Digital audio solution based on configurable processor Xtensa LX

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All media are converting to digital formats, and now even music is almost digital. TV and video are quickly being converted to full digital format. Digital photos will soon eliminate obsolete film. In the next few years, this conversion will be comprehensive, and the digital media format will become the standard format. Therefore, the electronics industry needs to face the development of a digital media format conversion engine to provide voice and images to media users.

As the first fully digital media, audio technology is the most in-depth in terms of conversion. Today's audio systems must support multiple digital audio formats, from the earliest format to the latest format. As digital audio formats become more advanced, their technology is becoming more complex, with the goal of getting better sound by using fewer bits. With the use of a large number of digital audio media formats (including MP3, AC3, AAC, and WMA) and various voice codecs for mobile phones, the conversion of digital audio formats requires some firmware programmable processor. The use of dedicated hardware can be overly complicated for the conversion of many different digital audio formats.

A wide variety of programmable processors can be used for the conversion of digital audio formats. For PCs, general-purpose 32-bit or 64-bit CPUs are typically used to process audio data, as this is already common in PCs. These CPUs have now reached a few GHz, so the bandwidth is enough to handle the conversion of audio data formats easily. However, the cost is as high as several hundred dollars, and the power consumption is several tens of hundreds of watts. Therefore, the general-purpose CPU does not. Suitable for low power, battery powered devices.

Low-power, low-cost digital signal processors (DSPs) can also be used to implement digital audio codecs in many consumer devices. In general, low-cost DSPs are 16-bit DSPs that lack sufficient bit precision to properly handle today's complex 16-bit audio data encoding and decoding, not to mention the more advanced 20-bit audio format. Moreover, DSPs generally lack instruction and input/output functions associated with task control in media products, so a control processor is typically required in such designs. The consequence of this is that even a simple DSP-based audio player must use a dual-processor architecture, which complicates the design and increases the risk of flaws in the design.

A configurable processor core can be used to build an audio processor that combines the high performance and low power of the DSP with the good control of a general purpose processor. Tensilica's Xtensa-based HiFi audio engine is an example of such a processor. The HiFi audio processor is based on a 32-bit Xtensa V processor core with 24-bit audio-specific instructions. The processor core has been adopted in a variety of products, including mobile phones, portable audio players, camcorders, digital cameras, and personal video recording (PVR).

Tensilica's Xtensa LX configurable processor is available to enhance the performance of the original HiFi audio engine, providing users with a more powerful audio processor called the Xtensa HiFi 2 audio engine. The engine can run more complex digital audio codec algorithms with narrower bandwidths (and thus lower power and energy). The Xtensa LX processor has some configurable features that allow for these improvements.

In particular, the Xtensa LX processor has a feature called FLIX (Variable Length Instruction Extension) that allows the processor to mix existing 16-bit and 24-bit Xtensa instructions to generate custom wide instructions. Each FLIX format instruction can place multiple independent operations into the operation slots in each wide instruction word. The Xtensa HiFi 2 audio engine extension of the Xtensa LX processor adds more than 300 dedicated audio DSP instructions, including 24-bit MAC (multiply/accumulate) instructions and stream encoding instructions. The introduction of these instructions greatly reduces complex audio codec algorithms. The number of execution cycles.

Some new dedicated audio instructions are 24-bit instructions, while others are 64-bit FLIX format instructions. The FLIX format instructions of the HiFi 2 audio engine include two operating slots. All HiFi 2 audio engine instruction extensions can directly utilize the compiler generated with the processor generated by Tensilica's automated Xtensa Processor Generator. As shown in Figure 1.


Figure 1: HiFi 2 instruction format

The HiFi 2 Audio Engine extends the hardware added to the Xtensa LX processor to include two MAC units. Each MAC unit can perform 24x24-bit and 32x16-bit multiplication, and both MAC units are pipelined, so a new execution result can be generated every clock cycle. The added hardware also includes a Huffman encoder and decoder, a bitstream processor and two dedicated register files for processing 24-bit audio data. One of the register files includes eight 48-bit registers, each of which can store two 24-bit data values. The other register file includes four 56-bit registers that hold the extended precision operation results produced by the two MAC units. as shown in picture 2.


Figure 2: HiFi 2 block diagram

The performance of the digital audio codec algorithm ported to the Xtensa HiFi 2 audio engine architecture shows that the design requires only a small processor bandwidth to support high quality audio codecs. For example, an MP3 decoder requires only 13 to 15 MHz of processor bandwidth when playing music stored in a 48 kHz/128 kbps stereo format. At a similar bit rate, the MP3 encoder requires only 38 to 40 MHz of processor bandwidth. Similar results can be obtained with the AAC-LC encoder/decoder and WMA decoder.

The Xtensa LX processor uses a wide range of clock gating techniques that go beyond the technology used in the Xtensa V processor design to reduce power consumption. This feature, along with the low clock rate required to implement digital audio codecs on the Xtensa HiFi2 audio engine architecture, results in a very efficient audio processor. The Xtensa LX processor with the Xtensa HiFi 2 audio engine extension uses TSMC's 130nm LV manufacturing process and consumes only 91μW/MHz. The original Xtensa V processor-based HiFi audio engine consumes 207μW/MHz.

When the AAC-LC encoder algorithm is implemented, the HiFi 2 audio engine operates at 38 MHz; the original Xtensa HiFi audio engine operates at 85 MHz. The result of these improvements is that the Xtensa HiFi 2 audio engine consumes 3.5mW when performing the AAC-LC encoder algorithm; the original Xtensa HiFi audio engine consumes 17.6mW while performing the same task. Although the power consumption of the original Xtensa HiFi audio engine is already low, the power consumption of the Xtensa HiFi 2 audio engine is still five times lower.

The Xtensa HiFi 2 audio engine architecture has three advantages over the original Xtensa HiFi audio engine. First, the Xtensa HiFi 2 audio engine reduces the processor bandwidth for advanced digital audio codecs. Second, when implementing these digital audio codec algorithms, power consumption is reduced due to the reduced bandwidth required and because the Xtensa LX processor on which Xtensa HiFi 2 is based has more than the previous Xtensa V processor. Extended clock gating. Finally, the Xtensa HiFi 2 audio engine architecture based on Xtensa LX requires less gate counts than the Xtensa HiFi engine based on the Xtensa V processor (the combined processor count is reduced at 200MHz clock rate). About 20%).

By adding dedicated audio commands, Tensilica's Xtensa LX processor provides an efficient, low-power, high-performance platform for building digital audio products. Such a platform can perform a variety of audio codec algorithms and, if needed, perform control-like tasks that go beyond traditional DSPs. Moreover, the Xtensa HiFi 2 audio engine can deliver much lower cost and clock frequency for the same performance. Therefore, the power consumption level can be greatly reduced compared to a general-purpose CPU.

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