Rockbox

[EDIT: Attached patch for experimenting with AS3543 settings.]

When playing back audio on a Sansa ClipV2 player, there is a regular background noise that's audible with sensitive headphones and that's quite annoying during quite music passages. The background noise is a hissing sound that repeats about 4 times per second. The noise can be heard when there is no disk activity and even at very low volume or when playback is muted. It stops as soon as audio is paused.

The noise is not present in the original firmware (OF).

This is a long-known issue, but hasn't been formally tracked before. It is related to FS#11907, but different in that the background noise is present when there's no disk (SD) activity. However, I did have the impression that disk activity intensifies the noise.

This item serves to collect the various approaches people have tried to eliminate the noise, and ideas for future exploration.

I ran a series of experiments on the assumption that the audio hardware is misconfigured is some way. I worked through various settings of the AS3543 component based on its datasheet. Here is what I have tried; but nothing helped to eliminate the noise:

* Higher-quality audio settings as proposed in  FS#11304  (AUDIOSET2_HPH_QUALITY_HIGH, AUDIOSET3_HP_BIAS_150)
* Disabled DAC gain control (AUDIOSET1_DAC_GAIN_on)
* Bypassed mixer and disabled mixer gain control (HPH_OUT_R_HP_OUT_DAC, AUDIOSET2_AGC_off, AUDIOSET2_SUM_off)
* Mute SDI data (AUDIOSET1_SDI_MUTE)
* Enable and control DAC attenuation (AS3543_DAC_ATTEN_on)
* Disable common mode buffer (phantom ground) (AUDIOSET3_HPCM_on)
* Disabled various unused mixer components (AUDIOSET3_MICMIX_off, AUDIOSET3_ADCMIX_off, AUDIOSET3_LINMIX_off)
* Used external clock source for DAC (AS3543_DAC_EXT_MCLK)
* Enable headphone detection (HPH_OUT_L_HP_DET_ON)
* Resetting all reserved audio registers to 0

[EDIT: I have attached a patch that allows experimenting with these settings by uncommenting the relevant source lines at compile time or extending the provided debug function to toggle the setting at run time.]

Possible next steps (suggested by various people on IRC):

* As the noise is not present on the other AMSv2 targets (Clip+, FuzeV2), a dump of the AS3543 settings (as3514_regs[] array) on these targets would be helpful. I have attached a patch that adds a debug-menu function for doing this (needs a logf build).

* Someone with a JTAG could read the AS3543 configuration set by the OF.

* The issue could be unrelated to the AS3543; it could be a power-management or frequency-scaling issue.

Also attached: A dump of as3514_regs[] (logf.txt; all values in hex).
[EDIT: I've removed the patch to dump these settings; see a later comment in this task for a more up-to-date patch to do this.]

Other ideas?

 b/apps/debug_menu.c               |   27 +++++++++++++++++++++++++++
 b/firmware/drivers/audio/as3514.c |   34 ++++++++++++++++++++++++++++++----
 b/firmware/export/as3514.h        |   25 ++++++++++++++++++++++---
 3 files changed, 79 insertions(+), 7 deletions(-)