mu-law companding

hi all,
I'm doing some tests with mu-law companding. I have a speech signal
with Fs = 16 kHz, 16-bit. I test on 2 methods to encoding in 8/4/2
bit:
- 1st methode: using uniform quantization 256/ 16/ 4 levels.
- 2nd methode: using mu-law to compand sigal before, then using
uniform quantization 256/ 16/ 4 levels, then using mu-law inrerse.

In all three cases, the SNR with mu-law companding is always lower
than do the quantization directly. can any one help?
Thks
.