July 9, 2009

10359 Stereo: Rounding & Scaling

by Oleg Dzhimiev

1. [In Progress] “Verification of everything”.

The results of running 1 frame through the cross-correlation block are more or less ok now:

  • took coefficients’ product bits [31:16] (that is 16 bit rounding)

Sample 1:

070909_source1

Fig.1 Source – test frame.

070909_result1

Fig. 2 Result (there are some errors on the top of the result image near the areas of color change).

Sample 2:

070909_source2Fig. 3 The picture from the sensor w/o a lense (a bit dirty – dust on scotch).

img_090709_2

Fig. 4 Result, again there are some errors – overflows are possible.

TODO:

1. Adjust the more suitable rounding for fft-coeffs multiplication – perform rounding after multiplication + perform rounding in IFFT?

Current Bits Calculation: 12bits -> FFT(8 stages with rounding) -> 12bits -> MULT(16 bits rounding) -> 8bits -> IFFT(no rounding) -> 16bits.

Try: 12 -> FFT (8 cut) -> 12 -> MULT (8 cut)-> 16 -> IFFT(8 cut) -> 16 – and there should be no errors.

2. Test frames from different channels.


One response to “10359 Stereo: Rounding & Scaling”

  1. andrey says:

    Oleg, why do you have just 12 bits? Multipliers are 18 bits, memory – also 18-bit wide (using “parity”).

    Do you use saturation when accumulating (adding/subtracting)?

    When I was implementing FFT on DSP I used group scaling after each pass – calculated the maximal value and optionally scaled by 2 on the next pass (keeping track of this scaling). On average (total “energy”) the values grow sqrt(2) on each pass if I remember that right.

    So I would recommend to try to increase precision while fighting overflows at the same time:
    – saturate values when accumulating;
    – watch fro maximal absolute value on each pass;
    – downscale (shift right by 1 bit) after passes with maximal value above threshold. Scaling can be done while reading data during the next pass.

    Andrey

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