Steve Leibson — Tensilica
If you're facing the design of an SOC with on-chip digital audio, this article will help you make the engineering tradeoffs and will take you to a successful project tape out. To do that, we'll look at your alternatives, key decision factors you should be evaluating, and the consequences of your decisions.
The Audio Codec
The core element in all digital-audio applications is the codec. Short for coder/decoder, the codec defines how analog audio is digitized and compressed into a bit stream and how the bit stream is later decompressed to reproduce the analog audio channels. The first compression algorithm to see widespread use in consumer products was MP3, first developed in 1991. Since then, many other audio standards have been introduced for better quality sound.
There are four types of choices for implementing audio codecs.
Alternative 1, using one processor to perform all system functions including the user interface, I/O, and running the digital-audio codec, has several advantages. First, there's most likely a general-purpose processor available, so the digital-audio codec is just another task running on that processor. The only incremental cost is perhaps a bit of additional instruction memory. Second, the processor can implement multiple audio codecs using firmware, so you can create a multifunction product. Finally, this design approach accommodates new codecs as they're invented.
There are some disadvantages to this approach however. Digital audio performance is extremely sensitive to glitches. The ear picks up every audio imperfection. Processor multitasking, as employed for this design alternative, increases the probability of audio glitches because the processor's bandwidth is not fully devoted to audio playback.
In addition, most general-purpose processors lack audio-specific features, so they execute audio codecs inefficiently. The consequence is increased clock rate. General-purpose processors need to execute more instructions per second to compensate for the inefficiency of their general-purpose instructions in audio applications.
The second design alternative pairs a relatively low-performance processor with a hardware codec and lets the codec hardware handle all of the audio processing. The processor can feed audio samples to the hardware audio codec over the bus or the codec might DMA audio samples directly out of memory.
Using a hardware codec has advantages. It is the most efficient way to implement one codec in terms of silicon area and energy consumption. However, each new codec requires an additional hardware block. So if your product must support three audio-codec standards, you must add three hardware blocks to the design as shown in Figure 1.
Figure 1. A design with three hardware codec blocks.
Next, if there's a change in the codec specification or a bug in the codec algorithm, you must respin the chip to fix the problem because a hardware audio codec isn't programmable. Also, you can't change a hardware codec to support a new digital-audio codec standard. You must design a new block, add it to the system design, and respin the chip.
Another approach to implementing digital audio is to run codec firmware on a general-purpose DSP, under the direction of the system's host processor. Most DSPs have integral hardware multipliers that greatly improve DSP execution efficiency on digital-audio firmware. Also, DSPs run firmware so they easily accommodate multiple digital-audio codec standards with relatively modest increases in memory size and therefore silicon.
Using DSPs to run audio codecs has disadvantages as well. Most DSPs are very poor targets for C compilers, so software codecs written in C will not easily run on a DSP. Also, 16- and 32-bit DSPs are not ideal for audio processing. Although most audio codecs today work with 16-bit audio samples, intermediate calculations need headroom to avoid round-off errors so 16-bit DSPs have problems with complex audio algorithms (they clip and distort the sound) unless the audio algorithms use double-precision integer math, which is inefficient and thus increases clock rate. Conversely, 32-bit DSPs are overkill. Audio algorithms don't need and can't make use of 32-bit multipliers; 24-bit DSPs are really optimum for audio algorithms.
The Audio-Specific RISC Processor
The fourth alternative is an audio-specific processor, which is a general-purpose processor with audio-specific extensions that make the processor especially efficient at executing audio-codec firmware while retaining the characteristics that make the general-purpose processor a good compiler target. Figure 2 shows a system that uses an audio-specific processor to run the audio codecs.
Figure 2. An Audio-Specific Processor system block diagram
This design approach has several advantages. The audio-specific extensions let the processor execute audio algorithms and deliver the required general-purpose performance while running at a much lower clock rate, which drastically cuts energy consumption. This implementation easily supports multiple audio standards so it's a good approach for multi-standard audio products. It also supports new codecs as they appear, through the addition of firmware. The one disadvantage of this approach is that the concept of an audio-specific processor is somewhat unfamiliar, so let's remedy that situation right now.
Tensilica's HiFi 2 Audio Engine exploits the extensibility and configurability of Tensilica's Xtensa 32-bit RISC processor architecture to create a general-purpose processor that's very efficient at executing audio firmware. One of the key extensions in the HiFi 2 Audio Engine is a pair of 24-bit hardware multipliers, which really speed the audio calculations.
However, multipliers alone won't get the cycle count down for audio algorithms so the HiFi 2 Audio Engine can also execute one or two operations per cycle; some of its audio-specific extensions perform two operations simultaneously. This feature further cuts cycle count. Wide 48- and 56-bit registers that can store 24-bit stereo sample pairs are another important extension. With the addition of these registers, the processor handles stereo audio data as a native data type. In all, Tensilica added 300 audio-specific instructions to the Xtensa RISC processor to create a more efficient audio-algorithm execution engine.
Hardware alone is not sufficient to make a processor into an attractive audio component for SOC design. Your product needs audio codecs and you don't want to develop these codecs yourself. (You don't have the time.) Although you can find some audio codecs on the Internet, they're not optimized so they're not efficient. In addition, you will not find code for licensed algorithms such as Dolby digital audio codecs on the Internet. Over 30 audio codecs are available for Tensilica's HiFi 2 Audio Engine. Note that all of these codecs are written in C. The processor's general-purpose RISC instructions combined with the audio-specific instructions allow the firmware writers to keep the codecs in C, which improves code maintainability while keeping clock rates low.
The HiFi 2 Audio Engine is an extension to Tensilica's configurable, extensible Xtensa LX2 processor core. Tensilica has taken that set of extensions and predefined a processor core called the Diamond 330HiFi Audio Engine. This processor core is preconfigured and it runs all of the HiFi 2 audio codecs. Tensilica's HiFi Audio Engines have already shipped in tens of millions of products from a variety of end product and semiconductor manufacturers. The largest current application, of course, is in cell phones. Future product applications will include video products, consumer radios, and ultra-mobile PCs.
The first consideration for designing audio into portable applications is whether the product will support just playback or playback with audio enhancements. These enhancements include multiband spectral equalization, bass enhancements, 3D synthesized audio, MIDI synthesis, and so on. HiFi 2 Audio Engine clock rates for various audio codecs are well below 100 MHz. Adding audio enhancements to the processing load can add 100 to 200 MHz to the required processor clock rate, which can have unforeseen consequences.
For example, if you need to synthesize the processor core for 200MHz instead of 50MHz operation, the logic-synthesis tool will meet the speed constraints by using more and bigger buffers and by adding redundant logic to speed signals along critical logic paths. In doing so, the synthesis tool creates a processor that consumes more energy. Running this processor at the higher clock rate also increases energy consumption. So, even though it sounds counter-intuitive, consider using a separate processor for the audio effects, to keep the clock rates down. This approach also lets you power down the effects processor when playback is all that's required.
You can also influence energy consumption by carefully selecting a memory strategy for the SOC's audio processor. Simple audio designs with just one or only a few supported audio codecs may need only local memory. If you use memory caches, the memories will be bigger because caches require tag arrays. Without caches, you need not power up the processor's cache-control logic, so there's energy to be saved by using local memory instead of cache.
More complex audio applications that use more sophisticated audio codecs may require so much memory that cache is a better choice. With a configurable processor like Tensilica's Xtensa LX2 with the HiFi 2 Audio Engine extensions, you can elect to have or not have both caches and local memory. Tensilica's preconfigured Diamond 330HiFi processor core has both caches and local memory. You can use both or you can leave the cache disabled for audio applications.
Paths to Low Power Audio
The key paths to lowest-power on-chip audio are low processor clock rate, low bus traffic, and optimal use of local memories and cache. You want to keep the audio traffic off of the main bus to avoid turning the bus into an artificial bottleneck and to minimize energy consumption. And finally, you want to make sure you can perform 24-bit audio processing on a comprehensive set of available audio codecs, to ensure good-quality audio and a timely product release.
Search for Tensilica IP here
Tensilica, Inc. is the recognized leader in configurable processor technology and has leveraged that technology to become the leading supplier of licensable controllers and DSP cores for mobile audio and video applications. Tensilica offers the broadest line of controller, CPU, network, and specialty DSP processors in the market today - including full software tool-chain and modeling support - in both, an off-the-shelf format via the Diamond Standard Series cores and with full designer configurability with the Xtensa processor family. The modern design behind all of Tensilica's processor cores provides semiconductor companies and system OEMs with the lowest power, smallest area solutions for high-volume products including mobile phones and other consumer electronics, networking and telecommunications equipment, and computer peripherals.
Find the component you need without hours of searching.