Phew! Another long tradeshow season is behind us! Reflecting on this I remember how in the old days I would often stop at the booths of manufacturers of consoles and mixers with a simple question, “What is the CMRR in your inputs and outputs?” I would get one of three responses. Those truly world-class manufacturers would tell me, “I know what that is…,” and look it up in the Owner’s Manual or Installation Manual for that console. The second response would be, “We’ll get back to you…,” and then they would go off and figure out what CMRR is. The third response, from those inexpensive mixer manufacturers, was, “Huh???” Read more
If you’ve read the last few blogs, you’ll know we’ve been discussing balanced lines, differential signals, and common-mode noise rejection. But one of the key factors we really haven’t discussed is the cable itself.
In the last blog, we talked about how electromagnetic noise signals, called common mode noise, hits both wires in our twisted pair and cancels out. But, to be truthful, the noise is cancelled out only if the two signals are identical. The closer to identical they are, the greater the rejection or common mode rejection ratio, CMRR will be.
We have a saying in cable manufacturing: physicals equal electricals. What this means is that the two wires need to be physically identical if we want the noise signals to also be identical, and the noise rejection to be 100%.
Well, aren’t the two wires in a twisted pair ‘automatically’ identical? After all, we twist those wires together. Isn’t that enough? Sure, twisting helps a lot, but it doesn’t make the two wires identical. Those two wires should also be the same AWG (gauge) size. Read more
We’ve been talking in the last few blogs about balanced lines. These are cables or circuits where the signals travel on pairs of wires, or pairs of traces on a circuit board. With cables, the pairs are usually twisted together. On a perfect balanced line, the audio signal is equal intensity but opposite polarity on the two wires. Just think of it like cars on the racetrack. If you’re zooming down the track, and you look across to the other side, those cars are going in the opposite direction.
Just like a racetrack, the signal is a circle, we call it a circuit, and we call these signals differential signals because they are moving in different directions. The fact that we twist the wires together makes it difficult to recognize it as a circle, but it still is. Read more