The Ins & Outs Of X.By Paul WiffenIn recent months, SOS's on-line Mac forum has highlighted the difficulty with achieving the same number of channels of audio I/O under Mac OS X and Logic that were possible under OS 9. We investigate the problems and provide some solutions.Most Mac users I know are now running Mac OS X in day-to-day use, and I have been using it personally with far fewer problems than I ever encountered with OS 9, so I am always interested when I come across musicians who are still on OS 9 to discover the reasons why they haven't yet switched over.

And there are quite a few people out there whose music Macs remain stubbornly on OS 9, even if they have other computers that have migrated to OS X.Why is this? Well, there are some people who simply don't feel the need to upgrade yet, as their OS 9-based music system is meeting all their requirements. This is particularly the case amongst musicians who haven't yet got into using CPU-hungry software synths and samplers, and are still predominantly running external hardware MIDI instruments with older, slower Macs.In keeping with this, most Powerbooks I see these days are running OS X for music and audio. After all, if you want a portable system, you're more likely to want to be unencumbered by hardware and use software instruments, and to do this, you'll need a recent, powerful machine, the majority of which now only run OS X. In contrast, the majority of the non-portable Macs I see are still running OS 9.

Many owners cite the lack of availability of a particular plug-in as their reason for not wanting to move over, or sometimes a freeware or shareware program which has not yet been ported across.A more commonly cited reason, now that most plug-ins have been ported to Audio Units or VST under OS X, is that the developers (not surprisingly) want to charge something extra to 'upgrade' to the OS X version, whereas the users (also not surprisingly) don't want to pay for something which gives them no additional functionality, just the ability to run under the new OS. Whilst this might seem a rather penny-pinching attitude when applied to individual pieces of software, when you look at the number of plug-ins that people may have acquired on an OS 9 system over the years and how many different update charges it may entail to replicate this exact setup of plug-ins under OS X, it becomes understandable.However, the biggest single reason that I hear for not moving the main studio computer across to OS X is that it would currently cause a reduction in the number of available I/O channels. Indeed, the main complaint that SOS hears from end-users migrating from OS 9 to OS X is that they were not aware that this reduction of channels would happen until they embarked on the procedure and found themselves with fewer I/O channels available. I decided to investigate how this comes about, and to see what, if anything, can be done to counteract or work around this reduction in the number of inputs and outputs available to the user.Core Audio was designed to be a central part of OS X, a single I/O engine for use by all applications which would operate at as low a level as possible in the OS to reduce the amount of to and fro between applications and hardware drivers, and thus also reduce both processing demands and latency. In this it has been very successful, and most PCI, USB and Firewire audio interfaces on the market now offer their best performance under Core Audio in OS X. Drivers have become more streamlined, if they are needed at all — many of the simpler USB devices are class compliant, requiring no bespoke driver whatsoever, but registering their presence and configuring themselves automatically on connection.

What's more, installation of even the most complex devices has become much simpler and more reliable. It is a long time since I have failed to get an audio I/O device to work on the Mac, a fairly regular occurrence in the OS 9 days.Firewire interfaces such as MOTU's 828 and M-Audio's Firewire 1814 work well under Mac OS X, but Logic users cannot address multiple 828s under Core Audio, as they could in OS 9. No, that's not a misprint — MADI is a digital interfacing protocol specified by the AES10-1991 standard, which up to now has not made much of an impression on the computer interfacing market, having been developed mainly for high-end multitrack tape machines (it was first specified back in 1989). However, RME have had the smart idea of using MADI as the interface on a high-performance PCI card because it has the capability to send 64 channels of 24-bit, 48kHz audio (or 32 channels of 24-bit, 96kHz) down a single cable.

You can then access these channels via eight ADAT optical connections on an external breakout box (RME's ADI648).MADI can use two different connectors (optical connectors and co-axial BNCs), so to be an all-purpose MADI interface, the RME card has to offer both. The BNCs and optical connectors, together with the stereo monitor output which has been a feature of all the Hammerfall cards, take up all the room on the back of the PCI card, so RME have resorted to attaching a secondary back plate, as they did on the original 9652 Hammerfall card. This plugs into the main card, rather than another PCI slot, but it nevertheless uses up the slot space for another full PCI card when installed, as you can work out from the picture on the previous page.

This time the secondary expansion board has the small DIN connector for the dual MIDI I/O, and the word-clock input and output, together with an LED to show the lock status of the word-clock in. There is also a word-clock termination switch, in case your computer is the last device in the clock chain and requires termination. If you don't need the MIDI or the word-clock connections on the card, you can dispense with the add-on expansion board completely, which will save you a PCI slot.You could be forgiven for thinking that the DSP MADI is not compatible with Macs at all, as no Mac drivers are supplied in the box, nor is there mention of Mac compatibility in any of the accompanying documentation. It takes a trip to the RME web site, to find the OS X driver, which worked perfectly when I installed it. Why RME continue to hide their Macintosh light under a bushel, I don't know!

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The OS X MADI Control Panel installed with the Hammerfall DSP MADI.Once the driver was downloaded and installed, the card appeared instantly as an available device in the test Mac's System Preferences Sound control panel, Audio MIDI Setup Audio Devices and MIDI Devices pages and the Core Audio devices sub-menu in the version of Logic I was using to test all of this. The installation includes an HDSP MADI Control Panel, and the Totalmix HDSP MADI Mixer software, which offers Mute, Solo and Volume level for all 64 input and output channels with monitoring and matrix functionality.The Control Panel (shown, right) is where you select which of the physical card input types you are using (optical or co-axial). You also determine the sync setting here (in other words, whether you want the card to act as sync Master or use an external reference, either via the MADI connection itself, or from the dedicated word-clock input).

There's also an option to use the MADI specification's original 56-channel mode, or the more recent 64-channel mode, which was added in an update to the MADI specification in 2001, by limiting the sample rate to 48kHz. Configuring the Hammerfall DSP MADI under OS X's Audio Devices control panel. Note all the I/O channels!Obviously, if this is set up incorrectly, you'll be unable to use all possible 64 channels, and the 56-channel mode is selected as the default, which is a little odd (it certainly threw me at first!). 96kHz operation is also possible, although as you'd expect, you can then only use half the maximum number of channels (28 or 32, depending on the previous setting).You can see how the card appears in the Audio MIDI Setup page (see right). This is the first time I have had so many I/O channels available under OS X that they could not all be displayed on the screen at the same time!Having set up the MADI PCI to work nicely with the ADI648 breakout box, it now only remained to connect its eight ADAT Outs to a suitable digital device. The Yamaha DM2000 digital mixer is one of the few which can take enough ADAT cards to allow all eight ADAT Outs of the ADI648 to be run simultaneously, so this seemed the logical choice for testing.

In no time at all, there was audio coming into the Yamaha desk right across all 64 channels, which means that the full capabilities of the Yamaha mixer could be driven by Logic 's output. Similarly, the eight ADAT Inputs of the ADI648 could be fed with 64 individual signals so that Logic could be set to record on 64 channels simultaneously. Of course, practical situations where you might need this many simultaneous input channels are rare, but the important thing is that I was no longer limited by Core Audio's restrictive practices!However, all is not lost — there are workarounds for the problem, albeit ones which rely on certain combinations of hardware and software. For example, if you are using MOTU's Digital Performer software under OS X, then you can use up to four 828 interfaces to give you 32 channels of I/O. This is achieved by holding down the Shift key when selecting the audio devices from the list of those available within DP.

You can even mix and match devices from different manufacturers under Digital Performer — the hardware in question does not have to be of MOTU origin, despite the theories of some posters on the SOS forum, who have speculated that this is a MOTU ploy to encourage OS X users to buy their software and hardware. Surely, MOTU would be only too glad to be able to sell multiple 828s to Logic 's user base, for example, as they used to?It seems to me that if MOTU can get this support of multiple Core Audio drivers working in Digital Performer, it should not be beyond the capabilities of Emagic to do the same in their sequencer (especially as they are now owned by the company that developed and controls the Core Audio standard). What's more, Emagic were proud of the fact that they supported multiple drivers under OS 9, so you would think that they would want to offer their users the same flexibility that MOTU have achieved under OS X.The fundamental sticking point is that you cannot select multiple Core Audio drivers inside Logic in the same way that you can in Digital Performer. This lack of ability to select multiple Core Audio drivers at the level of the Mac OS remains a mystery, because it's clearly still possible at application level. Digital Performer is not the only one capable of working around it — ever since I saw the first OS X version of Ableton's Live, I've been impressed by the program's ability to have different devices for Core Audio input and output.

Just because you are recording into your Powerbook via a USB device with mic preamps, you might not necessarily want to output via the same device. All contents copyright © SOS Publications Group and/or its licensors, 1985-2020. All rights reserved.The contents of this article are subject to worldwide copyright protection and reproduction in whole or part, whether mechanical or electronic, is expressly forbidden without the prior written consent of the Publishers. Great care has been taken to ensure accuracy in the preparation of this article but neither Sound On Sound Limited nor the publishers can be held responsible for its contents.

The views expressed are those of the contributors and not necessarily those of the publishers.Web site designed & maintained by PB Associates & SOS.

Core Audio is the digital audio infrastructure of iOS and OS X. It includes a set of software frameworks designed to handle the audio needs in your applications. Read this chapter to learn what you can do with Core Audio.

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Core Audio in iOS and OS X

Core Audio is tightly integrated into iOS and OS X for high performance and low latency.

In OS X, the majority of Core Audio services are layered on top of the Hardware Abstraction Layer (HAL) as shown in Figure 1-1. Audio signals pass to and from hardware through the HAL. You can access the HAL using Audio Hardware Services in the Core Audio framework when you require real-time audio. The Core MIDI (Musical Instrument Digital Interface) framework provides similar interfaces for working with MIDI data and devices.

You find Core Audio application-level services in the Audio Toolbox and Audio Unit frameworks.

• Use Audio Queue Services to record, play back, pause, loop, and synchronize audio.

• Use Audio File, Converter, and Codec Services to read and write from disk and to perform audio data format transformations. In OS X you can also create custom codecs.

• Use Audio Unit Services and Audio Processing Graph Services (represented in the figure as “Audio units”) to host audio units (audio plug-ins) in your application. In OS X you can also create custom audio units to use in your application or to provide for use in other applications.

• Use Music Sequencing Services to play MIDI-based control and music data.

• Use Core Audio Clock Services for audio and MIDI synchronization and time format management.

• Use System Sound Services (represented in the figure as “System sounds”) to play system sounds and user-interface sound effects.

Core Audio in iOS is optimized for the computing resources available in a battery-powered mobile platform. There is no API for services that must be managed very tightly by the operating system—specifically, the HAL and the I/O Kit. However, there are additional services in iOS not present in OS X. For example, Audio Session Services lets you manage the audio behavior of your application in the context of a device that functions as a mobile telephone and an iPod. Figure 1-2 provides a high-level view of the audio architecture in iOS.

A Little About Digital Audio and Linear PCM

Most Core Audio services use and manipulate audio in linear pulse-code-modulated (linear PCM) format, the most common uncompressed digital audio data format. Digital audio recording creates PCM data by measuring an analog (real world) audio signal’s magnitude at regular intervals (the sampling rate) and converting each sample to a numerical value. Standard compact disc (CD) audio uses a sampling rate of 44.1 kHz, with a 16-bit integer describing each sample—constituting the resolution or bit depth.

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• A sample is single numerical value for a single channel.

• A frame is a collection of time-coincident samples. For instance, a stereo sound file has two samples per frame, one for the left channel and one for the right channel.

• A packet is a collection of one or more contiguous frames. In linear PCM audio, a packet is always a single frame. In compressed formats, it is typically more. A packet defines the smallest meaningful set of frames for a given audio data format.

In linear PCM audio, a sample value varies linearly with the amplitude of the original signal that it represents. For example, the 16-bit integer samples in standard CD audio allow 65,536 possible values between silence and maximum level. The difference in amplitude from one digital value to the next is always the same.

Core Audio data structures, declared in the CoreAudioTypes.h header file, can describe linear PCM at any sample rate and bit depth. Audio Data Formats goes into more detail on this topic.

In OS X, Core Audio expects audio data to be in native-endian, 32-bit floating-point, linear PCM format. You can use Audio Converter Services to translate audio data between different linear PCM variants. You also use these converters to translate between linear PCM and compressed audio formats such as MP3 and Apple Lossless. Core Audio in OS X supplies codecs to translate most common digital audio formats (though it does not supply an encoder for converting to MP3).

iOS uses integer and fixed-point audio data. The result is faster calculations and less battery drain when processing audio. iOS provides a Converter audio unit and includes the interfaces from Audio Converter Services. For details on the so-called canonical audio data formats for iOS and OS X, see Canonical Audio Data Formats.

In iOS and OS X, Core Audio supports most common file formats for storing and playing audio data, as described in iPhone Audio File Formats and Supported Audio File and Data Formats in OS X.

Audio Units

Audio units are software plug-ins that process audio data. In OS X, a single audio unit can be used simultaneously by an unlimited number of channels and applications.

iOS provides a set of audio units optimized for efficiency and performance on a mobile platform. You can develop audio units for use in your iOS application. Because you must statically link custom audio unit code into your application, audio units that you develop cannot be used by other applications in iOS.

The audio units provided in iOS do not have user interfaces. Their main use is to provide low-latency audio in your application. For more on iPhone audio units, see Core Audio Plug-ins: Audio Units and Codecs.

In Mac apps that you develop, you can use system-supplied or third-party-supplied audio units. You can also develop an audio unit as a product in its own right. Users can employ your audio units in applications such as GarageBand and Logic Studio, as well as in many other audio unit hosting applications.

Some Mac audio units work behind the scenes to simplify common tasks for you—such as splitting a signal or interfacing with hardware. Others appear onscreen, with their own user interfaces, to offer signal processing and manipulation. For example, effect units can mimic their real-world counterparts, such as a guitarist’s distortion box. Other audio units generate signals, whether programmatically or in response to MIDI input.

Some examples of audio units are:

• A signal processor (for example, a high-pass filter, reverb, compressor, or distortion unit). Each of these is generically an effect unit and performs digital signal processing (DSP) in a way similar to a hardware effects box or outboard signal processor.

• A musical instrument or software synthesizer. These are called instrument units (or, sometimes, music devices) and typically generate musical notes in response to MIDI input.

• A signal source. Unlike an instrument unit, a generator unit is not activated by MIDI input but rather through code. For example, a generator unit might calculate and generate sine waves, or it might source the data from a file or network stream.

• An interface to hardware input or output. For more information on I/O units, see The Hardware Abstraction Layer and Interfacing with Hardware.

• A format converter. A converter unit can translate data between two linear PCM variants, merge or split audio streams, or perform time and pitch changes. See Core Audio Plug-ins: Audio Units and Codecs for details.

• A mixer or panner. A mixer unit can combine audio tracks. A panner unit can apply stereo or 3D panning effects.

• An effect unit that works offline. An offline effect unit performs work that is either too processor-intensive or simply impossible in real time. For example, an effect that performs time reversal on a file must be applied offline.

In OS X you can mix and match audio units in whatever permutations you or your end user requires. Figure 1-3 shows a simple chain of audio units. There’s an instrument unit to generate an audio signal based on control data received from an outboard MIDI keyboard. The generated audio then passes through effect units to apply bandpass filtering and distortion. A chain of audio units is called an audio processing graph.

If you develop audio DSP code that you want to make available to multiple applications, you should package your code as an audio unit.

If you develop Mac audio apps, supporting audio units lets you and your users leverage the library of existing audio units (both third-party and Apple-supplied) to extend the capabilities of your application.

To experiment with audio units in OS X, see the AU Lab application, available in the Xcode Tools installation at /Developer/Applications/Audio. AU Lab lets you mix and match audio units to build a signal chain from an audio source through an output device.

See System-Supplied Audio Units in OS X for a listing of the audio units that ship with OS X v10.5 and iOS 2.0.

The Hardware Abstraction Layer

Core Audio uses a hardware abstraction layer (HAL) to provide a consistent and predictable interface for applications to interact with hardware. The HAL can also provide timing information to your application to simplify synchronization or to adjust for latency.

In most cases, your code does not interact directly with the HAL. Apple supplies a special audio unit—called the AUHAL unit in OS X and the AURemoteIO unit in iOS—which allows you to pass audio from another audio unit to hardware. Similarly, input coming from hardware is routed through the AUHAL unit (or the AURemoteIO unit in iOS) and made available to subsequent audio units, as shown in Figure 1-4.

The AUHAL unit (or AURemoteIO unit) also takes care of any data conversion or channel mapping required to translate audio data between audio units and hardware.

MIDI Support in OS X

Core MIDI is the part of Core Audio that supports the MIDI protocol. (MIDI is not available in iOS.) Core MIDI allows applications to communicate with MIDI devices such as keyboards and guitars. Input from MIDI devices can be stored as MIDI data or used to control an instrument unit. Applications can also send MIDI data to MIDI devices.

Core MIDI uses abstractions to represent MIDI devices and mimic standard MIDI cable connections (MIDI In, MIDI Out, and MIDI Thru) while providing low-latency input and output. Core Audio also supports a music player programming interface that you can use to play MIDI-based control or music data.

For more details about the capabilities of the MIDI protocol, see the MIDI Manufacturers Association site, http://midi.org.

The Audio MIDI Setup Application

The Audio MIDI Setup application lets users:

• Specify the default audio input and output devices.

• Configure properties for input and output devices, such as sampling rate and bit depth.

• Map audio channels to available speakers (for stereo, 5.1 surround, and so on).

• Create aggregate devices. (For information about aggregate devices, see Using Aggregate Devices.)

• Configure MIDI networks and MIDI devices.

You find Audio MIDI Setup in the /Applications/Utilities folder.

A Mac Core Audio Recording Studio

A traditional—non-computer-based—recording studio can serve as a conceptual framework for approaching Core Audio. Such a studio may have a few “real” instruments and effect units feeding a mixing desk, as shown in Figure 1-5. The mixer can route its output to studio monitors and a recording device (shown here, in a rather retro fashion, as a tape recorder).

Many of the pieces in a traditional studio can be replaced by software-based equivalents—all of which you have already met in this chapter. On a desktop computing platform, digital audio applications can record, synthesize, edit, mix, process, and play back audio. They can also record, edit, process, and play back MIDI data, interfacing with both hardware and software MIDI instruments. Mac apps rely on Core Audio services to handle all of these tasks, as shown in Figure 1-6.

As you can see, audio units can make up much of an audio signal chain. Other Core Audio interfaces provide application-level support, allowing applications to obtain audio or MIDI data in various formats and output it to files or output devices. Core Audio Services discusses the constituent interfaces of Core Audio in more detail.

Core Audio lets you do much more than mimic a recording studio on a desktop computer. You can use it for everything from playing sound effects to creating compressed audio files to providing an immersive sonic experience for game players.

On a mobile device such as an iPhone or iPod touch, the audio environment and computing resources are optimized to extend battery life. After all, an iPhone’s most essential identity is as a telephone. From a development or user perspective, it wouldn’t make sense to place an iPhone at the heart of a virtual recording studio. On the other hand, an iPhone’s special capabilities—including extreme portability, built-in Bonjour networking, multitouch interface, and accelerometer and location APIs—let you imagine and create audio applications that were never possible on the desktop.

Mac Development Using the Core Audio SDK

To assist audio developers, Apple supplies a software development kit (SDK) for Core Audio in OS X. The SDK contains many code samples covering both audio and MIDI services as well as diagnostic tools and test applications. Examples include:

• A test application to interact with the global audio state of the system, including attached hardware devices (HALLab).

• A reference audio unit hosting application (AU Lab). The AU Lab application is essential for testing audio units you create, as described in Audio Units.

• Sample code to load and play audio files (PlayFile) and MIDI files (PlaySequence).

This document points to additional examples in the Core Audio SDK that illustrate how to accomplish common tasks.

The SDK also contains a C++ framework for building audio units for OS X. This framework simplifies the amount of work you need to do by insulating you from the details of the Component Manager plug-in interface. The SDK also contains templates for common audio unit types; for the most part, you only need override those methods that apply to your custom audio unit. Some sample audio unit projects show these templates and frameworks in use. For more details on using the framework and templates, see Audio Unit Programming Guide.

Note: Apple supplies the C++ audio unit framework as sample code to assist audio unit development. Feel free to modify and adapt the framework based on your needs.

The Core Audio SDK assumes you will use Xcode as your development environment.

You can download the latest SDK from http://developer.apple.com/sdk/. After installation, the SDK files are located in /Developer/Examples/CoreAudio. The HALLab and AU Lab applications are located in /Developer/Applications/Audio.

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