Transport Methods

In Uncategorized by tfwm

ANALOG AUDIO

Signal formats, audio, video, data, digital, analog, computers and networks. Fiber optic cables, twisted pair, category 5, category 6. 220, 221, whatever it takes!

Who can keep everything straight and sort through the confusion? It can be especially difficult when trying to figure out what signal format is appropriate, or which signals might be compatible.

What is the best way to transport audio, video or other information? Which of the formats is the most cost effective? And which is most reliable? What do they all mean and where do they fit in the big picture?

Anyone who is involved in audio, video or lighting production has probably run into some new type of signal or transport method. Of all the people I know in the industry, the number of people who actually understand and have a grasp of all the available formats could be counted on one hand. Not to say that most of us aren’t familiar with at least some of the signal formats and transport methods. But to know and understand all of them is a daunting task.

This is the first in a series of articles that will attempt to help sort out the mess and make some sense of the signal types. The goal is to break down the jungle of signal types by discipline. We want to spend a little time with a simple explanation of each of the widely used signal formats, explaining the characteristics of each one, the advantages/disadvantages of each format, and the appropriate uses.

The goal is to dispel misconceptions, clarify and explain, and to correct some of the obvious misuses of certain terms. For example, most everyone who works or participates in our industry has a computer with a monitor. Depending on the vintage of the hardware, the monitor is likely connected by either a “DVI” cable or a “VGA” cable. You can go into any computer store and ask for a VGA cable and the sales staff will know what you are looking for. But do you know that the term “VGA” is a descriptor for a video resolution of 640×480 and not a type of cable, or even a video signal format. Our “VGA cable” is actually an analog video cable with HD15 connectors that will transport component video signals including Red, Green and Blue color information along with Horizontal and Vertical synchronization signals (RGBHV). Virtually everyone today who uses a “VGA” cable will not use it for a true VGA signal. Instead, we use them for an analog video signal of much higher resolution and refresh rate than that specified by the definition of VGA.

In this series of articles, we will describe various signal types and formats in:

Analog Audio
Digital Audio
Analog Video
Digital Video
Data and Control – as related to the AV and Lighting industry

When looking at each of these five categories, we will also discuss the various types of transport methods for each signal.

Some of our discussions will relate directly to manufacturer specific formats while others will be based on industry defined or adopted standards from the various governing organizations such as the AES (Audio Engineering Society), SMPTE (Society of Motion Picture and Television Engineers), IEEE (Institute of Electrical and Electronics Engineers) and others. This series may get a little technical for some, probably be overly simple for others, and I’m sure we’ll leave more than one format out of the discussion. However the goal is to help the basic audio and video operator achieve a better understanding of the myriad of acronyms that make up all these new (and old) audio, video and control signal types.

So stay tuned. We’ll see how all this unfolds. Hopefully we’ll make some sense of all this mess at our journey’s end. Without further ado, we bring you the first part of this article series, involving analog audio connections.

Analog Interconnections
During the writing of this article, much discussion took place with industry folks regarding the idea of signal protocol or formats. If you think about it, analog is the old-school blue collar form of audio signal transport. It may not be as sexy as one of the new digital audio transport formats, but it gets the job done very reliably. So while other categories in this series will talk about different formats or transport methods, analog audio is just a plain-Jane garden variety varying amplitude voltage signal that is generally transported over a pair of good ol’ copper wires. Sometimes we even throw a shield around the wires, and sometimes we twist them.

Level Ranges
The first issue with analog audio is the electrical level at the connection. In a sound system we commonly talk about “Speaker Level”, “Line Level”, and “Microphone Level”. These are not three exact levels but rather three different level ranges that we never want to mix.

Speaker Level is the highest, and is found at the output of power amplifiers or powered mixers. Such an output is intended to drive a loudspeaker. We never want to plug a Speaker Level output into a Line or worse yet a Microphone Level input. Connecting a Speaker Level output to a Microphone Level input is very likely to damage the Microphone Level input. Connecting a Speaker Level output to a Line Level input is not recommended, and may damage the Line Level input.

The input cables for speakers that contain their own power amplifiers (called powered speakers or active speakers) are Line Level cables, not Speaker Level cables.

Line Level is lower, and is found at the outputs of mixers, MP3, DVD, CD, and cassette players, computer audio outputs, keyboard instruments, and many other places. Line Level is the most common interconnect level going between the electronic devices that make up our sound systems. There are two sub-categories of Line Level: professional, which is somewhat higher in level, and consumer or instrument, which are somewhat lower in level. Connecting a Line Level output to a Microphone Level input is not recommended, and may damage the Microphone Level input.

Microphone Level is the lowest, and is found at the outputs of microphones and direct boxes (sometimes called DI for Direct Injection). Microphone Level has the widest range of any of these three level categories because the sensitivity of microphones varies greatly, as does the acoustic level we use them to pick up.

Microphones
The most sensitive microphones are generally what are called “condenser” or “capacitor” microphones. All such microphones require power in order to operate. The power runs a small amplifier circuit inside the microphone, which is why these microphones tend to be the most sensitive. The power may come from a battery inside the microphone, or from a special power supply the microphone must plug into, or it can come from “phantom power” sent over the microphone cable from our mixer. If a very sensitive condenser microphone is used to pickup a very loud sound (for example inside a kick drum), then the output level can approach Line Level. For most common applications, the output of a condenser mic tends to be in the upper portions of the Microphone Level range, and may require a preamp gain of from 10 to 40 dB to reach Line Level.

Less sensitive are “dynamic” microphones. These microphones do not require power to operate, and their output level generally falls into the middle of the Microphone Level range, and may require a preamp gain of 25 to 55 dB to reach Line Level.

Least sensitive tend to be ribbon microphones, unless they are “active” versions with built-in amplifiers like condenser mics. Their output usually falls at the bottom of the Microphone Level range, and may require a preamp gain of 50 to 80 dB to reach Line Level. Ribbon mics used to record a string quartet or similar fairly quiet music may require around that maximum gain.

Speaker Level connections have been made using many different connector types. Some power amplifiers are provided with screw terminals or binding posts that will accept bare wire ends so there is no connector on the end of the speaker wire. More common these days is to have a connector of some sort on the end of the speaker wire. Older amplifiers may be designed to accept double banana plugs (sometimes called GR plugs), or 1/4” plugs. A downside of the double banana plug is that it can easily be plugged in backwards and reverse the loudspeaker polarity. A downside of the 1/4” plug is that it is also commonly used for guitars and other instruments, and for Line Level inputs. Therefore it is too easy to accidentally connect power amplifier outputs to instrument or Line Level inputs or outputs and damage the equipment. Fortunately, more recently Neutrik developed the Speakon connector for Speaker Level connections, and many modern power amplifiers and loudspeakers are equipped with these connectors. They have many advantages, but foremost is they are a connector that is only used for Speaker Level connections. This greatly reduces the chances of accidental incorrect connections and the resulting damage to equipment.

Another advantage of Speakon connectors is that they have been designed to accept heavier gauge wires that are often required for Speaker Level connections. How heavy the conductors in a speaker wire need to be depend on several factors, but the main ones are; how long the wire is, and what impedance speaker(s) are connected to the far end of the speaker wire. For example for sound reinforcement, and with just a single 8 ohm loudspeaker at the end of a cable I recommend 18 gauge wire for up to 31 feet, 16 gauge wire for up to 49 feet, 14 gauge wire for up to 78 feet, 12 gauge wire for up to 124 feet, and 10 gauge wire for up to 198 feet. To calculate the minimum wire size for different situations you can use this Excel spreadsheet for low impedance speaker lines: http://www.soundfirst.com/LZ_DF_calculator.zip

For 25 or 70 volt speaker systems use this Excel spreadsheet: http://www.soundfirst.com/CV_DF_calculator.zip

Speaker cables may be of many different physical constructions, but cables with the pair of wires twisted together are preferred over cables with the two conductors in parallel (sometimes called ZIP cord). Twisted pair speaker wires are far less likely to induce interference into nearby Microphone Level or Line Level cables, and are less likely to pick up radio frequency (RF) interference. Speaker cables benefit very little from a shield around the cable, and therefore most speaker cables are made unshielded.

When we talk about Line and Microphone Level connections we also must discuss balanced and unbalanced interconnections.

Unbalanced interconnections are the simplest variety and have only a single insulated conductor in the center of the wire that is completely surrounded by an outer conductive layer called a shield. This arrangement of one wire in the center and a shield around it is called a coaxial type cable. Coaxial cables of different designs are used for most musical instrument cables, unbalanced Line and Microphone cables, and even cable TV and video connections.

Coaxial cables used for musical instrument cables need to be very flexible so they don’t tangle when the musician moves, and need to be designed so they don’t make noise when hit or stepped on. Because of the high impedance of many musical instrument outputs, it is possible for the interconnect cables to act as microphones and pick up noise when the cables are bent, hit, or crushed. Better instrument cables are designed to avoid this problem.

Unbalanced interconnections are susceptible to picking up various sorts of interference including hum and buzz. To greatly reduce the chances of interference, particularly on longer interconnections, balanced interconnections were developed.

Balanced interconnections require two wires rather than one in the cable, and that the wires be twisted. Most balanced cables have a shield around the twisted pair (STP or shielded twisted pair).

A balanced interconnection requires that the balanced input be primarily sensitive to the difference between the electrical signals on each wire of the pair, and reject or ignore signals that are the same or common to both wires. How well the balanced input performs this task is called the Common Mode Rejection Ratio (CMRR). CMRR is expressed in dB, and the higher the number the better.

In order for a balanced connection to do its job of rejecting noise and interference, the input must have equal impedance to shield from both wires in the pair with respect to signals that are the same on both wires in the pair (this is called common mode input impedance). How close to equal these impedances are will help determine how effective the input is at rejecting noise and interference. It also helps if these impedances are relatively high. Similarly the output must have equal impedance to shield from both wires in the pair with respect to signals that are the same on both wires in the pair (this is called common mode output impedance). How close to equal these impedances are will help determine how much the output helps the noise and interference rejection capabilities of the input it drives.

There is a common myth that a balanced output must have equal signal levels with opposite polarities on the two wires of the output pair in order to be balanced. While there are some advantages to such an output, equal signal levels has nothing to do with the ability of a balanced output to reject interference. It is quite possible to have a balanced output with unequal signal levels on the two sides of the pair, or even signal only on one wire of the pair.

(Vance Breshears wrote the introduction to this article. In addition to being a systems design consultant and the Audio Advisor for Technologies for Worship Magazine, Vance is just as confused with all the different signal formats as anyone else.)