I'm using an FSA9485 Multimedia Switch (MMS) for audio recording via USB headset. So far, just noise is recorded...so I'm wondering if the MICBIAS is working correctly.

The MMS schematic shows MICBIAS applied on the codec side of the MMS, which is how we've implemented it:

[MMS datasheet]

The headset MIC pin is connected to VBUS_IN of the micro USB connector. I've measured voltage at VBUS_IN, and don't see any amount of bias voltage. At the codec, MIC/MICBIAS is roughly 1.2-1.4V.

Will an "ear bud" style microphone work with no bias? I presume the bias voltage should be present at the connector so that it can properly bias the headset mic.

Link to headset

EDIT: Turns out I wasn't contacting the micro USB VBUS or GND pin properly with my multimeter and the 1.4V bias is present at the connector. It's working properly for me with the bias.


1 Answer 1


Analog headsets (typically using TRRS plugs, or two TRS plugs, or abusing a micro-USB connector like your headset) do need a bias voltage, so not getting any recorded signal might very well be caused by missing bias. I can not find a data sheet for your USB multiplexer that deserves that name, just a product brief by Fairchild called "data sheet". The block diagram hints that the chip is likely able to pass the voltage from the MIC pin to the USB-plug headset at VBUS, but it does not say whether it needs any specific initialization for that. If you use that chip, you should have a more detailed data sheet that explains how a USB headset with microphone on VBUS is autodetected (if at all), and what you need to enable that. Furthermore, if you don't see a bias voltage at VUSB, I am confident that you also won't get sound input from VUSB back to the MIC pin.

  • \$\begingroup\$ Thanks. The data sheet I have (although 40 pages) is somewhat limited for this; not much info on bias other than the reference schematic above. Appreciate the guidance regarding bias, I'm new to audio \$\endgroup\$ Commented Sep 26, 2016 at 18:07
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    \$\begingroup\$ I don't think you need guidance about bias. If the FSA chip were in headset mode, it would pass the bias voltage from the MIC pin to VBUS_IN. You need guidance on how to get the chip into headset mode. \$\endgroup\$ Commented Sep 26, 2016 at 21:44

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