Home > Articles > Programming > Mac and iOS Programming

An Overview of Core Audio

This chapter looks at what’s in Core Audio and where to find it. Then it broadly surveys some of its most distinctive conventions, which you’ll get a taste for by writing a simple application to exercise Core Audio’s capability to work with audio metadata in files.
This chapter is from the book

Core Audio is the engine behind any sound played on a Mac or iPhone OS. Its procedural API is exposed in C, which makes it directly available in Objective-C and C++, and usable from any other language that can call C functions, such as Java with the Java Native Interface, or Ruby via RubyInline. From an audio standpoint, Core Audio is high level because it is highly agnostic. It abstracts away both the implementation details of the hardware and the details of individual audio formats.

To an application developer, Core Audio is suspiciously low level. If you’re coding in C, you’re doing something wrong, or so the saying goes. The problem is, very little sits above Core Audio. Audio turns out to be a difficult problem, and all but the most trivial use cases require more decision making than even the gnarliest Objective-C framework. The good news is, the times you don’t need Core Audio are easy enough to spot, and the tasks you can do without Core Audio are pretty simple (see sidebar “When Not to Use Core Audio”).

When you use Core Audio, you’ll likely find it a far different experience from nearly anything else you’ve used in your Cocoa programming career. Even if you’ve called into other C-based Apple frameworks, such as Quartz or Core Foundation, you’ll likely be surprised by Core Audio’s style and conventions.

This chapter looks at what’s in Core Audio and where to find it. Then it broadly surveys some of its most distinctive conventions, which you’ll get a taste for by writing a simple application to exercise Core Audio’s capability to work with audio metadata in files. This will give you your first taste of properties, which enable you to do a lot of the work throughout the book.

The Core Audio Frameworks

Core Audio is a collection of frameworks for working with digital audio. Broadly speaking, you can split these frameworks into two groups: audio engines, which process streams of audio, and helper APIs, which facilitate getting audio data into or out of these engines or working with them in other ways.

Both the Mac and the iPhone have three audio engine APIs:

  • Audio Units. Core Audio does most of its work in this low-level API. Each unit receives a buffer of audio data from somewhere (the input hardware, another audio unit, a callback to your code, and so on), performs some work on it (such as applying an effect), and passes it on to another unit. A unit can potentially have many inputs and outputs, which makes it possible to mix multiple audio streams into one output. Chapter 7, “Audio Units: Generators, Effects, and Rendering,” talks more about Audio Units.
  • Audio Queues. This is an abstraction atop audio units that make it easier to play or record audio without having to worry about some of the threading challenges that arise when working directly with the time-constrained I/O audio unit. With an audio queue, you record by setting up a callback function to repeatedly receive buffers of new data from the input device every time new data is available; you play back by filling buffers with audio data and handing them to the audio queue. You will do both of these in Chapter 4, “Recording.”
  • OpenAL. This API is an industry standard for creating positional, 3D audio (in other words, surround sound) and is designed to resemble the OpenGL graphics standard. As a result, it’s ideally suited for game development. On the Mac and the iPhone, its actual implementation sits atop audio units, but working exclusively with the OpenAL API gets you surprisingly far. Chapter 9, “Positional Sound,” covers this in more detail.

To get data into and out of these engines, Core Audio provides various helper APIs, which are used throughout the book:

  • Audio File Services. This framework abstracts away the details of various container formats for audio files. As a result, you don’t have to write code that specifically addresses the idiosyncrasies of AIFFs, WAVs, MP3s, or any other format. It enables your program to open an audio file, get or set the format of the audio data it contains, and start reading or writing.
  • Audio File Stream Services. If your audio is coming from the network, this framework can help you figure out the format of the audio in the network stream. This enables you to provide it to one of the playback engines or process it in other interesting ways.
  • Audio Converter Services. Audio can exist in many formats. By the time it reaches the audio engines, it needs to be in an uncompressed playable format (LPCM, discussed in Chapter 2, “The Story of Sound”). Audio Converter Services helps you convert between encoded formats such as AAC or MP3 and the uncompressed raw samples that actually go through the audio units.
  • Extended Audio File Services. A combination of Audio Converter Services and Audio File Stream Services, the Extended Audio File APIs enables you to read from or write to audio files and do a conversion at the same time. For example, instead of reading AAC data from a file and then converting to uncompressed PCM in memory, you can do both in one call by using Extended Audio File Services.
  • Core MIDI. Most of the Core Audio frameworks are involved with processing sampled audio that you’ve received from other sources or captured from an input device. With the Mac-only Core MIDI framework, you synthesize audio on the fly by describing musical notes and how they are to be played out—for example, whether they should sound like they’re coming from a grand piano or a ukulele. You’ll try out MIDI in Chapter 11, “Core MIDI.”

A few Core Audio frameworks are platform specific:

  • Audio Session Services. This iOS-only framework enables your app to coordinate its use of audio resources with the rest of the system. For example, you use this API to declare an audio “category,” which determines whether iPod audio can continue to play while your app plays and whether the ring/silent switch should silence your app. You’ll use this more in Chapter 10, “Core Audio on iOS.”

As you develop your application, you’ll combine these APIs in interesting ways. For example, you could use Audio File Stream Services to get the audio data from a net radio stream and then use OpenAL to put that audio in a specific location in a 3D environment.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020