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Reconcile magnitude scaling comment with code.

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antirez 2024-01-21 10:06:19 +01:00
parent de61bd564f
commit 5579c4afe2
1 changed files with 11 additions and 5 deletions
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dump1090.c
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@ -304,12 +304,18 @@ void modesInit(void) {
memset(Modes.data,127,Modes.data_len);
/* Populate the I/Q -> Magnitude lookup table. It is used because
* sqrt or round may be expensive and may vary a lot depending on
* the libc used.
* sqrt or round may be expensive and performance may vary a lot
* depending on the libc used.
*
* We scale to 0-255 range multiplying by 1.4 in order to ensure that
* every different I/Q pair will result in a different magnitude value,
* not losing any resolution. */
* Note that we don't need to fill the table for negative values, as
* we square both i and q to take the magnitude. So the maximum absolute
* value of i and q is 128, thus the maximum magnitude we get is:
*
* sqrt(128*128+128*128) = ~181.02
*
* Then, to retain the full resolution and be able to distinguish among
* every pair of I/Q values, we scale this range from the float range
* 0-181 to the uint16_t range of 0-65536 by multiplying for 360. */
Modes.maglut = malloc(129*129*2);
for (i = 0; i <= 128; i++) {
for (q = 0; q <= 128; q++) {