Mastered for iTunes : Music as the Artist and Sound Engineer Intended

Mastered for iTunes: Music as the Artist and Sound Engineer Intended

Whether you’re a major label or a small indie, you provide the most important ingredient for iTunes—the music itself. It’s our job to faithfully and accurately deliver your songs and albums to fans around the world exactly as you intend them to be heard. We’ve designed our tools to facilitate the best possible results, ones that live up to your highest standards for music available on the iTunes Store. To achieve this transparency, you need tools and technologies from us to ensure delivery of the highest quality master recordings possible into our ecosystem. With over 315 million iOS devices capable of playing your music, there's never been a better time than now for us to communicate, codify, and distribute updated information to you about the best tools and processes used to produce the millions of AAC files delivered daily to our mutual customers in over 50 countries around the world.

Innovation and Excellence in Sound

Apple celebrates a rich history and tradition of innovation and excellence in sound for computing as well as content creation. The original Mac was engineered fully capable of supporting audio without additional hardware or software, making it one of the

first personal computers ever to ship with sound. In 2002, even before the launch of the iTunes Store, Apple received a GRAMMY Award® for technical excellence in music, the first and only such award ever given to a personal computing company. When iTunes launched, the, decision was made to standardize on AAC instead of the more popular MP3 format simply because AAC clearly provides superior audio quality compared to other codecs at similar bit rates. In working with Dolby and Fraunhofer, there have since been further improvements to AAC to get it to the level of excellence experienced on iTunes today. If you follow the guidelines outlined in this document and audition sample AAC encodes on Apple devices, you can achieve dynamic range that’s superior to red book audio and a final product that’s virtually indistinguishable from the original recording.

Mastering for Digital Delivery

Digital distribution is no longer an afterthought. It is today's dominant medium for consuming music and as such needs to be treated with utmost care and attention. For decades, the standard for consumer digital audio has been the compact disc, and most mastering has been done with CDs in mind.

In recent years the quality of digital music delivery has vastly increased, as has the number of digital music sales, with iTunes being a key driver of those sales. With more than 16 billion downloads encoded as AAC to date worldwide, AAC is the new standard for digital music. It only makes sense to create masters specifically for this format.

Frequency is the number of vibrations per second and is measured in Hertz (Hz). Human hearing spans a range from roughly 20Hz to 20kHz.

Bit rate is different than bit depth. Bit rate indicates how much data is being used per second and is calculated using the sample rate and bit depth. An iTunes Plus AAC file uses a sample rate of 44.1kHz and is encoded with a target bit rate of 256 kilobits/second. It utilizes Variable Bit Rate (VBR) encoding, which uses each bit strategically, dynamically allocating less data for simple sections and more data for complex passages.

Dynamic Range, when used as a general audio term, refers to the range of possible volumes—the difference between the softest and the loudest parts.

In digital audio, aliasing refers to audible artifacts created when higher frequencies are sampled at an insufficient rate. The result is distortion. A visual metaphor for aliasing can be found in the “wagon wheel effect”—a rapidly spinning wagon wheel filmed at a low frame rate can appear to be moving backward.

AAC (Advanced Audio Coding) is a format for compressing and encoding digital audio. AAC achieves the portability and convenience of compressed and encoded digital audio while retaining audio quality that’s indistinguishable from larger digital files, such as audio from CDs.

The iTunes catalog was initially offered in 2003 as 128 kbps AAC files, many of which were encoded from the original CD masters. They sounded great—in fact, these downloads led the industry in sound quality. More than 100 million songs were sold in this format in a little over a year, changing the landscape of legal digital music forever.

But innovation didn’t stop there. Recently, using the most advanced AAC encoder, the iTunes catalog was upgraded to iTunes Plus: a variable bit rate (VBR) 256 kbps AAC encoding format. iTunes AAC encoders are now able to transparently encode high definition audio, creating files that retain the small footprint, portability, and ease of use iTunes is known for. And they sound amazing.

High Resolution Digital Recording

Digital audio, such as that on a CD, generally uses Linear Pulse Code Modulation (LPCM, often referred to simply as PCM) to represent audio signals. LPCM works by taking snapshots of the analog audio signal and assigning each a numerical value.

The resolution of an LPCM recording is determined by the sample rate (how many times per second samples are taken) and the bit depth (how many bits are used to represent each sample). Higher sample rates can capture higher frequencies, and higher bit depths can accurately represent a greater dynamic range.

The standard for CDs is 16-bit 44.1kHz resolution, meaning that the analog signal is sampled 44,100 times per second and each sample is given a value between -32,768 and 32,767. This resolution is often referred to as 44/16.

The Nyquist sampling theorem states that to accurately represent a signal one must use a sampling rate double that of the highest frequency being represented. The highest frequency audible to humans is around 20kHz; therefore a sampling rate of over 40kHz is required to accurately capture the audible range of frequencies. Compact discs’ 44.1kHz rate is adequate for this need.

Even so, many experts feel that using higher resolution PCM files during production provides better-quality audio and a superior listening experience in the end product. For this reason, 96/24 resolution is quickly becoming a standard format in the industry, and it’s also common to see higher resolution files, such as 192/24.

Challenges with Encoding Higher Resolution Recording

An inherent challenge of working with high resolution audio has been that both the sample rate and the bit depth must be reduced to match the specifications used in mainstream

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