STM CircleOS forum / Primer2 Audio Quality

Eduardo · 2010-02-11 17:02:32

Hallo.

I like the MP3 Project on the Primer2 very much.
However I am hearing some background noise. Looking at the schematics I found out that the Audio Codec is supplied from two L6298 chips. Both a running in low power mode.
I have had a very bad experience with a former design using a similar low power mode with a switching regulator. The voltage went up and down like crazy.
I think it's worth trying to switch one or both converters to low noise mode to see if the noise disappears. A good way to do this might even be a sw switch using a general purpose IO pin from the Codec or the STM32. The idea is changing the level on the pin "SYNC" from high to low for low noise operation.
Unfortunately I don't have the tools for changing the connection to the tiny pins at this moment. If somebody wants to try...

Cheers,

Eduardo

repzak · 2010-02-12 11:13:35

Hello,

Please specify if you use speaker or headphone.

Kasper

ntrf.zns · 2010-02-12 23:02:29

Not only power suply can cause background noise.

I have a realy wierd interference in my headphones when i'm pluging them into my primer 2. I live near the old factory with antenas on top of it. Most of them are still in use. In some parts of my appartment i can hear some kind of radio station! In russia we have a huge mess with radio transmissions.

Try moving around the city. Since Primer 2 is only a dev board it might be affected to nearest radio trasmitters.

Eduardo · 2010-02-14 21:16:48

Hallo.

I am using headphone out with a fairly sensitiv SONY headphone.
The noise is related to sw activities in the primer. This is similar to the noise you get when you connect a headphone to a cheap Audio card in a PC. You can hear it when you move a window over the screen.

I don't think it has anything to do with radio in this case. I know the effect and I am far away from any radio station.

The week after the 22nd I may try the power supply theory, hooking a scope etc.

E.

repzak · 2010-02-15 11:11:20

Hello,

Did you run with battery and no USB connections?

Kasper

slimjim · 2010-02-17 22:28:37

you can get a better signal to noise ratio by overriding the limits imposed as default by CircleOS, I set mine this way and found the sound to be much more acceptable, both with headphones and external amplifier. Also as repzak mentioned, powering the Primer 2 from the usb is terrible for audio, all sorts of high frequency clock signals and weird little phasing noises are present which dissappear when running on the internal battery.

Code to increase the SNR and switch off extraneous whatsits:

AUDIO_SPEAKER_OnOff(OFF);
AUDIO_SetMode (AUDIO_CIRCULAR_MODE,LG_16_BITS,FRQ_44KHZ,STEREO); // Set Audio mode re raw files, 16 bit, 44.1khz, stereo
AUDIO_WriteRegister(0x0F,0x00); // switch buzzer off!!!!
AUDIO_WriteRegister(8,3); // Left Channel headphone amp min attenuation(0x08,00000000)
AUDIO_WriteRegister(9,3); // Right Channel headphone amp min attenuation(0x09,00000000)
AUDIO_WriteRegister(19,1); // Bass Gain +2.5dB (0x13,00000101)
AUDIO_WriteRegister(20,0); // Compressor off, 0dB attenuation (0x14,0)

-James

[edit] You could also try sheilding the guts of the Primer 2 with some adhesive foil connected to ground somewhere to reduce radio frequency interference, or rebox it in a metal chassis...

Last edited by slimjim (2010-02-17 22:49:39)

mdce · 2010-02-23 08:33:22

Hello,
Also take into consideration that the sleeve of the audio connector is NOT ground but "VCMHP".
This is not good when connecting to external amplifier.

# 1 2010-02-11 17:02:32 Primer2 Audio Quality

Primer2 Audio Quality

# 2 2010-02-12 11:13:35 Primer2 Audio Quality

Re: Primer2 Audio Quality

# 3 2010-02-12 23:02:29 Primer2 Audio Quality

Re: Primer2 Audio Quality

# 4 2010-02-14 21:16:48 Primer2 Audio Quality

Re: Primer2 Audio Quality

# 5 2010-02-15 11:11:20 Primer2 Audio Quality

Re: Primer2 Audio Quality

# 6 2010-02-17 22:28:37 Primer2 Audio Quality

Re: Primer2 Audio Quality

# 7 2010-02-23 08:33:22 Primer2 Audio Quality

Re: Primer2 Audio Quality

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