IRC Audio

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What is -10 and +4 and what's a
balanced line?

Professional level audio lines differ from consumer lines in two ways. First, consumer lines tend to run about 14 dB lower in level than pro lines. Second, professional lines run in differential, or balanced, configuration. In a single-ended line, the signal travels down one conductor and returns along a shield. This is the simplest form of audio transmission, since it is essentially the same AC circuit you learned about in high-school physics. The problem here is that any noise or interference that creeps into the line will simply get added to the signal and you'll be stuck with it. In a differential line, there are three conductors. A shield, a normal "hot" lead, and a third lead called the "cold" or "inverting" lead, which carries a 180-degree inverted copy of the hot lead. Any interference that creeps into the cable thus affects both the hot and cold leads equally. At the receiving end, the hot and cold leads are summed using a differential amplifier, and any interference that has entered the circuit (called "common-mode information" since it is common to both the hot and cold leads), gets canceled out. Differential lines are thus better suited for long runs, or for situations where noise or interference may be a factor.

What is meant by "impedance matching"? How is it done? Why is
it necessary?

What are ground loops and how do I avoid them?

One of the most difficult troubleshooting tasks for the audio professional is finding the source of hum, buzz and other interfering signals in the audio signal. Often these are caused by "ground loops." This unfortunate and inaccurate term (it need not be in the "ground" path, and the "loop" is not what causes the problem) is poorly understood by most users of audio equipment. A better name for this phenomenon is "shared path coupling" because it happens when two signals share the same conductor path and couple to each other as a result. Another semantic problem that should be addressed early on is the idea that "ground" is one place where all currents go. It's not, there's nothing special about calling a signal "ground," current still flows through any path that's available to it. Referring to the discussion above regarding unbalanced signal paths, recall that there must be a complete circuit from the output of some device, through the input of another device and back to the "return" side of the output if any current is to flow. Current doesn't flow by itself, it must have a complete path. If there are multiple paths over which the current might flow, the current will be divided among them with most of the current flowing through the path having the least resistance. Any available path, regardless of the resistance in it, will carry some of the current, it's not a case of all the current following the path that has least resistance. For example, suppose we have two units connected together through a small piece of coaxial cable, and the units are also connected together at the wall outlet through their grounded power cords -- the ground pins are connected to the chassis at each end. The audio signal goes along the center of the coaxial cable, and part of it might come back along the shield of the coax, but part will also go through the ground wire of one unit and back through the ground wire of the other unit. A problem arises when some other signal is also flowing through this same return path. The other signal might be another audio signal, video, data, or power. All of the currents in a wire add together, and the resistance of the wire causes a voltage to appear in proportion to the current flowing. All of these voltages add together, so there is a little bit of the video signal added to the audio, some of the power signal added to the video, some of the power signal added to the audio, etc. In rare instances, the "loop" of wire formed by the intended ground return path. The happenstance lower resistance return path formed by mounting hardware, power cords, etc. can form a magnetic pickup as well, so that magnetic fields radiated by transformers, CRT's, etc. can also induce a current in the "loop," which makes yet another source of noise voltage. This shared path coupling is a constant problem with unbalanced audio systems. Lots of different methods have been tried to get around the problem, many of them dangerous. Clipping off the ground leads of equipment so there is no common power line path between them simply makes any fault or leakage current follow some other path. Back through the signal cable to some equipment that has a ground -- perhaps through the user's body, if all the ground pins have been removed. The only general solution to "ground loop" coupling with unbalanced equipment is to connect all the chassis together with a very low resistance path (copper strap or braid, for example), on the principle that since the resistance is so low, any leakage current will produce a correspondingly low signal voltage. It may also be effective to interrupt the ground path of shield conductors over signal wires; force the return path to go through the designated common strap while leaving the shield in place only for electrostatic screening. With balanced equipment, no current should be flowing in the shield conductors, and in fact performance should be identical with the shield left disconnected at one end (preferably the receiver end). Therefore balanced systems should be impervious to shared path coupling or "ground loop" problems. But in fact they aren't, because most signals inside a given piece of equipment are unbalanced, and there are often return paths internal to the equipment that can be shared with return paths between other units of equipment connected to it. Especially with mixed digital, video and audio signals and high gain, high negative feedback amplifier circuitry, this can be a big problem � small currents can create big effects.

What is the "Pin 1 problem" and how do I avoid it?

This is a special case of "ground loop" or shared path coupling. Suppose you have a mixer, whose balanced output is connected to an amplifier's balanced input through a correctly wired cable. Both units are powered from the AC mains and one or both have some small amount of AC leakage current that travels to ground through all available ground paths -- including the shield of the cable that connects the two units. So far so good, no harm done because the circuit is balanced and any common mode voltage from current flowing through the shield will be canceled by the amplifier input. However... a small part of this leakage current also travels through the shield of the wire going from the back panel XLR connector to the PC board, through some "ground" traces on the PC board, and back out through the power line ground cable. No problem so far, except that some gain stage on that same PC board also uses that piece of ground trace in its negative feedback loop, and some part of that leakage signal will be added to the signal in that gain stage. It might be video, or data, or other audio signal(s), or (most commonly) power. The solution to this variant of shared path coupling is the same sort of approach that applies to other unbalanced signals: give the leakage current a very low resistance path to follow, and remove as many of the shared paths as possible. Within a unit of equipment, all the XLR connectors' pin 1 terminals should be connected to ground with very low resistance (big) wire or traces, and preferably all of the ground connections should be made at one point, the so-called "star ground" system. A brute force approach is to assume that the back panel is the star ground, and wire every connector's pin 1 solidly to the panel as directly as possible, and lift all the ground wires but one that go from the connectors to the circuitry. In this way, all the external leakage currents will be conducted through the back panel and out of the way. Rather than running them through the ground traces on the PC board where they will mix with internal low level signals in high gain stages. Individual wires can be run from points on the circuit board that need to be at "ground" potential to a common point on the back panel, which is designated a "zero signal reference point" (ZSRP). Equipment that has a reputation for being "quiet" and easy to use in many different applications is often found to be wired this way, while equipment that is "temperamental" if often found to be wired in such a way that leakage currents are easily coupled to internal signal lines. There's a simple test that can be done to check equipment susceptibility to this problem. Connect the output, preferably balanced and floating, of an ordinary audio oscillator to the pin 1 of any two XLR connectors on the equipment. Now operate the equipment through its various modes, gain settings, etc. You may be surprised to find the audio oscillator's signal appearing in many different places in the equipment.

How do I 'ring out' a system?

Ringing out a 'system' simply means creating an equal opportunity environment for frequency. Hence this environment is where feedback at any given frequency is no more likely to feedback than any other frequency, (i.e. flat). In a 'rung out' system the response of the system should be flat verses the response of the microphone that is primarily driving the system with consideration to the various problems that a room can contribute. This is primarily true in monitors where often the mix is centered on a particular microphone and less true at front of house where you don't want to base your mix off the response of a SM58. Front of house will want to be truly 'flat' to be rung out. Any microphone characteristics, which are undesirable (like many common live mics), can be dealt with on the channel EQ's. When 'ringing' out a system, whether front of house or a monitor mix, the best place to start is with your voice. Using a microphone which you are used to the sound of, listen to your voice over the system and using the equalization available, use at least a third octave, remove any obvious frequencies that sound out of balance. A good way to do this, especially if you are not use to identifying frequencies, is to 'bump' up each band and get a feel for what it sounds like and then make the decision whether or not it is in balance with the rest. When adjusting the EQ be careful not to remove too much of any frequencies. Abuse of the EQ can cause many more problems than it solves regarding feedback, as well as create problems with the overall balance and gain structure. In general if the speakers and system in general are of high quality and the room is not too awkward you probably wont be pulling any more than 4 or 5 db out of 6 or so bands. If you notice your pulling out more than 10 bands with 6+ db, it might be best to flatten and start over. Many times a RTA can come in handy for doing this. Often after starting off with the technique above I will use a RTA to check for any problem areas. Sometimes just your voice alone won't make you aware of certain acoustical problems of a room. Also keep in mind that the response of a system, and a room, is going to change with the sound pressure level. When your listening to the system turn it up a bit, get closer to the volume that the show will be performed at. Spending time adjusting the system prior to show time can reduce many headaches during the show. One way I often judge whether the system is 'rung out' is to turn it up with a live microphone hooked up a bit past the typical maximum level. When the system just starts to feedback listen to the frequencies that ring. If it is apparent that there is more than one frequency feeding back at the same time then you at a good point. This indicates that there is not a single frequency in the bandwidth which will feedback before any others, this is the goal of ringing out the system. The more full bandwidth the ring the less likely that a particular frequency will feedback later on. Getting use to doing this is one of the keys to being good at it. Try and keep in mind what various types of speakers sound like as you encounter them and what areas of the bandwidth they are problematic in. This will help you when time is tight.

How do I soundproof a room?

Despite what you may have seen in the movies or elsewhere, egg crates on the wall don't work! First, understand what "soundproofing" means. Here we mean the means and methods to prevent sound from the outside getting in, or sound from the inside getting out. The acoustics within the room are another matter altogether. There are three very important requirements for soundproofing: mass, absorption, and isolation. Actually, there are also three others: mass, absorption, and isolation. And to finish the job, you should also use: mass, absorption, and isolation. Sound is the mechanical vibration propagating through a material. The level of the sound is directly related to the size of those vibrations. The more massive an object is, the harder it is to move and the smaller the amplitude of the vibration set up in it under the influence of an external sound. That's why well isolated rooms are very massive rooms. A solid concrete wall will transmit much less sound then a standard wood-framed, gypsum board wall. And a thicker concrete wall transmits less than a thinner one: not so much because of the distance, but mostly because it's heavier. Secondly, sound won't be transmitted between two objects unless it's mechanically coupled. Air is not the best coupling mechanism. But solid objects usually are. That's why well isolated rooms are often set on springs and rubber isolators. It's also why you may see rooms-within rooms: The inner room is isolated from the outer, and there may be a layer of absorptive material in the space between the two. That's also why you'll also see two sets of doors into a recording studio: so the sound does not couple directly through the door (and those doors are also very heavy!). If you are trying to isolate the sound in one room from an adjoining room, one way is to build a second wall, not attached to the first. This can go a long way to increasing the mechanical isolation. Try using two sheets of drywall instead of one on each wall, and use 5/8" drywall instead of 3/8", it's heavier. But remember: make it heavy, and isolate it. Absorptive materials like foam panels from Auralex and such can only control the acoustics in the room: they will do nothing to prevent sound from getting in or out to begin with. There is a very good reference on Studio and Control Room design with lot's of good information on soundproofing and materials located on the web at the following URL: www.auralex.com

What is "phantom power"?

Phantom power is DC voltage (Usually 48 volts) that is supplied via the mic cacle without affecting to operation of any other mic on the system.

All electret and condenser mics require power for the impedance converter stage, if nothing else. You can use a battery inside or the mixer/ preamp etc. can supply the power using phantom. If that is not avaiable an external/outboard Phantom power supply is required.