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Shift up the samples into -1..1, not much different, but we get an extra bit of resolution at the low end.

This commit is contained in:
George Norton 2023-09-02 09:22:41 +01:00
parent 354d8e09bb
commit 3beab578b7
1 changed files with 5 additions and 5 deletions
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10
firmware/code/fix16.inl
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@ -32,11 +32,11 @@ static inline fix3_28_t norm_fix3_28_from_s16sample(int16_t a) {
/* So, we're using a Q3.28 fixed point system here, and we want the incoming /* So, we're using a Q3.28 fixed point system here, and we want the incoming
audio signal to be represented as a number between -1 and 1. To do this, audio signal to be represented as a number between -1 and 1. To do this,
we need the 16-bit value to map to the 28-bit right-of-decimal field in we need the 16-bit value to map to the 28-bit right-of-decimal field in
our fixed point number. 28-16 = 12, so we shift the incoming value by our fixed point number. 28-16 = 12 + the sign bit = 13, so we shift the
that much to covert it to the desired Q3.28 format and do the normalization incoming value by that much to covert it to the desired Q3.28 format and
all in one go. do the normalization all in one go.
*/ */
return (fix3_28_t)a << 12; return (fix3_28_t)a << 13;
} }
/// @brief Convert fixed point samples into signed integer. Used to convert /// @brief Convert fixed point samples into signed integer. Used to convert
@ -69,7 +69,7 @@ static inline int32_t norm_fix3_28_to_s16sample(fix3_28_t a) {
/* When we converted the USB audio sample to a fixed point number, we applied /* When we converted the USB audio sample to a fixed point number, we applied
a normalization, or a gain of 1/65536. To convert it back, we can undo that a normalization, or a gain of 1/65536. To convert it back, we can undo that
by shifting it but we output 24bts, so the shift is reduced. */ by shifting it but we output 24bts, so the shift is reduced. */
return (a >> 5); return (a >> 6);
} }
static inline fix3_28_t fix3_28_from_flt(float a) { static inline fix3_28_t fix3_28_from_flt(float a) {