code:rs8encode
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| code:rs8encode [2010/04/20 21:19] – fsphil | code:rs8encode [2010/07/08 15:07] (current) – Move static tables into PROGMEM fsphil | ||
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| This is the Reed-Solomon encoder as used on the [[projects: | This is the Reed-Solomon encoder as used on the [[projects: | ||
| - | 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>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.</ |
| + | |||
| + | 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:// | For Phil's original code see: http:// | ||
| - | <code c>/ | + | <code c>/* Reed-Solomon encoder |
| * Copyright 2004, Phil Karn, KA9Q | * Copyright 2004, Phil Karn, KA9Q | ||
| * May be used under the terms of the GNU Lesser General Public License (LGPL) | * May be used under the terms of the GNU Lesser General Public License (LGPL) | ||
| */ | */ | ||
| + | #include " | ||
| #include < | #include < | ||
| + | #include < | ||
| #include " | #include " | ||
| Line 35: | 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[] = { | + | |
| 0x01, | 0x01, | ||
| 0x6F, | 0x6F, | ||
| Line 58: | Line 58: | ||
| }; | }; | ||
| - | const uint8_t index_of[] = { | + | PROGMEM |
| 0xFF, | 0xFF, | ||
| 0x04, | 0x04, | ||
| Line 77: | Line 77: | ||
| }; | }; | ||
| - | const uint8_t poly[] = { | + | PROGMEM |
| 0x00, | 0x00, | ||
| 0x18, | 0x18, | ||
| Line 93: | Line 93: | ||
| for(i = 0; i < NN - NROOTS - pad; i++) | for(i = 0; i < NN - NROOTS - pad; i++) | ||
| { | { | ||
| - | feedback = index_of[data[i] ^ parity[0]]; | + | feedback = pgm_read_byte(& |
| if(feedback != A0) /* feedback term is non-zero */ | if(feedback != A0) /* feedback term is non-zero */ | ||
| { | { | ||
| for(j = 1; j < NROOTS; j++) | for(j = 1; j < NROOTS; j++) | ||
| - | parity[j] ^= alpha_to[mod255(feedback + poly[NROOTS - j])]; | + | parity[j] ^= pgm_read_byte(& |
| } | } | ||
| Line 103: | Line 103: | ||
| memmove(& | memmove(& | ||
| if(feedback != A0) | if(feedback != A0) | ||
| - | parity[NROOTS - 1] = alpha_to[mod255(feedback + poly[0])]; | + | parity[NROOTS - 1] = pgm_read_byte(& |
| else | else | ||
| parity[NROOTS - 1] = 0; | parity[NROOTS - 1] = 0; | ||
| } | } | ||
| - | }</ | + | } |
| + | </ | ||
| <code c>/* rs8.h, Reed-Solomon encoder/ | <code c>/* rs8.h, Reed-Solomon encoder/ | ||
code/rs8encode.1271798360.txt.gz · Last modified: 2010/04/20 21:19 by fsphil