Crackling sound on USB soundcard

[ArthurJam]

Hello,

I have a raspberry pi 2 with a USB soundcard and i can't get clean sound on it.
After many search and meny try, i dont know what to do.
If someone can help me, i would appreciate.

I post an audio file to show you the problem : https://clyp.it/j3hnfvvt

pi@raspberrypi ~ $ lsusb
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp.
Bus 001 Device 004: ID 08bb:2704 Texas Instruments Japan
Bus 001 Device 005: ID 148f:7601 Ralink Technology, Corp.

pi@raspberrypi ~ $ aplay -l
**** Liste des Périphériques Matériels PLAYBACK ****
carte 0: DAC [USB Audio DAC], périphérique 0: USB Audio [USB Audio]
  Sous-périphériques: 1/1
  Sous-périphérique #0: subdevice #0
carte 1: ALSA [bcm2835 ALSA], périphérique 0: bcm2835 ALSA [bcm2835 ALSA]
  Sous-périphériques: 8/8
  Sous-périphérique #0: subdevice #0
  Sous-périphérique #1: subdevice #1
  Sous-périphérique #2: subdevice #2
  Sous-périphérique #3: subdevice #3
  Sous-périphérique #4: subdevice #4
  Sous-périphérique #5: subdevice #5
  Sous-périphérique #6: subdevice #6
  Sous-périphérique #7: subdevice #7
carte 1: ALSA [bcm2835 ALSA], périphérique 1: bcm2835 ALSA [bcm2835 IEC958/HDMI]
  Sous-périphériques: 1/1
  Sous-périphérique #0: subdevice #0

pi@raspberrypi ~ $ uname -a
Linux raspberrypi 3.18.13-v7+ #784 SMP PREEMPT Sat May 9 15:57:36 BST 2015 armv7l GNU/Linux

Thank you.

[ArthurJam]

I dont know why but the sound going more bad when i touch the piscreen.
I got more noise and crack.

[notro]

Since you mention piscreen, I assume you use the fbtft drivers. This is a similar case: #888 (comment)

[notro]

You can verify if this is the problem by not loading the piscreen device.

[ArthurJam]

Hello,

Yes i use the fbtft driver.
I still have the problem when i remove the piscreen.
But the fbtft driver us maybe loaded.
Can you give me a method to unload the driver correctly.

Thx notro for your answer and for yours works.

[notro]

Can you give me a method to unload the driver correctly.

That depends on how you load it.
If you use a Device Tree overlay, just uncomment it in /boot/config.txt
If you use fbtft_device, uncomment it in /etc/modules or wherever it is loaded.
Verify that the driver is not loaded by looking in the kernel log: dmesg
This line should not be present:

graphics fb1: XXXX frame buffer, XXX KiB video memory, ...

Or you can unload the module after it is loaded:
fbtft_device: sudo modprobe -r fbtft_device
DT overlay: sudo modprobe -r fb_xxxx (use lsmod to find the name)

[ArthurJam]

Thx i will try.

[ArthurJam]

The sound going better after unload the fbtft.
It s not perfect when i stress the raspberry like starting a browser but the sound is more clear.
Do you know how i can use my piscreen, it s for a personnal project ?

[notro]

You can try txbuflen=128 (256,512,1024,2048). If you use fbtft_device, you can use it directly as an argument.
If you use a DT overlay, you need to customize the source and add: txbuflen = <128>;
Lowering this value will slow down your display.

You will see that the change has taken effect in the log:
graphics fb1: XXX frame buffer, X KiB video memory, X KiB buffer memory,

https://github.com/notro/fbtft/wiki/fbtft_device
https://www.raspberrypi.org/documentation/configuration/device-tree.md

[ArthurJam]

Ok, i will try to make my own overlay.
I found the solution on this page : https://github.com/notro/fbtft/wiki/FBTFT-RPI-overlays

Can you tell me in which section I must add this line : txbuflen = <128>;
My overlay will be based on : https://github.com/notro/fbtft/blob/master/dts/overlays/rpi/piscreen-overlay.dts

I'm sorry for my incompetence.
Thx.

[notro]

Add it alongside fps, buswidth and regwidth:

            piscreen: piscreen@0{
                compatible = "ilitek,ili9486";
                reg = <0>;
                pinctrl-names = "default";
                pinctrl-0 = <&piscreen_pins>;

                spi-max-frequency = <32000000>;
                rotate = <270>;
                bgr;
                fps = <30>;
                buswidth = <8>;
                regwidth = <16>;
                txbuflen = <128>;

If you add this to overrides (DT overlay argument), then you can tweak the value easily:

    __overrides__ {
[...]
        txbuflen =      <&piscreen>,"txbuflen:0";
    };

dtoverlay=piscreen,txbuflen=256

[ArthurJam]

Ok,
I make the custom overlay and i have a better sound now.
I try with multiple txbuflen value.
Do you know what the default value with piscreen orginal file ?

[notro]

The fbtft default is 4k=4096. txbuflen is the size of the transmit buffer, how much is transferred in one SPI message.

[ArthurJam]

Thanks.
Is it a material restriction ?
Or should I have a hope to get a greater buffer one day ?

[notro]

It's probably a buffer underun in the usb audio device. But I know to little about the processor internals to say anything about where the bottleneck is.