LE Audio Codec Sample Rate Configuration Issue with nRF5340-Based Hardware

[similingus]

I'm working with an nRF5340-based custom hardware setup that includes an audio codec and a microphone, and I'm testing LE Audio functionalities. My setup involves running the Audio Application Unicast server provided by Nordic (Ref: docs.nordicsemi.com/.../index.html) with a custom device tree and driver software for the additional hardware components.

I'm facing an issue with audio quality related to sample rate settings. Specifically, when configuring the AUDIO_SYSTEM_SAMPLE_RATE in /nrf5340_audio/src/audio/Kconfig to AUDIO_SAMPLE_RATE_48000_HZ, and the LC3 codec sample rate is settled with 16000 Hz or 24000 Hz, the audio transmitted from my device becomes severely distorted into loud noise on the receiving device, such as a phone. The noise is so loud that it sounds like it's hitting the maximum volume. Other combinations of LC3 sample rates and AUDIO_SYSTEM_SAMPLE_RATE do not produce this issue.

choice AUDIO_SYSTEM_SAMPLE_RATE
	prompt "System audio sample rate"
	default AUDIO_SAMPLE_RATE_48000_HZ
	help
	  This configuration reflects the system sample rate, but the audio data may be resampled to
	  another sample rate before encoding, and after decoding.

Why do the configurations cause such noise, and how can I avoid this problem?

I’m using NCS version 2.6.1.

Thank you for your assistance!

[Edvin]

Hello,

I reached out to our Audio team, and they said:

1: The ACLK of a receiving nRF5340 is adjusted ever so slightly to keep playback in sync. It could be that the DA7212 does not like these subtle changes. 

2: Try connect his custom HW to the codec (CS47L63) on an audio DK and see if the same thing happens. - Or try some other HW codec for that matter to definitely see if this is a FW or HW issue. 

3: It would also be useful to  have a look at the signal with a logic analyzer, which can decode the audio stream. Run a pure sine wave through and check the signal integrity.

Best regards,

Edvin

[similingus]

Hi Edvin,

Sorry for the delayed reply.

Regarding item 1, should I try adjust the ACLK value for my HW codec? If so, could you advise on how to find an appropriate clock frequency?

In the meantime, I will also look into items 2 and 3.

Best Regards,

Miho

[Edvin]

Hello,

I am not sure it is possible to adjust it on the HW codec. That is not how I interpreted the feedback from our LE Audio team. Did you get to look into items 2 and 3?

Best regards,

Edvin