What's the right processor then?
Which processor is best for a given digital audio product depends on a broad mix of factors. Key selection criteria typically include speed, price, energy efficiency, on-chip integration, development tools, and the availability of application software modules.

Speed
Processor speed is a key factor affecting product performance. The processor must be fast enough to implement all of the required algorithms, operating in their most demanding modes, in real-time. Additional performance beyond this level may not be helpful, though.

Although clock rate has an impact on processor speed, it is not a reliable indicator of processor speed. Processors that are highly specialized for audio signal processing can generally achieve real-time performance on audio tasks at lower clock speeds than can general-purpose processors. A lower processor clock rate is desirable, as it may help reduce system complexity and external component costs. For example, lower clock speeds can result in less costly board design and a reduction in EMI effects.

Price
The audio processor is often one of the most costly components on the bill of materials, and thus its cost is closely scrutinized. But bear in mind that the choice of processor can affect the total system cost in complex ways. For example, a specialized processor may be expensive compared to a general-purpose processor, but the specialized processor may include a number of required peripherals, reducing system chip count and resulting in a lower total bill of materials cost.

Energy Efficiency
Processor energy efficiency is a key concern for portable audio products, since it directly affects battery life. Increasingly, processors that target portable applications are incorporating sophisticated power management features, such as software controlled on-the-fly frequency and voltage scaling. Lowering the operating frequency alone, or in combination with lowering the supply voltage, can result in reduced power consumption. System designers can optimize power consumption by tailoring the processing power available (via frequency and voltage scaling) to the demands of current workload. Line-powered devices are not immune from energy efficiency considerations: higher energy dissipation may require heat sinks, larger power supplies, or costly (and noisy!) cooling fans—which are unlikely to be acceptable in an audio product.

Software and Development Tools Availability
A digital audio device will require a number of software components, or sub-systems. Commonplace components include communications protocol stacks like TCP/IP, device drivers for USB ports and other peripherals, file system managers, the user interface, audio compression/decompression algorithms, and audio post-processing algorithms such as reverb, echo, room correction, and 3D surround sound. Developing highly optimized implementations of audio processing algorithms, such as audio compression/decompression and post-processing, is difficult and time consuming. (Read the accompanying article "Building Good Audio Software" for details). In some cases, the processor vendor or third party supplier offers audio software modules optimized for a particular processor. If the available off-the-shelf modules correspond to the key functions needed in the product, their availability can dramatically reduce development time and the specialized know-how required.

When evaluating off-the-shelf software modules, it is important to consider not only the functionality provided, but also the degree of optimization, level of quality, and whether the modules are provided in source or object code form. Some modules are distributed only in object code form, making it difficult to understand their operation and fix programming errors. Other modules may be hindered by inadequate documentation; for example, they may lack accurate data on the processor resources used. Even when most of the required software modules are available off-the-shelf, inevitably some new software will be required. Solid software development tools are critical for efficient software development and integration. For testing audio software, tool support for streaming test data to and from the processor can be extremely helpful. Signal-processing-oriented tools features, such as data visualization, can be important for audio application development, but aren't found in most tools.