Obtains the actual format based on the input primary format and sub-format.
The actual format is 4-byte long. The higher 4 bits of the first byte are reserved, and the lower 4 bits are c0 format. The second and third bytes describe the sub-format, and the fourth byte describes the primary format, as shown in the following:
/*
* ---------------------------------------------------
* | 4 bits | 4bits | 2 bytes | 1 byte |
* |------------|-------------|----------------|--------|
* | reserved | c0 -format | sub-format | format |
* ---------------------------------------------------
*/