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.
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