Corey Bailey
           Audio Engineering
5.1 Channel Audio (or sound) This is a listening environment that involves the use of six separate speakers. The placement configuration is: Left, Center, Right, Left Surround, Right Surround and a Subwoofer. The left, Center and Right speakers are placed in front of the listener and the Left and Right Surround speakers are placed to each side or slightly to the rear of the listener. The Subwoofer is generally placed somewhere to the front of the listener. The exact placement of the Subwoofer is dependent on its individual design and the acoustics of the listening room. The term “5.1” refers to the individual channels with the “.1” referring to the subwoofer since its bandwidth is limited to the lower part of the audio frequency spectrum. AM This is an abbreviation that stands for Amplitude Modulation. We all know what AM Radio is (at least, in the U.S.A.), but understanding what Amplitude Modulation is will take some explanation: Radio transmission uses a set frequency called a Carrier Frequency. This carrier frequency is demodulated (eliminated) by your receiver. In the case of AM radio, the carrier frequencies used are in the Kilohertz range (100,000 cycles per second). Amplitude Modulation simply means that the carrier frequency is modulated by the amplitude (volume or strength) of the signal being transmitted. Once your radio receiver strips out the carrier frequency, what’s left is the signal being transmitted. The number base used to tune the station is representative of the carrier frequency used by that particular radio station. XYZ 950 would mean that the AM radio station XYZ is transmitting with a carrier frequency of 950 Kilohertz (kHz).   Bandwidth Defined simply as a given range of frequencies (audible or not). We humans, for example, have an audible hearing bandwidth ranging from approximately 20 Hz (cycles per second) to 20 Kilohertz (kHz). Clipping Audio, as we know it and are used to hearing it, consists of symmetrical sine waves whether it’s a clarinet, a cricket, or a cymbal crash. Whenever the volume of undistorted audio is increased beyond the limit of a given piece of electronic equipment or a medium such as digital recording, the nice round tops of those sine waves are flattened or “clipped off.”    The result is very noticeable. dB dB is the symbol abbreviation of a decibel. The decibel is a unit of measurement expressed as a mathematical logarithm and is one tenth of a bel, which itself, is a unit of measurement that is seldom used. For example: A change in power ratio by a factor of ten is a 10 dB change. A change in power ratio by a factor of two is approximately a 3 dB change. The bel and the decibel originated at Bell Telephone Laboratories as a unit of measurement for the signal loss (or gain) in telephone circuits. The bel was so named for the founder of Bell Telephone, Alexander Graham Bell. It’s worth noting here that Bell Telephone Laboratories established the ground rules and practices that have been used by professional audio installations since the early part of the twentieth century and are still in use today. There are a few flavors of dB in use. Some examples are: dBm (The “m” for milliwatt) is a measurement of power where “0” dBm equals one milliwatt of power. 0 dbm is 0.775 Volts RMS referenced into a 600 ohm resistor or load. dBu In this case, the “u” stands for unterminated or unloaded. 0 dbu is 0.775 Volts RMS not referenced into a given termination or load. dBV This signifies a reference level of 1 volt RMS, regardless of the load and thus, will vary as the load varies. DBFS This is actually a definition as opposed to a reference level. It refers to the decibel level below “O” VU, full scale. It came about with the advent of digital recording where the absolute maximum level is 0VU or full scale digital. For example: The professional audio standard most often used for digital audio recording is -20DBFS which is a reference level of 20dB below full scale digital or 0VU. Feedback  Most commonly associated with that bone jarring, teeth rattling howling and screeching that occasionally happens at live venues when the volume of a particular microphone is turned up too high or the microphone itself is placed too close to one of the PA speakers. This is known as acoustic feedback and it happens because microphones are very sensitive to small changes in atmospheric pressure while speakers are designed to effect very large changes in atmospheric pressure. When a particular speaker is reproducing the amplified output of a microphone, feedback can happen when the former is placed in close proximity to the latter because, in effect, you have created a closed loop. Ironically, feedback at the electronic component level is what makes all this possible. In the world of electronics, amplifier gain is controlled by routing a small, carefully controlled, amount of the signal from the output of an amplifier back to its input……Feedback! FM The abbreviation for Frequency Modulation, FM is the household acronym in the U.S.A. that refers to the commercial radio band which is broadcast using a carrier frequency between 87.5 and 108 Megahertz (millions of cycles per second or MHz).  The 87.5 to 108 MHz band is fairly common throughout the world with a couple of exceptions: Japan and Russia, where a wider frequency spectrum is used. Frequency Modulation is accomplished by modulating the carrier frequency with the frequencies of the signal being transmitted. Once your radio receiver strips out the carrier frequency, what’s left is the signal being transmitted. An interesting benefit of FM radio transmission is that the noise power decreases as the signal power is increased. Thus, a 50,000 watt transmitter will have a better signal-to- noise ratio than a 10,000 watt transmitter.   Harmonic Distortion and Total harmonic distortion (THD) Harmonic Distortion is a consequence of electrical amplification. The actual harmonic component measured at the output of an audio amplifier is: Total Harmonic Distortion plus Noise (THD+N). First, we need to understand just what a harmonic is: A harmonic is any frequency that is a multiple of a fundamental frequency. A harmonic can be any multiple or any number of multiples of the fundamental frequency. Imagine a guitar string. If you pluck the string exactly in the middle, it vibrates at its fundamental frequency plus a second harmonic frequency resulting from the string being divided in two when plucked. Each half of the string also vibrates at the same time as the full length string is vibrating, creating a second harmonic,  which, in this case, is twice that of the fundamental frequency. Now, as it turns out, all electronic amplifiers create harmonics of the fundamental frequencies being amplified. The type and amount of harmonics generated are a consequence of amplifier design and the amount of amplification involved. When the harmonic distortion of an amplifier is measured, a pure tone is applied to the input of the amplifier by a piece of equipment known as a distortion analyzer  (makes sense). The output of the amplifier being tested is fed back to the distortion analyzer, which removes the pure tone, measures what's left, and usually displays the results in decibels. The displayed results are all of the harmonics generated by the amplifier plus any noise added by the process of amplification: Total Harmonic Distortion + Noise.   Hertz (Hz) No, this is not about the car and truck rental company. Hertz, abbreviated Hz, represents the definition of frequency. Frequency, in this case, is referring to cycles per second (cps). Hertz is the surname of Heinrich Hertz, a German-born PhD physicist who, in the late Nineteenth century, was able to conclusively prove the existence of electromagnetic waves. The most common multiples of Hertz are: Kilohertz, is one thousand cycles per second (KHz). Megahertz, is a million cycles per second (MHz). Gigahertz, is one billion cycles per second (GHz) Intermodulation Distortion (IMD) This gets a little more complicated than simple harmonic distortion because IMD is the byproduct of non-linear amplification and is not harmonically related to the signal being amplified. As an amplifier creates harmonics of the fundamental signal, some of those harmonics themselves create sum and difference frequencies. This process is called….you guessed it!….intermodulation. Those frequencies created by the process of intermodulation are measured as intermodulation distortion. Using a distortion analyzer, IMD is measured by feeding two pure tones into the amplifier under test, subtracting them at the amplifier’s output, and measuring what’s left. Loudness Control The switch on your stereo system that compensates for the apparent loss of bass and treble at lower volume settings. Our hearing is level or loudness-dependent. As volumes decrease below normal speaking levels, we tend to loose sensitivity at the lower and higher frequencies, leaving only the midrange (2-5 kHz) which, curiously, is where most of the energy of the human voice resides. Two researchers by the names of Fletcher and Munson documented this phenomenon in the 1930’s. Thus, the loudness control on your home audio equipment is designed to create what is known as an equal loudness contour that compensates for our level-dependent hearing. Minus 10 (-10) Technically speaking, Minus 10 is 10 dB below the reference point of 0 dBu. So, what is 0 dBu you ask? 0 dBu is 0.775 Volts RMS not referenced into a given termination or load. That said, Minus-10 dBu is actually the “0” reference level for consumer analog equipment. The reference voltage (for those still interested) is 316 millivolts (Mv) or 0.316 Volts. MONO or: Monaural Defined simply as “single-channel.” Although there was some early experimentation with stereo recording, nearly all of the recordings produced from the beginning of recorded sound up through the middle 1950’s were monaural.  Peak Music Power More of an advertising slogan than an actual reference measurement, Peak Music Power is measured with an amplifier operating at 100% power. And, this is assuming 100% efficiency, which few (if any) production model amplifiers can achieve. In this case, the peak power rating is referring to the maximum output power that can be achieved for an instant without destroying the amplifier under test. Manufacturers will often connect the amplifier under test to a very large, brute-force power supply in order to measure the maximum peak output rating. Obviously, this is not practical or even sustainable. Manufacturers love this rating as it typically overrates the product by about 30%. Pink Noise Yes, some types of noise are defined by color. Pink Noise is all of the frequencies combined within a given spectrum. And, all of those frequencies within that given spectrum have the same power. The spectrum for Pink Noise used in the audio realm is usually 20Hz to 20kHz. Pink Noise is useful for measuring how flat (or equal) the frequency response is of a given piece of audio equipment. Pink Noise was routinely used for calibrating analog recorders. It takes a specially designed generator to produce Pink Noise, and, in order to view it in real time, a spectrum analyzer must be used. Pink Noise sounds somewhat like a waterfall. Incidentally: The sound of a waterfall (up close) would be defined as white noise, or all of the frequencies combined not within a given spectrum and not necessarily of equal power. Quadraphonic This is a discrete 4 channel system that consists of 2 front channels and 2 rear channels. “Quad,” as it was known, was actually introduced with 4 channel reel- to-reel audio tapes in the late 1960’s. By the early 70’s Quad formats started appearing for vinyl record albums. Eventually, there were three competing Quadraphonic formats for records, and the resulting confusion in the consumer market may have led to its ultimate commercial failure. One of the formats, the Sansui SQ system, was resurrected by Dolby Laboratories in the mid 1970’s and was successfully used as a theatrical sound format for feature films. It’s known as “Dolby Stereo.” Dolby reassigned the audio channels to: Left, Center, Right and one Surround channel. This is the analog format that still survives on today’s feature film releases. R.M.S. An abbreviation for the term “Root Mean Square.” This mathematical formula is useful for calculating the statistical average of a given quantity. It is used in several scientific disciplines such as the physical sciences, statistics, and electronics. In electronics, the RMS formula is used to measure an audio power amplifier’s ability to provide continuous power into a given load such as your speakers. The RMS power rating of a power amplifier is achieved using a single tone that is fed into the amplifier input at a specified voltage while the amplifier is terminated into its designed load. The signal at the output is measured using a meter calibrated to measure RMS voltages. The RMS rating of an amplifier is the power rating that matters most because it tells you the power that can be safely and continuously delivered to your speakers. If the manufacturer’s specifications do not include the RMS power rating, you can closely calculate it by multiplying the advertised peak power rating by 0.707. VU The abbreviation for “Volume Unit.” The term VU is most commonly associated with a VU meter, which is a reference meter calibrated in decibels. “0” VU on a VU meter originally corresponded to 0 dBm, or 0.775 Volts RMS referenced into 600 ohms.   In today’s professional audio world, “0” VU is often +4 dBu, which is 1.23 volts RMS at 1000 Hz. The VU meter was introduced in the late 1930's and originally contained two scales: dB on the top scale and “percentage of modulation” on the lower scale, calibrated from 0 to 100. The percentage of modulation was a useful reference for the broadcast and feature film industries. References Howard M. Tremane, “Audio Cyclopeia” Second Edition, Howard W. Sams, 1973  Rudolph F. Graf, “Dictionary of Electronics” Howard W. Sams, 1974 Glenn D. White, “The Audio Dictionary” University of Washington Press, 1987 Wikipedia, Return to TOP of page © 2012 Corey Bailey
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