Formation of CRC-32 for SDT to ts. file

Question

My task is to capture the videostream from the server and write it to ts. files. I ran into a problem when trying to write a SDT package to ts. I form the packet manually by bytes and I can’t figure out how to calculate 4 bytes of CRC-32 for the end of SDT. (used golang)

I tried to get the CRC based on the checksum of the data already written to the SDT packet, but it doesn't work. In the byte-by-byte parsed SDT, CRC was different from what I calculated.

Answer 1

If you're talking about MPEG-2, then the CRC definition is:

width=32 poly=0x04c11db7 init=0xffffffff refin=false refout=false xorout=0x00000000 check=0x0376e6e7 residue=0x00000000 name="CRC-32/MPEG-2"

There is an example in RFC 4326:

00 3f 86 dd 00 01 02 03 04 05 60 00 00 00 00 0d
3a 40 20 01 0d b8 30 08 19 65 00 00 00 00 00 00
00 01 20 01 0d b8 25 09 19 62 00 00 00 00 00 00
00 02 80 00 9d 8c 06 38 00 04 00 00 00 00 00 7c
17 17 63

where the last four bytes of that is the CRC-32/MPEG-2 (in big-endian order) of all of the bytes that precede it.

Here is example C code to calculate that CRC:

#include <stddef.h>
#include <stdint.h>

uint32_t crc32mpeg2(uint32_t crc, void const *mem, size_t len) {
    unsigned char const *data = mem;
    if (data == NULL)
        return 0xffffffff;
    for (size_t i = 0; i < len; i++) {
        crc ^= (uint32_t)data[i] << 24;
        for (unsigned k = 0; k < 8; k++) {
            crc = crc & 0x80000000 ? (crc << 1) ^ 0x4c11db7 : crc << 1;
        }
    }
    return crc;
}

(When called with mem equal to NULL, the initial CRC value is returned.)

Answer 2

Here is the same (Golang) code as provided by Mark Adler (in case anyone needs it):

func CalcCRC32(data []byte) uint32 {

    var crc uint32 = 0xffffffff

    if data == nil {
        return crc
    }

    for _, v := range data {
        crc ^= uint32(v) << 24

        for j := 0; j < 8; j++ {
            if (crc & 0x80000000) != 0 {
                crc = (crc << 1) ^ 0x04C11DB7
            } else {
                crc <<= 1
            }
        }
    }

    return crc
}