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--- code:rs8encode [2010/04/19 11:58] – fsphil
+++ code:rs8encode [2010/07/08 15:07] (current) – Move static tables into PROGMEM fsphil
@@ Line 1: / Line 1: @@
 ====== Reed-Solomon Encoder (255,223) ======
-This is the Reed-Solomon encoder as used on the HADIE module. It was originally written by Phil Karn, KA9Q and was modified slightly for use on the AVR. It takes a pointer to (223 - pad) bytes of data and writes 32 bytes of correction data.
+This is the Reed-Solomon encoder as used on the [[projects:HADIE]] module. It was originally written by Phil Karn, KA9Q and was modified slightly for use on the AVR. It takes a pointer to (223 - pad) bytes of data and writes 32 bytes of correction data, which allows for up to 16 bytes to be corrected by the decoder.
-<code c>/* rs8encode.c, Reed-Solomon encoder
+<del>This code is not as effective in practice as it should be as fldigi is likely to drop bytes if the signal is weak or noisy. A dropped byte means almost all subsequent bytes are incorrect and the Reed-Solomon decoder will fail. Ideally fldigi should return padding bytes when it can't adequately decode bytes from the signal, giving the Reed-Solomon decoder a much better chance of correcting the gaps.</del>
+I have since modified fldigi to estimate the number of missing bytes in the data stream, allowing the block decoder to fill any gaps and give the Reed-Solomon decoder a chance to correct it. This has not yet been tested in flight.
+For Phil's original code see: http://www.ka9q.net/code/fec/
+<code c>/* Reed-Solomon encoder
  * Copyright 2004, Phil Karn, KA9Q
  * May be used under the terms of the GNU Lesser General Public License (LGPL)
  */
+#include "config.h"
 #include <string.h>
+#include <avr/pgmspace.h>
 #include "rs8.h"
@@ Line 31: / Line 39: @@
 }
-/* These tables are read-only, and could be stored in flash only with PROGMEM
+PROGMEM uint8_t alpha_to[] = {
- * if RAM usage becomes an issue. They eat up about 646 bytes.
-*/
-const uint8_t alpha_to[] = {
 x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80,0x87,0x89,0x95,0xAD,0xDD,0x3D,0x7A,0xF4,
 x6F,0xDE,0x3B,0x76,0xEC,0x5F,0xBE,0xFB,0x71,0xE2,0x43,0x86,0x8B,0x91,0xA5,0xCD,
@@ Line 54: / Line 58: @@
 };
-const uint8_t index_of[] = {
+PROGMEM uint8_t index_of[] = {
 xFF,0x00,0x01,0x63,0x02,0xC6,0x64,0x6A,0x03,0xCD,0xC7,0xBC,0x65,0x7E,0x6B,0x2A,
 x04,0x8D,0xCE,0x4E,0xC8,0xD4,0xBD,0xE1,0x66,0xDD,0x7F,0x31,0x6C,0x20,0x2B,0xF3,
@@ Line 73: / Line 77: @@
 };
-const uint8_t poly[] = {
+PROGMEM uint8_t poly[] = {
 x00,0xF9,0x3B,0x42,0x04,0x2B,0x7E,0xFB,0x61,0x1E,0x03,0xD5,0x32,0x42,0xAA,0x05,
 x18,0x05,0xAA,0x42,0x32,0xD5,0x03,0x1E,0x61,0xFB,0x7E,0x2B,0x04,0x42,0x3B,0xF9,
@@ Line 89: / Line 93: @@
         for(i = 0; i < NN - NROOTS - pad; i++)
         {
-                feedback = index_of[data[i] ^ parity[0]];
+                feedback = pgm_read_byte(&index_of[data[i] ^ parity[0]]);
                 if(feedback != A0) /* feedback term is non-zero */
                 {
                         for(j = 1; j < NROOTS; j++)
-                                parity[j] ^= alpha_to[mod255(feedback + poly[NROOTS - j])];
+                                parity[j] ^= pgm_read_byte(&alpha_to[mod255(feedback + pgm_read_byte(&poly[NROOTS - j]))]);
                 }
@@ Line 99: / Line 103: @@
                 memmove(&parity[0], &parity[1], sizeof(uint8_t) * (NROOTS - 1));
                 if(feedback != A0)
-                        parity[NROOTS - 1] = alpha_to[mod255(feedback + poly[0])];
+                        parity[NROOTS - 1] = pgm_read_byte(&alpha_to[mod255(feedback + pgm_read_byte(&poly[0]))]);
                 else
                         parity[NROOTS - 1] = 0;
         }
-}</code>
+}
+</code>
 <code c>/* rs8.h, Reed-Solomon encoder/decoder