AD9398 audio issue

Noise issue occurs on audio output from AD9398.

Noise issue is that small pop noise occurs one time per 5 seconds. And you can hear it on 10kHz sine wave more easily than 1kHz sine wave.

The condition is following.

-All pictures(480p,1080i, VGA)

-All sampling rate(32, 44.1, 48kHz)

-To set FIFO Reset UF, FIFO Reset OF(0x59[2:1]) effects this issue.

   It reduces the frequency of occurrence but does not solve this issue.

-To set N_CTS_Disable(0x58[3]) effects this issue.

   It can kill the noise occasionally.

-The register settings is attached.

   The settings are based on your instruction of datasheet page12.

Please give me your advice to solve this issue.
  • 0
    •  Analog Employees 
    on Apr 26, 2011 6:52 PM

    Some questions for you:

    Q1. What are you using as an A/V source?

    Q2. How do we know that the data from A/V source is okay?

    Q3. Have you listened to this signal on another HDMI receiver?

    Q4. Do you see this same behavior with AD9398 Evaluation Board?


  • 0
    •  Analog Employees 
    on Apr 26, 2011 9:18 PM

    Unless you have compelling reasons to set I2S to ‘left-justified’, we recommend you set this to 'normal' mode (set R0x2E to 0x18 instead of 0x58.)

    Also, we have determined that the best operation with audio is to ‘reset’, or clear the N/CTS values whenever a change of mode takes place. This is done by writing a 0x00 to (surprisingly) a read-only register – 0x7F. This will assure that the AD9398 updates the N and CTS values from the source. We think this may clear up the issue you are experiencing.

    The theory of operation for this audio is that the source will continue to monitor the calculation of 128*fs = fTMDS_clock*N/CTS. It is inevitable that this will drift slightly back and forth, and the CTS will be altered by 1 or 2 counts during transmission of audio. This keeps everything in sync and prevents over-flow and underflow.

    Finally, please note that we have posted some additional documentation for AD9398 and similar devices at the following URL:


  • Hello, Matt-san.

    Thank you for your help.

    It seems that the settings of "HDMI Audio Settings for the AD9880 0707.pdf" and writing 0x00 to R0x7F would solve this issue with 480p and VGA.

    From now we will try with another resolution(1080i).


  • Hello, Matt-san

    We have tried with another resolutions. The result is attached(result.xls).

    Using "Optimized VGA/480p" settings makes an improvement for this issue. But it isn't perfect.

    It seems that the 'PLL Enable' register(R0x03[2]) and 'MCLK PLL Divisor' register(R0x58[6:4]) have an effect on this issue.

    Does this case become your hint?

    Also, what timing should we set the 'reset' by writing 0x00 to R0x7F? (only one time after register settings?)

    I'm looking forward to your advice.

    Best regards.


  • 0
    •  Analog Employees 
    on Apr 28, 2011 8:15 PM

    Hi Kikka,

    I'm glad to see you are making some progress.  But it seems you are still experiencing some issues.

    With regards your question about when to do a reset, we would recommend that you do a reset during any of the following conditions:

    1) the video mode changes (for example, from 480p to 1080i)


    2) audio frequency changes (for example, from 32KHz to 44.1KHz)

    We noticed that your chart includes resolution of 480i. What are you using as a source?  (Normally, this isn't supported on HDMI unless the source is doing pixel-doubling and communicating this correctly)

    Finally, can you provide some details about the test procedure (including what registers you are writing and when)?