So the microphone signal line isn't broken by the mute, got it.
Amazon Echo Flex: Microphone mute button appears to be real and functional
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Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#12The parallel flops are probably hardening against bit flips from ESD events. There aren't any other active protection devices and phantom flips probably showed up at some point during testing of earlier prototypes.
Maybe because they had a second flip-flop and CMOS is sensitive to floating inputs? So they just decided to use both in parallel? Just a guess.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#13So the microphone signal line isn't broken by the mute, got it.
No-- even better, the MEMS microphone is depowered.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#14The implication is that the microphone emits completely no signal without power and doesn't have any capacitor to last even a few seconds. Surely the most valuable data is just after the mute is turned on?
There's no massive electrolytic capacitor that would be needed to power it for a few seconds.
It's gonna be pretty damn fast. Standard ceramic decoupling cap-- very likely 0.1uF-- right next to it. 1/2 C V^2 at 3.3V is 0.5 microjoules. If the entire energy in the capacitor can go to effectively power the microphone (it can't), and the typical power consumption of a MEMS microphone is about 1 milliwatt, it might keep working for about 500 microseconds.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#15Disclosure: I'm an Amazon Devices PE, but not on Alexa devices.
Jeff (and others) have spoken publicly about this in the past, and this teardown is correct. The hardware is designed such that, if the privacy LED is on, the microphone is unpowered. In order to compromise that, one would need to physically alter/damage the hardware.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#16Colour me impressed! I would have expected a digital pin be used to control the microphone in software, which I wouldn't have been nearly as satisfied to see. Really nicely presented breakdown too.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#17Tldr; it is real.
Thanks. Annoying that clickbait titles are allowed here.
The writers don't write for our amusement here and according to our criteria on HN, they writes as it pleases them. They went to the length to do a useful breakdown of a circuit, and write about it.
They don't owe us some nice title, or some convenient summary so we don't read their post...
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#18Very nice writeup, I love the combination of fun writing style and hand drawn schematics with things quickly escalating to decapping chips. Also nice to see that the led is a real one, even if the micro does have the ability to override the mute switch... I guess the 80's scifi movies with red glowing eyes to indicate the 'evil' subroutene was activated got it pretty close!
Here, the red glowing eye turns off when the evil subroutine is activated.
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#19Colour me impressed! I would have expected a digital pin be used to control the microphone in software, which I wouldn't have been nearly as satisfied to see. Really nicely presented breakdown too.
> Colour me impressed! I would have expected a digital pin be used to control the microphone in software
Yup, and even if the same pin drives both the LED and the microphone power switch, there's various attacks (e.g. high frequency PWM 75% duty cycle-- enough to light the LED to nearly full brightness, but also probably enough to keep the mic bypass capacitor nearly fully charged).
Since the SOC can only turn off mute, and not turn it back on, any attempt to eavesdrop would leave evidence (a mute LED left off).
Re: Amazon Echo Flex: Microphone mute button appears to be real and functional
#20Conclusion:
> But, basically if one presses the switch it causes the flip-flops to change state and to then either add or remove power from the microphones.
> The 'mute' button appears to be very real and functional. When the button glows 'red' the power is removed from the microphones.