Alter audio-attenuating resistors around the Bluetooth module (R32 and R33)

[chipaudette]

See this post in the forum: https://forum.tympan.org/t/issues-with-external-mic-in-line-in-1/299

This person is correct that the 1K-to-ground resistors (R32 and R33) are too small to support a line input.

Solution Idea 1 (FAIL): Ideally, for a line-level input, the resistors to ground would be at least 10K. Is that acceptable for these resistors? What is their function? Their function is to form a voltage divider to reduce the signal strength from the Bluetooth module before it goes to the audio codec. Right now, we have a 10x divider. If we were to keep the 10x divider, we'd have to increase the series resistors (R30 and R31) to 100K. But these high values would interfere with the audio codec's ~20K input impedance and would knock down the signal too much.

Solution Idea 2 (SUCCEED?): Now that we have lived with the system, I do not believe that we need a 10x divider. I think that, worst case, the BT module puts out 0->3.3V whereas the audio codec can only take 0->1.8V. Therefore, we only need a ~2x voltage divider. Therefore, we can change R32 and R33 to 10K and leave R30 and R31 at their current value of 10K.

@biomurph, if we do a revision to RevD, can we change these two resistors to the higher value?

[biomurph]

Interesting idea @chipaudette
Is it possible for you to test this change? Or is it pretty obvious?

If it's obvious, and a high level of confidence that it will work, I can change the value of the R32 and R33, but it would have to get confirmed very soon as we are in production for the Rev E, which uses this same topology.

[chipaudette]

It is high confidence. If it can be done on RevE, that would be great. If not, we'll live.

[biomurph]

OK, I have a request in with my CM. I don't think it's too late to make the change.
Will update here and in the schematic if it's possible.