• State Not Answered
  • Locked Locked
  • Replies 2 replies
  • Subscribers 23 subscribers
  • Views 1454 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Fixed point rfft issue in CMSIS-DSP

Paul Chen

Hi all,

I tried the rfft for q31 fixed point. The scale of the output is different than what I thought.

I use 32 pt rfft. Input is constant 2^10=1024 (in Q31). The DC term output should be 2^10 * 2^5 / 2^4 = 2048 (in Q5.27), where multiplication of 2^5 is due to 32 pt fft, and division of 2^4 is due to input scale in each stage as mentioned below. All the terms other than DC one should be zeros.

However, my result for DC term is 1024 + j 0 (I expect 2048+j 0).

Anyone has any idea?

Paul

Code and results are attached.

Code:

void test_fft()
{
    arm_rfft_instance_q31 fft_st;
    arm_status status;
    int i;
    int32_t   len_fft=32;
    uint32_t  ifftFlagR=0;
    uint32_t  bitReverseFlag=1 ;
    static int32_t xfft[32];
    static int32_t yfft[64];
    for (i = 0; i < len_fft; i++)
    {
      xfft[i] = (int32_t) 1024;
    }
    for (i = 0; i < len_fft; i++)
    {
      am_util_stdio_printf("%d: %d\n", i, xfft[i]);
    }
    am_util_stdio_printf("\n");
    arm_rfft_init_q31(&fft_st, len_fft, ifftFlagR, bitReverseFlag);
    arm_rfft_q31(&fft_st, xfft, yfft);
    for (i = 0; i < ((len_fft>>1) + 1); i++)
    {
      am_util_stdio_printf("%d: %d + j %d  \n", i, yfft[2*i], yfft[2*i+1]);
    }
    am_util_stdio_printf("\n");
   
}
Results:
Input:
0: 1024
1: 1024
2: 1024
3: 1024
4: 1024
5: 1024
6: 1024
7: 1024
8: 1024
9: 1024
10: 1024
11: 1024
12: 1024
13: 1024
14: 1024
15: 1024
16: 1024
17: 1024
18: 1024
19: 1024
20: 1024
21: 1024
22: 1024
23: 1024
24: 1024
25: 1024
26: 1024
27: 1024
28: 1024
29: 1024
30: 1024
31: 1024
Output:
0: 1024 + j 0
1: 0 + j 0
2: 0 + j 0
3: 0 + j 0
4: 0 + j 0
5: 0 + j 0
6: 0 + j 0
7: 0 + j 0
8: 0 + j 0
9: 0 + j 0
10: 0 + j 0
11: 0 + j 0
12: 0 + j 0
13: 0 + j 0
14: 0 + j 0
15: 0 + j 0
0