-debounce option to get the fastest input GPIO response. As you likely already know, you'll need a Buffer Gate set to 28.0 to convert the GP input's 5.23 format to the integer format the index mux needs. The program execution through the Buffer Gate and the mux switching all happens in one sample interval.
Best regards,
Bob
Hey Bob, long time no see! How are you doing?
I guess you just saved me a significant amount of time and effort. So even having a mute switch on Sigma Studio that is triggered immediately with no latency, I still wouldn't see the 100uS muting as the DACs would still be putting out some Z^-x samples! Only solution then is to put some muting J-Fets right after the DAC, doing it the old fashioned way.
By the way, I'm working for Kicker now, that's one of the reasons I disappeared for a while, all the change that took place...
Have a great day my friend!
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