It’s known that:
“Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:
Class Ia 50 bits - most sensitive to bit errors
Class Ib 132 bits - moderately sensitive to bit errors
Class II 78 bits - least sensitive to bit errors”
Which exactly parameters of speech include into class Ia, into class Ib and II. There are many parameters at this coder such as LAR [0..7], D[0..3], G[0..3], X1max[0..3], X[0..12][0..3]. But I don’t know which exactly “bits of this block were more important for perceived speech quality than others”.