Noise on analog audio output

Technical Support
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1

If we connect an amplifier to the analog output of the Ixora board (with i.mx6 module) we get noise every time we show a simple blending effect with ~100x100 pictures on the LVDS display.
The display does not need to be connected to create the noise.
During the blending, we can see this on the power line (the signal with the higher amplitude):

921-ds1z-quickprint12.png
921-ds1z-quickprint12.png800×480 37.5 KB

We have the problem on Ixora (with active desktop speakers) and on our custom board (different power supply, different DC/DC, Mono amplifier on board).

How can we get rid of the noise while doing some graphics operations in Qt?

2

The Ixora board audio path is not optimized for high audio quality. High frequency noise (around 1MHz) is leaking from digital circuitry. The simplest way to fix it is to add a low pass filter between audio output and amplifier. Directly connected speakers or headphones act as a low pass filter by themself so the low pass filter is not required.

3

Hello Alex,
I forgot to mention that we have an audible noise, the EMC stuff is not relevant here. The frequency of the noise is pretty low.
We had it also really loud on the microphone input, but we could fix it here by using a linear regulator for AVCC, as described in the Apalis Carrier Board Design Guide.
For the audio output we currently have no solution.

4

Could you please provide version of BSP you are using and a command to play a sound. I’ll try to repro that issue on my side.

5

We are using BSP 2.7.
We created an image without X11/Wayland.
We are running Qt5 software with eglfs output plugin and we use QML-based blending effects for a small slide show.
The CPU load and video processing does not affect the noise, it just seems to be related to 2D/3D effects.
We don’t need to play sound to hear the noise.

6

Is the noise always there or does it occur when you start your application?

Could you provide the changes to local.conf and other files to do the build? Additionally an example application to reproduce the problem would be also helpful.

Thanks

7

The noise occurs only if the application runs.
One of our application developers created a simple program which shows the problem.

The local.conf looks like this (some unimportant lines removed)

DISTRO_FEATURES_remove = "wayland"
BB_NUMBER_THREADS ?= "${@oe.utils.cpu_count()}"
PARALLEL_MAKE ?= "-j ${@oe.utils.cpu_count()}"
MACHINE ?= "apalis-imx6"
DL_DIR ?= "${TOPDIR}/../../downloads"
DEPLOY_DIR = "${TOPDIR}/../deploy"
PACKAGE_CLASSES ?= "package_ipk"
EXTRA_IMAGE_FEATURES = "debug-tweaks package-management"
USER_CLASSES ?= "buildstats image-mklibs image-prelink"
PATCHRESOLVE = "noop"
BB_DISKMON_DIRS = "\
    STOPTASKS,${TMPDIR},1G,100K \
    STOPTASKS,${DL_DIR},1G,100K \
    STOPTASKS,${SSTATE_DIR},1G,100K \
    ABORT,${TMPDIR},100M,1K \
    ABORT,${DL_DIR},100M,1K \
    ABORT,${SSTATE_DIR},100M,1K" 
PACKAGECONFIG_pn-qemu-native = "sdl"
ASSUME_PROVIDED += "libsdl-native"
CONF_VERSION = "1"
INHERIT += "toradex-mirrors"
DISTRO = "angstrom"
IMAGE_FSTYPES += "tar.bz2"
PREFERRED_PROVIDER_psplash-support = "psplash-angstrom"
PREFERRED_PROVIDER_u-boot-fw-utils = "u-boot-toradex-fsl-fw-utils"
DISTRO_FEATURES_remove += "x11"
DISTRO_FEATURES_remove += "wayland"
DISTRO_FEATURES_append += "gles2"
PACKAGECONFIG_pn-qtmultimedia += "examples"
PACKAGECONFIG_pn-qtmultimedia += "gstreamer"
PREFERRED_VERSION_nodejs = "6.9.1"
PREFERRED_VERSION_nodejs-native = "6.9.1"
IMAGE_FSTYPES_append += " ext4.gz"

Our image is based on a console image with Qt5 added.

8

Thanks for the update. We will try to reproduce this on our side and share the results with you soon.

9

Hello @sst
We reproduced the issue on our side , We ran your application with desktop-speakers connected to audio output of the Ixora Board and volume set to highest level.

There was no audible sound at the output of speakers.

Could you provide us some details about your setup? Maybe you can make a picture? Which power supply are you using for the Ixora Board? What is exact hardware version of your module?

Please check this community post also.