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Posted by: Steve Lampen on August 02, 2012

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.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on July 27, 2012

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.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on July 19, 2012

Do you know the difference between a balanced line and an unbalanced line? If we're talking about analog audio, this difference between balanced and unbalanced is also the difference between consumer products, which are almost always unbalanced, and professional, which is almost always balanced. To go into the details of these differences would take a number of blogs, so I'm going to try and condense it here.

If you want to know more details, just invite me out to speak at your local group. Are you a member of the Society of Broadcast Engineers? Perhaps you joined the Society of Motion Picture and Television Engineers or the Audio Engineering Society. All three of these, and many other similar organizations, have local groups and often have lunch or dinner speakers. I would be delighted to fit you in my schedule and come give a presentation on balanced lines or many other subjects.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on July 12, 2012

You hear it everywhere: The world is going digital. But, hold on there, that might be true for video, but it's not true for audio. Just go visit the major networks in New York or LA. The majority of consoles they're running are still analog. Look in any broadcast or pro-audio catalog. Analog audio consoles still outsell digital audio by some huge number. And it's no wonder. The performance of analog cable was never a problem.

The limits in analog are based more on the boxes (preamps, consoles, distribution amplifiers) than on the cable itself. As long as you have reasonably low capacitance in the cable, at around 30 pF/ft. or 100pF/meter, you can go very far without significant loss. The real distance determinator is the source impedance of the device feeding that audio line. If you can get down to 100 ohms source impedance, you can go over 1,000 ft. (300m) before you're even 1 dB down at 20 kHz on that 30pF/ft. cable.

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Tags: Analog, Audio

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Blog Category: Broadcast AV

Posted by: Steve Lampen on July 06, 2012

Professional video connectors are crimped. But there's one professional connector that is still soldered, the venerable XLR. It has become the universal standard for audio wiring. It is made by many manufacturers including Neutrik, Switchcraft, Amphenol and many others. You can even find some made by ITT Canon, who invented the connector in the early 1950s. And now Belden is working on a video to show you how to solder a mic cable (or a line-level cable) into this connector and will post it as soon as it's available.

However, I wanted to add some comments which are not addressed in the video. Foremost is the tendency of some plastics to melt when heat is applied. And the problem is, the higher the performance of the cable, the more likely that plastic is to melt. The converse is equally true. The lower the quality (not the price, mind you, but the performance of the cable), the less likely it is to melt. If you have rubber insulated singles, such as Belden 8412 or our new super-strong 1776, you could hold a soldering iron on these wires for a long time. You might eventually burn through them, but it would take a while. Rubber, after all, is not a thermoplastic, it is a "thermoset" material.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on June 28, 2012

If you've ever done anything on the professional side of audio, you've probably run into a condenser microphone. These require power to work. Sometimes, that's a battery you put into the mic. Sometimes it's a separate dedicated power supply with a special multi-pin cable.

But most often these microphones use phantom power. This is power delivered by a mixer or console, which uses the same cable that the audio comes back on. Hence, the idea phantom because the cable is doing two things and the power seems to be delivered by magic.

Phantom power is set up to run on shielded balanced lines, because it uses both the twisted pair and the shield as the DC power delivery. Unbalanced microphones, therefore, cannot use this system - at least not as it is described below.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on June 21, 2012

Last time I promised to tell you how to calculate a voltage drop. This could be very valuable if you have a cable that is carrying both signal and DC power such as control cable, broadband cable for powering satellite dishes or fancy cables like SMPTE 311M that contain power conductors to remote power video cameras.

Phantom powering microphones is a bit more complicated and I will talk about that in a following blog. The hard part of all this will be finding out all the things you need to know before you even do any calculations. Don't be scared of the math. It's simple algebra. If you can add, subtract, multiply and divide, you're good to go.

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Blog Category: Broadcast AV

Posted by: Steve Lampen on June 14, 2012

One day, Georg Simon Ohm did an experiment. He built a voltaic pile - what we would now call a battery. To judge the voltage, he attached a wire to each end, held one in his hand and touched the other to his tongue. Ouch! He did notice one interesting thing. If he stuck with copper wires and he went to a larger wire, his tongue hurt more! I am not making this up! You can Google it.

That told him there must be some relationship between the size of the wire and the voltage running down that wire. He soon realized that this could be described in a formula, the formula we now call Ohm's Law. He told this to his scientific friends and they were aghast! The idea that there was a relationship between voltage and current and the size of the wire (resistance) was so controversial, that they begged him not to reveal it. And so these results were not published until his death. It is pretty much the same story as Copernicus and the sun-centered planets. And we're still using Ohm's Law today.

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