MPEG2 Presentation Time Stamps (PTS) calculation
I have an MPEG2 TS file and now I am interested in extracting PTS information from each picture frame. I know that PTS is described in 33 bits including 3 marker bits. But I don't know how this bitfield can be converted to more understandable form (seconds, milliseconds). Can anybody help me please
Answer
The MPEG2 transport stream clocks (PCR, PTS, DTS) all have units of 1/90000 second. The PTS and DTS have three marker bits which you need to skip over. The pattern is always (from most to least significant bit) 3 bits, marker, 15 bits, marker, 15 bits, marker. The markers must be equal to 1. In C, removing the markers would work like this:
uint64_t v; // this is a 64bit integer, lowest 36 bits contain a timestamp with markers
uint64_t pts = 0;
pts |= (v >> 3) & (0x0007 << 30); // top 3 bits, shifted left by 3, other bits zeroed out
pts |= (v >> 2) & (0x7fff << 15); // middle 15 bits
pts |= (v >> 1) & (0x7fff << 0); // bottom 15 bits
// pts now has correct timestamp without markers in lowest 33 bits
They also have an extension field of 9bits, forming a 42bit integer in which the extension is the least significant bits. The units for the base+extension are 1/27000000 second. Many implementations leave the extension as all zeros.