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Remove unnecessary zero-padding in DATV mod RS encoder

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This commit is contained in:
Jon Beniston 2021-03-27 09:55:46 +00:00
parent 2c2cf314b1
commit edcda403b6
1 changed files with 7 additions and 9 deletions
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16
plugins/channeltx/moddatv/dvb-s/dvb-s.cpp
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@ -86,7 +86,8 @@ void DVBS::scramble(const uint8_t *packetIn, uint8_t *packetOut)
// Using GF(2^8=256) so 1 symbol is 1 byte // Using GF(2^8=256) so 1 symbol is 1 byte
// Code generator polynomial: (x+l^0)(x+l^1)..(x+l^15) l=0x02 // Code generator polynomial: (x+l^0)(x+l^1)..(x+l^15) l=0x02
// Field generator polynomial: x^8+x^4+x^3+x^2+1 (a primitive polynomial) // Field generator polynomial: x^8+x^4+x^3+x^2+1 (a primitive polynomial)
// Add 51 zero bytes before 188-byte packet to pad to 239 bytes, which are discarded after RS encoding // Add 51 zero bytes before 188-byte packet to pad to 239 bytes, which are
// discarded after RS encoding. They don't change the result so can just be ignored
// n=255 k=239 // n=255 k=239
// RS adds 2t = 16 bytes (n-k) after original data (so is systematic) // RS adds 2t = 16 bytes (n-k) after original data (so is systematic)
// t=8 means we can correct 8 bytes if we don't know where the errors are // t=8 means we can correct 8 bytes if we don't know where the errors are
@ -104,19 +105,16 @@ uint8_t DVBS::gfMul(uint8_t a, uint8_t b)
// Reed Solomon encoder // Reed Solomon encoder
void DVBS::reedSolomon(uint8_t *packet) void DVBS::reedSolomon(uint8_t *packet)
{ {
uint8_t tmp[rsN]; uint8_t tmp[rsK];
const int zeroPadding = rsK - tsPacketLen; const int partityBytesIdx = tsPacketLen;
const int partityBytesIdx = zeroPadding + tsPacketLen;
// Zero pad from 188 to 239 bytes
memset(tmp, 0, zeroPadding);
// Copy in packet // Copy in packet
memcpy(&tmp[zeroPadding], packet, tsPacketLen); memcpy(&tmp[0], packet, tsPacketLen);
// Zero parity bytes // Zero parity bytes
memset(&tmp[partityBytesIdx], 0, rs2T); memset(&tmp[partityBytesIdx], 0, rs2T);
// Divide generator polynomial by input to compute parity bytes // Divide input by generator polynomial to compute parity bytes
for (int i = 0; i < rsK; i++) for (int i = 0; i < tsPacketLen; i++)
{ {
uint8_t coef = tmp[i]; uint8_t coef = tmp[i];