Editor’s note: This document has been updated to address questions and comments from our readers. Also, as of August, 2019 Apple has rebranded their iTunes music app and Mastered for iTunes content specifications and this document reflects and notes those updates. We appreciate any and all feedback and strive to provide useful, accurate and up-to-date information for our readers.
Mastering is a complicated process—a delicate blend of art and science that requires a well-trained set of ears, a deep technical understanding of digital and analog audio, and, perhaps most importantly, the ability to assess the artistic intent of a piece of music.
The mastering engineer’s job is to shape the sonic presentation of a finished mix and optimize playback across all systems and media formats. The subsequent mastered version of the song acts as the master copy from which all copies or duplicates are derived–files for online streaming, CDs, and maybe even vinyl.
Each distribution platform, analog or digital, has its own specifications and standards regarding file type, overall loudness, and included metadata. One of the most stringent sets of requirements, or “best practice” standards, comes from Apple via their Apple Digital Master specifications. In this blog, we’ll take a look at the requirements for earning the Apple Digital Master badge, what those requirements mean, and how to make sure you’re delivering the highest quality digital audio files possible. You can safely assume that if your masters meet the Apple Digital Master requirements, the masters will also be of superior quality for any other streaming or online distribution platform.
What is an Apple Digital Master?
MaApple Digital Masters, formerly referred to as Mastered for iTunes (MFiT), refers to a set of guidelines developed by Apple in order for mastering engineers to create high quality masters for streaming on the Apple Music service and for sale through the iTunes Music Store. Apple has recently announced a change to the iTunes store, but don’t fear—the iTunes music store is not going away, but will reside inside the new Apple Music app and the video content will be available via the new Apple TV app. The idea is that the masters we provide to Apple can be converted to Apple’s AAC format while retaining the original dynamic range and fidelity that exceeds 16bit 44.1kHz red book CD quality standard.
Apple uses an audio format known as Advanced Audio Coding (AAC) for compressing and encoding digital audio. While similar to mp3, AAC files offer improved encoding algorithms over MP3 files and typically provide better audio quality for similar sized encoded files. In 2003, when the iTunes Store was first launched, Apple used 128 kbps AAC files, but over time the iTunes catalog was upgraded to iTunes Plus, which uses a 256 kbps (VBR) AAC encoding format.
AC and MP3 descriptions.
The term kbps, or kilobits per second, describes to the data rate of an mp3 or AAC, and therefore also describes the size of the compressed audio file. A 16-bit 44.1 kHz uncompressed .wav (referred to Linear PCM) file has a data rate of 1.4 megabytes per minute, so a 3-minute song would have to stream just over 30 megabytes of data. A 256 kbps AAC file streams only about one-fifth of that data for the same song. Common bit rates include 128, 192, 256 and 320 kbps. iTunes Plus uses 256 kbps encoding.
The description VBR refers to variable bit rate, as opposed to CBR, or constant bit rate. Variable bit rate is simply an efficient way to create smaller files without sacrificing audio quality. The bit rate of a VBR file varies as a song streams to supply the required amount of data. When the audio is very simple, a lower bit-rate can be used and when the music is more complex, a higher bit rate is used. iTunes Plus uses a type of VBR encoding referred to as Average Bit Rate.
iTunes Plus 256 kbps (VBR) audio files typically sound as good or better than larger 320 kbps mp3 files. AAC files also support multi-channel audio formats, as well as wider frequency response and superior transient response, as compared to mp3 files.
The sample rate of an audio file describes the frequency response of the capture audio, and we want to reproduce frequencies up to at least 20 kHz. To reproduce 20 kHz, the sampling rate must be at least double the frequency, so a minimum sampling rate of 44.1 kHz must be used. Apple’s latest encoding methodology uses sample rate conversion (SRC) to resample your master file to a sample rate of 44.1kHz, regardless of the original sample rate of your supplied master .wav file. Apple uses an excellent algorithm to resample the audio in order to produce the best sounding 44.1kHz file.
The bit depth of an audio file determines the dynamic range of an audio signal. Each bit represents approximately 6 dB of dynamic range. The most common bit depths for audio files are 16-bit (CD quality), which provides 96 dB of dynamic range, and 24-bit (typical DAW file), which provides 144 dB of dynamic range. Regardless of the audio files bit depth, 32-bit floating-point processing has become common inside DAWs. Without getting into the pros and cons of 24 bit integer math vs 32-bit floating point binary math, we can safely say that, 32-bit floating point internal processing provides more accurate (better sounding) processing of audio inside a computer. Audio interfaces and analog to digital converters on the market today provide either 16-bit or 24-bit audio to our DAW, but the math inside our DAW may be done at the higher resolution that 32-bit floating point math provides. When outputting a 24-bit final master from a DAW running at 32-bit float, be sure to dither from 32-bit to 24-bit. Masters may be output from many DAWs at 32-bit float resolution, but be sure that 32-bit audio files can be accepted by the distributor or end user.
How Sample Rate and Bit Depth fit into Apple Digital Masters
In order for Apple to create the best sounding streaming files, they use a two-step process to convert supplied masters. According to Apple, the file is first sample rate converted at 32-bit float resolution and then the file is encoded into the AAC format. This process avoids clipping and retains the dynamic range of the original file. Additionally, this process avoids the necessity of adding dither to your master.
In order to ensure that your music may be encoded to AAC files as transparently and faithfully as possible, Apple has created a document that outlines the best practices for delivering files to Apple. Apple Digital Masters certified mastering engineers and providers can be trusted to follow Apple’s best practice recommendations, and Apple relies on the honor system to enforce their specifications. Below are the Apple Digital Master delivery recommendations:
• Provide high-resolution masters, ideally at 24-bit, 96 kHz. It’s important that you maintain the highest possible resolution throughout the production process, but upsampling files to a higher sample rate won’t add information or improve the sound of your project. This means if you create your masters at 24-bit, 44.1 kHz, you should not upsample to 96 kHz. Simply keep the files at 24-bit, 44.1 kHz.
• Avoid clipping at all costs. Make sure your tracks have sufficient headroom. Although Apple will not reject files for a specific number of clips, tracks with audible clipping do not qualify for Apple Digital Master status. Apple suggests leaving 1 dB of headroom below 0 DBFS as your ceiling when creating your final, mastered, .wav file. When using a digital limiter, simply set the ceiling to -1.0 dB (True Peak, or ISP enabled).
• Check your masters on the devices your audience will be using. Take into account the limitations of these devices, as well as the listening environment of your audience. There are many software tools to audition your master as an encoded AAC file, such as Ozone 8’s codec preview or Sonnox’s Codec Toolbox, among others.
In order to make sure your track qualifies for Apple Digital Master status, Apple has provided access to the actual tools they use to verify the integrity of your master.
The Master for iTunes Droplet is a standalone, drag-and-drop tool for encoding masters in Apple’s AAC format. It creates an AAC audio file from an AIFF or WAVE source file. If the sample rate of your .wav file is higher than 44.1 kHz, it’s downsampled to 44.1 using Apple’s optimized SRC. Alternatively, if you are familiar with using Terminal on Mac OS, creating AAC files can be done manually using the command-line utility afconvert. Examples of these command-line codes are included in the Apple Digital Masters document.
Afclip is another command-line utility that can be used to check files for clipping. Afclip generates a unique stereo audio file that uses the left channel for the original audio and the right channel for a graphic representation of each clipped sample so you can quickly identify when and where a file is clipping. It also produces a text readout with each instance of clipping, as well as a summary of how many total clipped samples the audio file contains.
AURoundTripAAC is an Audio Unit plug-in that lets you compare audio encoded using iTunes Plus AAC against its source file to quickly A/B the changes. It can be used in any audio unit host application, such as Logic or AU Lab.
The Audio to WAVE Droplet is a standalone tool that creates .wav files from any audio file natively supported on Mac OS X. It can be used to decode AAC files to 24-bit .wav files just like afconvert, and allows you to decode multiple files at once.
Delivery requirements for mastering engineers are constantly evolving. As technology advances and better sounding formats are introduced, mastering engineers must adapt to the new standards. Back when CDs were the de facto format, mastering engineers were only required to submit 16-bit 44.1 kHz files. Today, every distribution service and aggregator has their own set of standards for submitting digital audio files.
As a mastering engineer, it’s typically considered best practice to provide multiple files for clients. While the 16-bit 44.1 kHz file may be best for CD duplication, it is not a high resolution format and is not best for conversion to compressed streaming formats like AAC or mp3. Along with a properly dithered 16-bit file, you should provide your client with an additional high-resolution Apple Digital Master quality file to make sure your work will sound great on high fidelity playback systems, as well as when encoded to lossy distribution formats.
The Apple Digital Master Badge
Apple’s Digital Master rules are simply best practice guidelines. As previously mentioned, we are on the honor system to produce and deliver the best quality masters possible. Apple provides a list of Apple Digital Master providers to their content developers, but any facility can produce Apple Digital Masters as long as they understand and abide by Apple’s guidelines. Unfortunately, if you would like the iTunes store to display the “Apple Digital Master” badge for your music, things get a little complicated. Albums distributed by major labels will almost certainly display the badge, but if you use an aggregator like Tunecore, Distrokid or many others, you must do your homework to see how they handle your files and if they even provide the option for Apple Digital Master certification. Distrokid, for example, requires that you email them to ask for specific instructions for uploading your file and receiving the Apple Digital Master badge.
In the end, we all desire to distribute the highest quality version of our music possible. As encoding technologies improve and streaming bandwidth increases, the customer will receive better and better sounding files, and as long as we follow a best practice, like the Apple Digital Master standards, our master files will always be optimized for creating the best sounding product.