Dr. AIX's POSTS ā€” 25 July 2013

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I thought I would start a brief glossary of terms that relate to high-end audio production and playback systems. There’s seems to be a lot of confusion about some of these terms so let’s take a few terms at a time and see if we can get them straight. This is not meant to be a comprehensive list and I want to avoid being overly technical. Iā€™m going to focus on things that have particular relevance to high definition audio.

Amplification – increasing amplitude or adding gain to an audio signal in order to make the sound louder. We’re all familiar with the function of a power amplifier in an audio system. There are many different types of amplifiers.

Analog to Digital Converter or ADC – the component that converts an analog electrical audio signal to a digital representation. This can be to PCM (DXD) or DSD.

Attenuation – reduction of amplitude or gain. When you turn the volume down on your audio system, you are attenuating the level.

Bandpass Filter – a filter that allows a specified area of the entire audio frequency band to pass through without any changes. The low and high frequencies outside of the band are attenuated.

Corner Frequency – the point at which a filter or equalizer begins to affect the amplitude of an audio signal. It is the point that is 3 dB up or down from the starting level of the signal.

Equalization – is a fancy word for tone controls or timbral modification. Engineers use equalizers to adjust the bass, middle and high frequencies of an audio signal in order to balance the sound or to creatively alter the natural sound coming in from the microphones.

HD-Audio – High Definition Audio is a recording and playback system that meets or exceeds the capacity of human hearing with regards to frequency response and dynamic range. Standard analog tape recording (with or without noise reduction), vinyl LPs and compact discs do not meet the minimum specifications of HD-Audio and therefore are not high definition.

HD-Radio – a brand name of the iBiquity company that places low bandwidth digital radio stations between the analog bands usually associated with broadcast radio. The “HD” in HD-Radio does NOT stand for High Definition but rather for Hybrid Digital. The audio quality of HD-Radio is very poor and actually much less than a low fidelity MP3 file. They are usually 64 or 96 kbps (kilo bits per second).

High Pass Filter or HPF is analog or digital process that allows high frequencies to pass while attenuating low frequencies. HPFs are frequently used during recording sessions to remove low frequencies from getting into the microphones…especially from the mic stands themselves.

Low Pass Filter or LPF is an analog or digital process that allows low frequencies to pass while attenuating the higher frequencies.

Nyquist Frequency – is half of the sampling rate in a PCM digital system. For example, if the sampling rate is 44,100 Hertz then the Nyquist Frequency is 22,050 Hz. This is the highest frequency that can be captured by the system and it causes problems if any frequencies higher than the Nyquist are presented to the ADC.

PCM ā€“ Pulse Code Modulation is a digital encoding and decoding scheme that periodically samples the amplitude of an audio waveform and stores the amplitude in a binary word of varying length (typically 8, 16, 24 or 32 bits). Conversion from analog to digital and digital to analog requires a LPF.

Sample Rate – the number of samples taken in a specific amount of time (usually per second) in a digital audio system. This can apply to PCM (DXD) or DSD digital systems.

Word Length – the number of binary bits (or bit in a DSD system) used per sample to measure the amplitude of an incoming analog signal.

Let’s leave it at that for today. I’m traveling to Washington DC for the Capitol Audio Fest but will post again on Friday.

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About Author

Dr. AIX

Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

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