Transformer

Hi!


I have a delta-modulator card (Transmitter/receiver in one) for speech
application. In
the transmitter part there is a transformer (Shell core coil)..whose
input
(primary of the transformer) is labled with -- 300 to 34,00 HZ ---
and -3.5dBr/600 ohm ...
I want to know whats this rating? Does that mean that the microphone
driving the transformer
primary has a 600 ohm output impedance? Before the transformer
primary, there is a resistive
network like this:

A---------470 ohm-----
| |
| |
-3.5dBr/600 ohm 1.2K T/F primary
| |
B---------470 ohm-----|

Now does this mean that the impedance b/w A and B points should be 600
ohms? If the impedance of T/F primary is so selectec that 470+470+Zt=
1.2K then total impedance b/w A and B should be 600 ohm (1.2K ||
1.2K)...does this make sense? Can a driving microphone have 600
impedance? What is meant by -3.5dBr?

Please help.
rider