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Advances in hi-fi digital audio streaming

Posted: 13 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:audio CD? hi-fi? MP3? portable music players?

So new audio streamer devices will be required to:
???Provide an interface from the storage medium to the audio codec
???Implement FLAC decoding
???Perform faithful, low-noise and low-distortion conversion of the digital files to an analogue output

HDD to audio streamer
PCs offer a number of interfaces for transferring audio files to an audio streamer's DAC, with USB most commonly used with portable media players. The standard USB 2.0 interface implemented in today's PCs, however, suffers from excessive jitter that compromises sound quality to an unacceptable degree. In conventional USB, if the audio master clock is in the PC, then the audio streamer's DAC must lock on to the signal coming from the computer and reconstruct a new master clock using a variable-frequency oscillator, which cannot achieve the low jitter levels of a fixed-frequency master clock.

Audio-class USB supports asynchronous USB operation, which allows the audio streamer to become the master and the PC the slave. In this way the data can be pulled from the hard drive as required, and the audio player is not constrained by the performance of the PC clock.

As well as providing asynchronous operation, the latest USB Audio Class 2.0 standard overcomes a number of earlier limitations of USB Audio Class 1.0. USB Audio Class 2.0 supports 24bit audio, sample frequencies up to 192kHz and the ability to scale to large numbers of input/output channels.

An interesting way to implement USB Audio Class 2.0 is offered by semiconductor company XMOS, which supplies a software framework design to work on its configurable XS1-L devices. The design provides certified USB 2.0 Hi-Speed device and Audio Class 2.0 device class implementations with configurable digital audio interfaces and peripherals. In addition the framework is backwards-compatible with USB Audio 1.0. The USB Audio Class 2.0 interface supports the high-fidelity transfer of bit-perfect, 24bit, 192kHz audio from a PC (which performs FLAC decoding before transmitting the audio data).

The architecture is different if the intention is to support streaming FLAC playback independently of a PC. Here, the audio streamer downloads the FLAC file from an HDD or other networked storage device, and then decodes the file to provide a digital input to the DAC. Design teams may write their own decoding software in-house to run on the device's main processor, but an easier method is available from Freescale Semiconductor, which provides FLAC decoding software with its Kinetis K60 family of ARM Cortex-M4 microcontrollers.

Above, it has been shown why a PC's standard file-transfer interface is unsuitable for hi-fi audio reproduction. A PC's built-in sound card is similarly unsuitable, principally because of the low-grade DAC which such cards employ. This article has argued that audiophiles will want to buy a dedicated audio streamer device; such a device can use a high-performance DAC to convert digital files to an analogue output with very low distortion and noise. The WM8741 from Wolfson Microelectronics is specified for this type of application. As well as offering total harmonic distortion of -100dB at 48kHz and a signal-to-noise ratio of 123dB (non-weighted stereo at 48kHz), it provides many additional features including advanced digital filtering.

Design blueprint
The operation of an advanced, hi-fi digital audio streamer is demonstrated in a development platform from Future Electronics C the Audio Streamer Micro-Blox board. The board features Freescale's MK60N512VMD100 microcontroller which runs FLAC decoding software, and Wolfson Microelectronics' flagship WM8741 DAC buffered by high-end quad audio op amps. (The board also features other audio signal-chain and peripheral components suitable for designs targeting low-end embedded audio applications.)

The development board offers multiple inputs including USB, Ethernet and direct memory interfaces. Great care has been taken in the layout of the board and the selection of passive components and power devices (from Micrel) to produce a high-quality audio environment.

Death of the CD?
The music-buying public is already gravitating to the Internet, and now audiophiles have the potential to source high-quality music in a digital file format, FLAC, which provides hi-fi quality. Does this finally mark the end of the CD? It is worth remembering that the LP was said to be a dead medium as long ago as the 1980s and yet, while the numbers sold are far off their peak, recently sales have been growing year-on-year.

Likewise, some listeners continue to choose CDs C in fact, the technologies discussed above might even extend the life of the CD. First, the development of up-sampling/interpolation techniques has increased the perceived quality of CDs beyond their inherent 16 bits, and the sound quality is highly prized by some audiophiles.

Second, the ripping software that converts the data on a CD to FLAC format increases the quality of the audio output (compared to playback on a CD player), as the error correction is performed by the software, and is continuously repeated until a 100% accurate data file can be stored. By contrast, a CD player can only perform error correction on the fly, and thus produces a less than perfect output. For the audiophile, therefore, the best choice might be to continue owning (and even buying) CDs, but converting them to FLAC for enhanced playback on the kind of dedicated audio-streaming device described in this article.

About the author
Jim Porter is an audio market development manager at Future Electronics.

To download the PDF version of this article, click here.


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