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PCM2900C unwanted 1KHz tone on the output

Other Parts Discussed in Thread: PCM2900C, PCM2902

Dear Ti,

I've some unwanted 1Khz tone on the output of the PCM2900C USB codec. The codec is used in a mixer that i designed and users complain about a 1Khz tone. I did some research and some measurements and i want to share these results and want to know if this is a normal phenomenon.

First of all the 1Khz tone is only heard when the recording channel is somehow selected or addressed by a software program. The way i tested this is by selecting the recording devices tab of the sound devices window.

Reference measurement. Output of the mixer connected to my AP sys2522 with the audio devices windows selected on playback devices:

 As can be seen in the measurement is that the 1Khz is present at -70dB, if four chanels are full open than the level reaches -55dB ;(

Full bandwidth 48KHz 16Bit stereo out and 48KHz 16Bit Stereo in (recording) 


Altering the recording channel settings reduces the signal a small bit. First i switched back to mono recording:

48KHz 16 Bit out and 48KHz 16 bit mono in (Recording)

 Altered the playback channel settings back to 32KHz sampling rate:

 32KHz 16 Bit out and 48KHz mono in (Recording)


The lowest setting gives the lowest signal

32Khz 16Bit out and 11KHz 16bit in (recording) 

As long as any application doesn't address the recording of the codec there is not a problem. but most application do and also if the customer is recording something the 1Khz tone is very unwanted.

I also noted the SLAZ036A Errata noting somethin about a 1Khz tone when the devices is used in some of its modes. The errata only describes the problem of unwanted 1Khz tone in its tranfered data, so not in the output of the codec. 

I also compared my findings with a Behringer UCA202 that is equiped with a PCM2902 and found the same tone.

Is this problem know at TI and is there a fix ? please help

Kind regards Jaac 

  • Hi, Jaac,

    I've asked my colleague to look at this issue with you.


  • This is likely related to the USB audio 1ms (1kHz) isochronous packet rate, which on some USB ports can cause ripple on the USB power supply. Please check the power supply (assuming your circuit is bus-powered) when this unwanted tone occurs. If this is the root cause, you'll have to filter the power supply.

  • Hello,

    I've done some testing on the supply rails. Filtering the Vbus 5V with a inductor didn't improve the noise on the Vbus rail. 

    Vbus before PI filter, Host side.

    It made the Vbus even worse, as i expected it to be lower.

    Vbus after PI filter, Codec side:

    I've hooked up a bench power supply to the Vcci, pin 10, and st it to 3,6V and slowly turned up the current setting. Now i see some improvements! below the plots with  d without the regulator. the blue line is the Vcci line and the yellow line is the audio output.

    I will try a bigger cap on pin 10 to see if that will give some improvement before adding a regulator in the design.