Music as the Artist and Sound Engineer Intended
Whether you are a major label, an indie, an aggregator, a recording artist, or an audio engineer, you provide the most important ingredient for Apple Music and the iTunes Store — the music itself. It is our job to faithfully and accurately deliver this music to fans around the world exactly as you intend it to be heard.
We’ve designed workflows and protocols to facilitate the best possible results, ones that live up to your highest standards for audio quality. To achieve this transparency, you can use tools and technologies from Apple to ensure delivery of the highest-quality master recordings possible into our ecosystem.
With more than a billion active iOS devices, Mac computers, Apple TVs and HomePods in the world, there’s never been a better time to refine, codify, and communicate updated information to you about the best workflows and processes to produce the billions of files streamed and delivered daily to our mutual customers in over 100 countries around the world.
Note: Apple Digital Masters replaces Mastered for iTunes as the name of the program to reflect the fact that these audio advantages are available to our entire music catalog across the Apple ecosystem, whether streamed or downloaded. All Mastered for iTunes releases are now badged as Apple Digital Masters and the Mastered for iTunes software tools are still usable for Apple Digital Masters creation.
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 fact, 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. We worked with Dolby and Fraunhofer and have constantly iterated the Apple AAC encoder to reach the industry-leading quality it achieves today. And today Apple Music delivers lossless versions of these masters to our subscribers using the Apple Lossless Audio CODEC (ALAC.)
Following the guidelines and protocols outlined in this document to create and audition AAC encodes, you can achieve simply amazing results: dynamic range that’s superior to red book CD audio, and a final product that’s virtually indistinguishable from the original studio recording.
Technology Brief
Apple Digital Masters
Mastering for Digital Delivery
For decades, the standard for consumer digital audio was the compact disc (CD) and most mastering was done with CDs in mind. But with billions of streams and downloads encoded as AAC to date worldwide, AAC is now the standard for digital music. It only makes sense to create masters specifically for this format.
What is AAC?
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 much larger digital files. The iTunes catalog was initially offered in 2003 as 128 kbps AAC files, many of which were encoded from 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. We have iterated on our encoder for over fifteen years. Our AAC encoder is now able to transparently encode higher resolution audio—creating files that retain the small footprint, portability, and ease of use Apple 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) 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 sample rates during production provides better-quality audio and a superior listening experience in the end product. For this reason, higher sample rates of 48, 88.2, 96 and even 192 kHz have become standard. In addition, 24-bit recording has become the norm—increasing workable headroom and total dynamic range.
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 distribution, whether for CD or AAC. This can be done either with software (“in the box”) or by running the mix through an additional analog stage and resampling. There are pros and cons with each technique, but both can add noise and/or distortion.
Downsampling, as the word implies, is the process of using Sample Rate Conversion (SRC) to lower the sample rate (for example, from 96kHz to 48kHz). This process may create aliasing, an undesired effect. Dithering (or adding dither) is a technique used when reducing a file’s bit depth (for example, from 24-bit to 16-bit). It is an attempt to reduce the distortion inherent in this process. Dither is a tradeoff—the distortion is reduced at the cost of added noise. (Another bit depth reduction option is called “truncating,” where the additional bits are simply removed. This can cause quantization distortion.)
Improved Conversion and Encoding for AAC
Apple’s latest encoding methodology is a two-step process. The first step in the encoding path is to use state-of-the-art mastering-quality Sample Rate Conversion (SRC) to convert the source audio to a format that is compatible with the Apple AAC encoder. The second step is to use the Apple AAC encoder to create the final AAC file. This two-step process allows for the highest possible quality to be achieved in the final AAC file.
