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Reduce the size of tables for RFFT/RIFFT fixed point versions #25

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christophe0606 opened this issue Jun 30, 2022 · 1 comment
Open

Reduce the size of tables for RFFT/RIFFT fixed point versions #25

christophe0606 opened this issue Jun 30, 2022 · 1 comment
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enhancement New feature or request

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@christophe0606
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ARM-software/CMSIS_5#1275
@christophe0606 christophe0606 added the enhancement New feature or request label Jun 30, 2022
@jonnor
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jonnor commented Jun 15, 2024

This is a quite problematic issue for uses of RFFT on many smaller microcontrollers. Regardless of what length FFT is used, it always takes 32 kB FLASH space for the tables.

This is especially surprising since there are dedicated initialization functions for each RFFT length. Such as arm_rfft_init_64_q15 or arm_rfft_init_128_q15. However, each single one of them uses the same tables:

    S->pTwiddleAReal = (q15_t *) realCoefAQ15;                    \
    S->pTwiddleBReal = (q15_t *) realCoefBQ15;                    \

realCoefAQ15 and realCoefBQ15 are 16kB each.

@jonnor jonnor mentioned this issue Jun 15, 2024
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jonnor added a commit to jonnor/py32f0-template that referenced this issue Jun 15, 2024
@jonnor
Tool for generating RFFT twiddle tables
4dbebd8 
Workaround for ARM-software/CMSIS-DSP#25

A quick spot check of the output against
CMSIS-DSP realCoefAQ15 / realCoefBQ15 looks OK
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@jonnor @christophe0606