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Implement 16.16 fixed point feedback value calculation

This commit is contained in:
Reinhard Panhuber 2022-03-15 20:30:31 +01:00
parent 26339e694f
commit c9b444e771
1 changed files with 9 additions and 4 deletions
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13
src/class/audio/audio_device.c
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@ -310,7 +310,8 @@ typedef struct
#if CFG_TUD_AUDIO_ENABLE_FEEDBACK_DETERMINATION_WITHIN_SOF_ISR
uint8_t n_frames; // Number of (micro)frames used to estimate feedback value
uint8_t n_frames_current; // Current (micro)frame number
uint32_t feeback_param_factor; // TODO: Set this value within some new tud_audio_set_feedback_params_fm_fs function as feeback_param_factor = f_s / (f_cpu * n_frames)!
uint32_t feeback_param_factor_N; // Numerator of feedback parameter coefficient
uint32_t feeback_param_factor_D; // Denominator of feedback parameter coefficient
#endif
#endif
@ -2004,12 +2005,16 @@ bool audiod_xfer_cb(uint8_t rhport, uint8_t ep_addr, xfer_result_t result, uint3
}
#if CFG_TUD_AUDIO_ENABLE_EP_OUT && CFG_TUD_AUDIO_ENABLE_FEEDBACK_EP && CFG_TUD_AUDIO_ENABLE_FEEDBACK_DETERMINATION_WITHIN_SOF_ISR
// f_s max is 2^19-1 = 524287 Hz
// n_frames_min is ceil(2^10 * f_s / f_m) for full speed and ceil(2^13 * f_s / f_m) for high speed
// f_m max is 2^29/(1 ms * n_frames) for full speed and 2^29/(125 us * n_frames) for high speed
bool tud_audio_set_feedback_params_fm_fs(uint8_t func_id, uint32_t f_m, uint32_t f_s)
{
audiod_function_t* audio = &_audiod_fct[func_id];
uint8_t n_frame = 1; // TODO: finalize that
audio->n_frames = n_frame;
audio->feeback_param_factor = f_s / f_m / n_frame; // TODO: Check the 16.16 precision!
audio->feeback_param_factor_N = f_s << 13;
audio->feeback_param_factor_D = f_m * n_frame;
return true;
}
#endif
@ -2023,7 +2028,7 @@ void audiod_sof (uint8_t rhport, uint32_t frame_count)
// Determine feedback value - The feedback method is described in 5.12.4.2 of the USB 2.0 spec
// Boiled down, the feedback value Ff = n_samples / (micro)frame.
// Since an accuracy of less than 1 Sample / second is desired, at least n_frames = ceil(2^K * f_s / f_cpu) frames need to be measured, where K = 10 for full speed and K = 13 for high speed, f_s is the sampling frequency e.g. 48 kHz and f_cpu is the cpu clock frequency e.g. 100 MHz (or any other master clock whose clock count is available and locked to f_s)
// Since an accuracy of less than 1 Sample / second is desired, at least n_frames = ceil(2^K * f_s / f_m) frames need to be measured, where K = 10 for full speed and K = 13 for high speed, f_s is the sampling frequency e.g. 48 kHz and f_cpu is the cpu clock frequency e.g. 100 MHz (or any other master clock whose clock count is available and locked to f_s)
// The update interval in the (4.10.2.1) Feedback Endpoint Descriptor must be less or equal to 2^(K - P), where P = min( ceil(log2(f_cpu / f_s)), K)
// Ff = n_cycles / n_frames * f_s / f_cpu in 16.16 format, where n_cycles are the number of CPU cycles within n_frames
@ -2038,7 +2043,7 @@ void audiod_sof (uint8_t rhport, uint32_t frame_count)
if (audio->n_frames_current == audio->n_frames)
{
uint32_t n_cylces = tud_audio_n_get_fm_n_cycles_cb(rhport, audio->ep_fb);
uint32_t feedback = n_cylces * audio->feeback_param_factor;
uint32_t feedback = (n_cylces << 3) * audio->feeback_param_factor_N / audio->feeback_param_factor_D; // feeback_param_factor_N has scaling factor of 13 bits, n_cycles 3 and feeback_param_factor_D 1, hence 16.16 precision
tud_audio_n_fb_set(i, feedback);
audio->n_frames_current = 0;