HD-Audio By The Numbers: Part I Sample Rate

Specifications have always been a part of the audio hobby. I can remember looking at the THD (total harmonic distortion) or amplifier wattage numbers decades ago. These days specifications are also important…and equally misunderstood. Even if a particular listening experience or audio delivery format sounds great, knowing the raw potential of that recording is a factor in judging whether a track is really HD. Let’s begin with a discussion of sample rates.

PCM or Pulse Code Modulation is the world’s most common uncompressed encoding scheme. Yes, it’s true that there are more MP3 or AAC files downloaded, but they are lossy codecs and are not capable of HD delivery. PCM converts an incoming analog audio signal to a series of discrete sample values. The number of samples taken per second is called the sample rate. Typical sample rates are 44.1, 48, 88.2, 96, 176.4 and 192 kHz. There are even some systems in use studios today that use 384 kHz!

DSD, the encoding scheme developed by Sony and Phillips for use in the SACD format, uses a completely different encoding method but uses a sampling rate 64 or 128 times that of a standard compact disc. The DSD standard rate of 2.8224 MHz might seem a huge advance over PCM but it is not. Relying solely on the numbers for determining a format’s sound quality is as flawed today as it was back when I was searching for the equipment with the lowest THD specs.

The sample rate in a PCM digital is one of two critical specifications associated with the format. The other is the word length or the number of bits contained in each word. We’ll discuss word lengths in another post.

When I teach my advanced audio students about PCM encoding I draw a grid on the white board. I call it the “battleship” grid after the game of the same name. The vertical lines represent the samples and the horizontal lines are the amplitude values. I then draw a smooth sine wave over the grid.

sine_wave_sr

The important thing about the sample rate is that it determines the highest frequency that can be captured or digitized. There is a famous theorem by a man named Nyquist that provides the relationship between the sample rate and the max frequency. Simply dividing the sample rate in half gives the result. So a compact disc with its 44.1 sample rate is capable of capturing a 22.050 kHz tone or harmonic…and according to Nyquist it can do it without distortion. Essentially it takes two sample at the highest frequency to describe the waveform.

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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