/* Filename: CRC.C * Author: Patrick Persson, Teracom Nu * Version: 1.0, 940203 * Compiler: ANSI-C * Description: General Encoder/Decoder for CRC-32 * used in MPEG-2 systems */ #include #include typedef unsigned long int UIMSBF; UIMSBF Calculate_CRC(char *first,char *last) { int bit_count = 0; int bit_in_byte = 0; unsigned short int data_bit; unsigned short int shift_reg[32]; unsigned short int g[] = { 1,1,1,0,1,1,0,1,1,0,1,1,1,0,0, 0,1,0,0,0,0,0,1,1,0,0,1,0,0,0, 0,0,1 }; int i,nr_bits; char *data; UIMSBF crc; /* Initialize shift register's to '1' */ for(i=0; i<32; i++) shift_reg[i] = 1; /* Calculate nr of data bits */ nr_bits = ((int) (last - first)) * 8; data = first; while (bit_count < nr_bits) { /* Fetch bit from bitstream */ data_bit = (short int) (*data & (0x80 >> bit_in_byte)); data_bit = data_bit >> (7 - bit_in_byte); bit_in_byte++; bit_count++; if (bit_in_byte == 8) { bit_in_byte = 0; data++; } /* Perform the shift and modula 2 addition */ data_bit ^= shift_reg[31]; i = 31; while (i != 0) { if (g[i]) shift_reg[i] = shift_reg[i-1] ^ data_bit; else shift_reg[i] = shift_reg[i-1]; i--; } shift_reg[0] = data_bit; } /* make CRC an UIMSBF */ crc = 0x00000000; for (i= 0; i<32; i++) crc = (crc << 1) | ((UIMSBF) shift_reg[31-i]); /* Invert CRC before sending */ crc = ~crc; return(crc); } int Check_CRC(char *first,char *last) { int bit_count = 0; int bit_in_byte = 0; unsigned short int data_bit; unsigned short int shift_reg[32]; unsigned short int g[] = { 1,1,1,0,1,1,0,1,1,0,1,1,1,0,0, 0,1,0,0,0,0,0,1,1,0,0,1,0,0,0,0,0,1 }; int i,nr_bits,error; char *data; /* Preset the shift_registers to '1':s */ for(i=0; i<32; i++) shift_reg[i] = 1; /* Get nr_bits */ nr_bits = ((int) (last - first))*8; data = first; while (bit_count < nr_bits) { /* Fetch bit from bitstream */ data_bit = (short int) (*data & (0x80 >> bit_in_byte)); data_bit = data_bit >> (7 - bit_in_byte); bit_in_byte++; bit_count++; if (bit_in_byte == 8) { bit_in_byte = 0; data++; } /* Check if we reached the CRC_part, if then invert bits */ if ( bit_count > (nr_bits - 32) ) if (data_bit == 1) data_bit = 0; else data_bit = 1; /* Perform the shift and modula 2 addition */ data_bit ^= shift_reg[31]; i = 31; while (i != 0) { if (g[i]) shift_reg[i] = shift_reg[i-1] ^ data_bit; else shift_reg[i] = shift_reg[i-1]; i--; } shift_reg[0] = data_bit; } /* OR shift_regs, '0' = no errors and '1' = errors */ error = 0; for (i=0; i<32; i++) if (shift_reg[i] != 0) error = 1; return(error); } void main(void) { char data_bits[100]; unsigned short int g[100],preset[100]; int nr_bits,nr_shift_regs; char *data_ptr,*crc_ptr; int not_ok,i; UIMSBF crc; data_ptr = data_bits; nr_bits = 16*8; data_ptr = &data_bits[0]; /* PAT: 00 b0 11 00 64 c1 00 00 00 00 e0 22 00 01 e0 44 */ /* SA-2 bitstream (PAT): CRC in bitstream was 99 e7 4f 4e */ *data_ptr++ = 0x00; *data_ptr++ = 0xb0; *data_ptr++ = 0x11; *data_ptr++ = 0x00; *data_ptr++ = 0x64; *data_ptr++ = 0xc1; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0xe0; *data_ptr++ = 0x22; *data_ptr++ = 0x00; *data_ptr++ = 0x01; *data_ptr++ = 0xe0; *data_ptr++ = 0x44; crc_ptr = data_ptr; crc = Calculate_CRC(data_bits,crc_ptr); /* Add crc to last end */ *data_ptr++ = (char) ( (0xff000000 & crc) >> 24); *data_ptr++ = (char) ( (0x00ff0000 & crc) >> 16); *data_ptr++ = (char) ( (0x0000ff00 & crc) >> 8); *data_ptr++ = (char) ( (0xff0000ff & crc) ); data_ptr = data_bits; printf("Data bits (HEX): "); for (i=0; i<(nr_bits/8); i++) printf("%x ",(*data_ptr++ & 0xff)); printf("\n"); printf("CRC calculated to: %lx (Inverted %lx)\n",crc,~crc); not_ok = Check_CRC(data_bits,(crc_ptr+4)); if (not_ok) printf("CRC showed parity error !!\n"); else printf("CRC checked OK\n"); data_ptr = data_bits; nr_bits = 20*8; data_ptr = &data_bits[0]; /* Teracom_3 bitstream (PAT): CRC in bitstream was 97 c9 04 37 */ *data_ptr++ = 0x00; *data_ptr++ = 0xb0; *data_ptr++ = 0x15; *data_ptr++ = 0x00; *data_ptr++ = 0x7b; *data_ptr++ = 0xc1; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0xe0; *data_ptr++ = 0x37; *data_ptr++ = 0x00; *data_ptr++ = 0x01; *data_ptr++ = 0xe0; *data_ptr++ = 0x4d; *data_ptr++ = 0x00; *data_ptr++ = 0x02; *data_ptr++ = 0xe0; *data_ptr++ = 0x4d; crc_ptr = data_ptr; crc = Calculate_CRC(data_bits,crc_ptr); /* Add crc to last end */ *data_ptr++ = (char) ( (0xff000000 & crc) >> 24); *data_ptr++ = (char) ( (0x00ff0000 & crc) >> 16); *data_ptr++ = (char) ( (0x0000ff00 & crc) >> 8); *data_ptr++ = (char) ( (0xff0000ff & crc) ); data_ptr = data_bits; printf("Data bits (HEX): "); for (i=0; i<(nr_bits/8); i++) printf("%x ",(*data_ptr++ & 0xff)); printf("\n"); printf("CRC calculated to: %lx (Inverted %lx)\n",crc,~crc); not_ok = Check_CRC(data_bits,(crc_ptr+4)); if (not_ok) printf("CRC showed parity error !!\n"); else printf("CRC checked OK\n"); data_ptr = &data_bits[0]; /* GI's bitstream: PAT (PID 0) */ /* Their CRC was c7 75 37 e2 (faulty) */ *data_ptr++ = 0x00; *data_ptr++ = 0xb0; *data_ptr++ = 0x11; *data_ptr++ = 0x00; *data_ptr++ = 0x31; *data_ptr++ = 0xc1; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x00; *data_ptr++ = 0x01; *data_ptr++ = 0xe0; *data_ptr++ = 0x32; *data_ptr++ = 0x00; *data_ptr++ = 0x02; *data_ptr++ = 0xe0; *data_ptr++ = 0x33; crc_ptr = data_ptr; crc = Calculate_CRC(data_bits,crc_ptr); /* Add crc to last end */ *data_ptr++ = (char) ( (0xff000000 & crc) >> 24); *data_ptr++ = (char) ( (0x00ff0000 & crc) >> 16); *data_ptr++ = (char) ( (0x0000ff00 & crc) >> 8); *data_ptr++ = (char) ( (0xff0000ff & crc) ); data_ptr = data_bits; printf("Data bits (HEX): "); for (i=0; i<(nr_bits/8); i++) printf("%x ",(*data_ptr++ & 0xff)); printf("\n"); printf("CRC calculated to: %lx (Inverted %lx)\n",crc,~crc); not_ok = Check_CRC(data_bits,(crc_ptr+4)); if (not_ok) printf("CRC showed parity error !!\n"); else printf("CRC checked OK\n"); }