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What is the strange noise heard from the loudspeaker when in EFR mode?

When EFR was introduced the concern for testing mobiles was that they would not be able to handle the extra processing power required to support EFR. The 8922 tests this simply by setting the mobile to EFR mode; audio testing is not required, although the uplink data is looped back down to the mobile as a means of checking this.

The 8922 does not have an EFR speech CODEC and so continues to work as if on an ordinary speech call. Consequently, when the uplink data arrives, the 8922 tries to decode the speech as normal to send audio signals to the loudspeaker and front panel connectors. The different encoding of the EFR CODEC means this sounds like the PRBS chirping noise.

Since the uplink data is also looped back to the mobile before decoding you are able to speak into the mobile's microphone and hear the same signal from the mobile's loudspeaker.