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QC45 Echo Issue

Posted by Brandon Lackey on

We have received reports of customers hearing an "echo" in their sidetone audio (hearing your own voice in the headset).  To date, the issue seems to affect about 10% of our customers with the QC45.

After extensive testing and questioning of those who have experienced the issue, we have determined the issue is being caused by the Active Noise Reduction (ANR) of the QC45 conflicting with the sidetone provided by the aircraft.

Unfortunately, because the issue is being caused by the headphones, there is nothing we can do to address it.  However, the only thing affected is the quality of your own voice in your own headset.  You will sound normal to everyone else and all other incoming audio will sound normal to you.

Most people do notice any issue.  If it really bothers you, there is something that you as a BOSE customer can do.  You can call BOSE and complain!  Tell them they need to fix whatever change they made between the QC35 and QC45 that broke sidetone coming from an external source.

The truth is that the aviation application that we provide is not an insignificant portion of their market.  While they prefer you buy their $1k aviation headset, they also know that a lot customers buy BOSE headphones for the sole purpose of using it with our product.  It would be in their interest to fix it!

 

DIAGNOSIS

I was able to replicate the issue with our test equipment.
  1. By covering the external microphones on the headphones (the ones the headphones use for ANR), the issue cleared up.  Speaking softly also helped.  These both indicate the ANR being at play.
  2. Testing the microphone output separately from the QC45 sounded perfect.  So, it wasn't our microphone output.
  3. Plugging the QC45 directly into the panel (using the BOSE supplied cord), the issue persisted.  So, it wasn't our incoming audio circuit.
  4. Several different QC45s were tested and the issue occurred exactly the same on all.  We also tested the headphone of a customer who was experiencing a major delay and it sounded the same as our test headphones.  So, it wasn't a defect in some headphones.
  5. Tested all settings for the headphones with the Bose app and none seemed to make a change.  The only thing that helps is turning the ANR off.
  6. Our circuitry and components did not change, the only difference between the QC45 version and other HARMONY products is the shape of the housing.  If the issue was there, it would have appeared in all our other products.

Through process of elimination, the issue has to be within the headphones themselves.  I can find no other explanation.

 

DETAILED EXPLANATION

Sidetone and Delay:

The sidetone you hear in the aircraft is provided by the radio or intercom, the equipment feeds your transmission audio back into your headphones.  Due to electronic latency, the sound you hear can have a delay.

This infographic from the FAA details the potential delay envelope:

 

The FAA stipulates a maximum of 2.5mS delay for headsets.  Our headset has a 0.74mS delay, well within the limit.  However, the performance of the rest of the radio equipment will add to the delay time.

Active Noise Reduction:

ANR fundamentally works by reproducing sound through the headphones speakers that is 180 degrees out of phase with what is picked up by external microphones.  This "cancels" the amplitude of the sound wave resulting in no audible sound.  To put it a different way, if two cars (the sound waves) are moving perfectly towards each other (180 degrees out of phase) at exactly the same speed (amplitude) and collide, they will theoretically stop at the point of impact.  That is ANR.

Since ANR is reproducing audio through a circuit, it will have a delay.  The amount of delay how much is cancelled will depend on the technology and how aggressive the ANR is.  These are both determined solely by the headphone.

 

The problem:

The ANR is trying to cancel your voice, because it hears it in the environment, with whatever delay is inherent in the headphones.  The headphones are also reproducing the sound of your voice as sidetone at the same time.  If there is a difference in delay or amplitude, it will affect how it is cancelled, and therefore affect the quality (like the two cars now meeting at different speeds or angles, it won't be a clean stop).

I cannot say specifically if it is the technology behind the ANR that is causing the issue, or if there is some kind of delay in audio coming in through the auxiliary jack.  Either way, the outcome is the same.


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