31 Mar
2011
31 Mar
'11
12:54 p.m.
There are many ways of doing this subtaction. The
older modems I work on
tend to use a couple of op-amps. Modernt landline telephones often use a
disctrete transistor circuit(2 or 3 transistors [1]). Older telephones
both in the UK and US (and I assume elsewhere) use an audio transformer,
often called (at least in the UK), the 'induction coil'. There is such a
transformer in the 'Network' block I metnioned.
For those on the left side of the atlantic, the old ratshack book
"Understanding Telephone Electronics" has a diagram and explanation of
the hybrid, among other useful things.
De