Best place for audio processing adjustments.

[Jorritv3]

Hello all,

I am working on a project with 2 nrf5340 Audio DK's, and I have been working for a while now on getting myself familiar with the audio application, but I have a few questions. First of all I assumed it would be wise to develop on top of the provided audio application since writing the entire pairing and data transmission yourself would be near impossible, is this a correct assumption?

Secondly I have the walkie talkie demo running, and am now trying to encode and decode the data that I am sending over the line, for now I just want a simple xor operation to encrypt the data and decrypt it. Right now I am doing it before the codec encode in the audio_system.c and decrypting it after the decode, but I just get a horrible noise instead of the orginal audio. I tried a few other spots like inside the callback, but I saw another thread saying this was ill advised.

So my question is what would be the correct places to place the encrypt and decrypt of the data? 

[Einar Thorsrud]

I see. But then there should not be any difference with encryption or not, as the data into the decoder will be indentical to what it would be if ti was not encrypted. Is there a difference or did I misunderstand something? If this fails with encryption but not without it, you need to look into your encryption/decryption, as ther eseems to be an issue there. Can you verify that the decrypted data is identical to the data before encryption?

[Jorritv3]

I think you are misunderstanding, sorry for my unclear explanation.

I am doing an encryption after encoding, this would mean the encoded data that gets send to the other device should be encrypted. At the other device I do the decryption before the decoding, so the original sound should come out. 

When I do both encryption and decryption, I Do hear the original audio.
But when I Only do the encryption (So no decryption at the other side), I hear nothing.

I expect to hear noise since my data received at the other device it garbage. 

My question is why all PCM data gets set to 0 when decoding.

So at the decoding side encrypted data comes in, but that device shouldn't know it is encrypted, so how why does it produce no sound at all, instead of garbled noise.

[Einar Thorsrud]

Aha, then I se. I do not think that is the case. The codec will see that the input data is gabage and use error concealment. And as there is no data of value anywhere, the best concealment is simply no signal/silence.

[Jorritv3]

Hi,

Thank you for your reply, sorry for my late response.
But how does the codec know it is garbage?

[Einar Thorsrud]

I have not looked into the details, but there are several ways it could detect it. As it is encrypted and would look like just random noise, I expect it detects that the frame is invalid and sets  BEC_detect flag (see page 70 in LC3 spec v1.0.1). Also, I recommend you have a look at Appendix B Packet Loss Concealment.