Telex 38109-977 manual X X X X X X

In the late 1970’s, microprocessors became available and the first truly intelligent intercom system, the McCurdy 9400, was delivered. This was the first system that used data sent from the user stations as opposed to one wire per intercom key. As microprocessor technology improved, the 9400 was replaced by the 9500 series. This series was more dense, allowing a 50 X 50 system in 3RU. The technology was modern; a very conventional square array of switches allowing any input(s) to be switched to any output, but the implementation was somewhat limited by what is called the “square law” problem.

Briefly, in traditional matrix technology, in communications, audio, and video routing systems, the size (electrical and physical) of a matrix is related to the number of inputs and outputs, or “ports”, in a mathematical “square law” relationship.

Figure 4.3 A Comparison 3x3 vs. 9x9 Matrices

	3 X 3 Matrix
with 32 (9) Crosspoints
1	X X X
2	X X X
3	X X X
	1	2	3

				9 X 9 Matrix
		with 92 (81) Crosspoints
1 X X X X X X X X X
2	X X X X X X X X X
3	X X X X X X X X X
4	X X X X X X X X X
5	X X X X X X X X X
6	X X X X X X X X X
7	X X X X X X X X X
8	X X X X X X X X X
9	X X X X X X X X X
	1	2	3	4	5	6	7	8	9

If you examine Figure 4.3, you can see that the 3 x 3 matrix, which is needed to support a three-user intercom system, has nine crosspoints. The 9 x 9 matrix, for nine users has 81 crosspoints, so by tripling the number of users, the size of the matrix has increased from 9 to 81 crosspoints or nine times. As nine is equal to the threefold increase in number of ports squared, the term “square law” has come to represent the problem.

48 H a n d b o o k o f I n t e r c o m S y s t e m s E n g i n e e r i n g