• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 101 subscribers
  • Views 933 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

RTOS/TMDSLCDK138: Processing 40khz audio signal with the LCDK Omap L138

Nadav Benedek
Prodigy 240 points
Part Number: TMDSLCDK138
Other Parts Discussed in Thread: OMAPL138

Tool/software: TI-RTOS

Hi.

We bought the OMAP L138 LCDK + the X100 v2 emulator.

We ran the Mcasp Loopback demo, after defining the SAMPLE_RATE to be 96000u,

However, when we insert an input signal which is above 23-24khz nothing comes out.

Below, 23-24 khz - it passes alright.

I'm very afraid that some creative engineer decided to put a low pass filter somewhere at the output. 

Our system processes 40khz signals (ultrasound), so we must have it work.

We need your assistance to make it happen. Is it a code configuration thing, or a hardware filter that we need to disable?

  • 0
    TI__Guru** 114410 points

    Nadav,

    As I mentioned there is no low pass filter implemented that is causing this issue. If you see the audio setup on the LCDK. there is a ADC that converts the input audio signal to digital which is then inputed to the SOC using MCASP serializers. The SOC sends configuration to AIC codec using an I2C interface. By changing the sample rate to 96000u, you are probably setting the AIC codec correctly but to read the data correctly, you need to configure MCASP receive section appropriately.

    Please refer to section 22.3.12.2 of the OMAPL138 Technical Reference manual (TRM) for proper initialization. You can also refer to MCASP programming guide here:
    processors.wiki.ti.com/.../StarterWare_McASP

    Regards,
    Rahul

    PS: With a scope check the bit clocks and the framesync to see if this matches your setup.

  • 0 in reply to Rahul Prabhu
    Prodigy 240 points
    Hi Raul,

    1. I don't see a section 22.3.12.2 in this document: www.ti.com/.../spruh77c.pdf

    2. Can you please make our lives a bit easier, and tell us what should we change in the StarterWare Mcasp example, besides the 96000u constant, in order to make it work properly, so it will loop back all the frequencies as it should , and not only up to 23-24 khz?

    Thanks.
  • 0 in reply to Nadav Benedek
    Prodigy 240 points

    Raul, 

    You are right. There is no filter, and it passes the 40khz alright (although there is a lot of output noise, in every frequency)

  • 0 in reply to Nadav Benedek
    TI__Guru** 114410 points
    Nadav,

    Are you using the line In connector or the Mic in connector to connect the input? do you see the same issue with a simple audio loopback or only with the 40 Khz signal. I am not certain what is causing the noise and would need more information to determine if this is a hardware or a software issue.

    Regards.
    Rahul
  • 0 in reply to Rahul Prabhu
    Prodigy 240 points

    Rahul,

    1. On the LCDK, we use J10, which is the line_in and line_out. 

    2. It doesn't have nothing to do with the input. Even if we generate the sine signal ourselves in the code, we get the same results (=same noise).

    3. The same noise is shown in every frequency.

    4. Can you please run the loopback, and insert a sine signal (with the same voltage / amplitude) and post the result here, so we can compare the noise you see to our noise? We attach the noise that we see here. You can see that the output sine signal is very noisy.