MENU

The technological stars have aligned: why hi-fi digital audio streaming equipment might find a ready market

The technological stars have aligned: why hi-fi digital audio streaming equipment might find a ready market

Technology News |
By eeNews Europe



From as long ago as the 1980s, the storage and playback media of choice for audiophiles have been the audio CD and – for die-hard hi-fi enthusiasts – an even older technology, the vinyl long-playing (LP) record. For a short time, the music industry hoped that SACD (Super Audio CD), introduced in 1999, would replace the conventional CD because of its superior fidelity and dynamic range, as well as its higher storage capacity. In fact, SACD has never gained a strong foothold in the mainstream consumer market outside Asia, and the 16-bit, 44.1kHz CD remains the most popular physical medium for hi-fi music.

Until now, the existence of digital file compression formats – most notably MP3 – has not been thought to be relevant to the audiophile market. The main purpose of lossy file formats such as MP3 has been to enable the use of small portable music players. The MP3 codec was developed at the end of the 1980s and initially released in 1993. In its standard configuration, MP3 can compress an audio file by a factor of ten. This compression is important for two reasons:

  • a 40MB audio track on a CD may be reduced to just 4MB, enabling 1,000 tracks to be stored on a portable 4GB music player
  • a single track can be downloaded from the internet in less than 30 seconds via a broadband connection

MP3 and other lossy compression formats are well suited to personal music systems because the imperfections caused by the compression algorithms do not materially affect perceived audio quality. Limitations in the DACs, power supplies, amplifiers and earphones commonly used in portable music players compromise sound quality as much as the source file does. At the same time, consumers attach a high value to storage capacity, so the more tracks a player can hold, the better.

Various advances in hardware and software technology, however, have made it commercially viable to produce digital audio streaming equipment of hi-fi quality. This article outlines the key elements of the architecture of such equipment, and new board-level technology that developers should consider when evaluating technology for new product designs.

Technology trends supporting high-end digital audio streaming
Three changes in the technology of digital audio support the introduction of hi-fi digital streaming appliances. First, the cost of memory ($/GB) is continually falling, and it is now possible to buy a Hard Disk Drive (HDD) with >1TB of capacity for the same cost as a 1GB HDD would have cost several years ago. Second, consumer choice has expanded, with high-quality audio files in a variety of lossless compression formats such as FLAC available for purchase online. Software is also freely available that rips music on CDs to FLAC files.  Finally, the introduction of very high-speed broadband has made it possible to download lossless (uncompressed) audio files from the internet. So now, consumers can find CD-quality digital files online, download them quickly and store them on a PC or external HDD.

It follows that, by connecting a hi-fi amplifier and speakers to their PC or external storage device, audiophiles should be able to listen to high-quality digital audio files (see Figure 1). This in turn presents a new opportunity to consumer electronics OEMs: conventional computer hardware is unable to provide the high audio quality required. Indeed, it is hard to think of a more hostile environment to high-quality audio than the PC. Low-quality switching power supplies and various RF interfaces interfere with high-performance analogue outputs. And interfaces and signal processing circuitry introduce noise, jitter and distortion into the signal chain.

 

(Click on image to enlarge)

Fig. 1: options for playing back various types of audio files

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

Data transmission from 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 24-bit 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, 24-bit, 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.

A design blueprint for a hi-fi digital audio streamer
The operation of an advanced, hi-fi digital audio streamer is demonstrated in a development platform from Future Electronics – 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.

The Audio Streamer Micro-Blox will be available in Q4 2011, free on request to qualified customers of Future Electronics via www.my-boardclub.com.

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 – 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.

If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :    eeNews on Google News

Share:

Linked Articles
10s