The HRT Music Streamer II is an asynchronous-mode USB audio converter, capable of high-end audio reproduction for less than £140.

For most listeners, good digital sound from a computer is a fait accompli, a done deal. The built-in soundcard on a PC or laptop promises '20Hz to 20kHz' reproduction, so providing you play uncompressed WAV or lossless digital formats like FLAC or Apple Lossless (ALAC), you're getting sound as good as a CD player, right?

Well, not necessarily. Or less courteous to the efforts of PC makers, almost certainly not. For one thing, audio circuits hate computers.

Audio amplifiers and analogue circuits are easily upset by the interference from radio-frequency noise – such as the multi-megahertz clocks cycling all through a personal computer. So the first stride forward in sound quality is usually found by simply locating the digital-to-analogue converter (DAC) away from a noisy motherboard and its switch-mode power supply.

A quick-and-easy solution for this can be found in the shape of the USB audio adaptor. Working out of the box without additonal drivers or software on modern Macs and Windows PCs, these devices take raw digital data audio from the computer, with the minimum of fuss.

But audio data still needs some care in its transport, lest its precise timing is subtly smeared, leading to a form of audible distortion often dubbed jitter.

This can make for a harder, grittier sound with a looser sense of musical timing. In short, the sound may be rough-edged and glassy or vague and tuneless. Or all these things.

HRT Music Streamer II: Adaptive versus asynchronous

Until recently, sending sound over USB relied on a default transmission protocol called adaptive-mode USB audio. In short, the essential clock timing signals needed in D-A conversion are derived here from the computer's clocks.

Like the long-standing S/PDIF interface used to connect CD players to outboard converters, a bi-phase clock recovery system is essential to recreate the timing pulses required to reassemble the binary stream correctly.

It kind-of works, but perhaps not as well as a more up-to-date system, known as asynchronous mode USB audio.

Asynchronous mode USB audio now has the host (the computer) and the outboard converter's clocks free-running relative to each other.

Crucially, the converter device itself dictates when data packets are sent from the host, ensuring a timely flow of data with no chance of overflow or underflow in the data buffer.

Put another way, rather than passively receive whatever the computer dishes out, asynchronous mode devices such as the HRT Music Streamer II take a much more pro-active role, telling the computer when to send data packets. And the final D-A conversion is referenced to a precision local clock, right inside the unit.

In the case of the HRT Music Streamer II, the digital input receiver module is actually a transceiver – able to transmit instructions back to the host as well as receive music data packets. HRT employs a Texas Instruments TAS1020B chip, under the guidance of the company's own custom firmware to do this critical work. Set up this way, interface jitter is said to be eliminated.

After this, a Burr Brown PCM1793 chip (specified for 24-bit/192kHz operation) carries out the actual D-A conversion in the HRT Music Streamer II, from digital bitstream into analogue music.

Care has also been taken to ensure good isolation between the computer and audio system, lest unwanted noise intrude into those precious audio circuits. HRT quotes a greater-than 20 megaohm isolation figure.

HRT Music Streamer II circuit board

The HRT Music Streamer II features a Texas Instruments TAS1020B input transceiver running custom firmware (left), feeding a Burr Brown PCM1793 DAC (top right)

In practice, the HRT Music Streamer II appears as 24/96 device to the computer; a true audiophile standard. Which is not so surprising when you discover the pedigree of the HRT Music Streamer II.

HRT stands for High Resolution Technologies, an offshoot between Kevin Halverson, chief designer at US high-end audio brand Muse Electronics, and Mike Hobson of audiophile record label Classic Records. Both Halverson and Hobson have been championing the benefits of 24/96 audio since at least the early days of DVD in the late 1990s.

Kevin Halverson told us, 'Our goal has been to bring to the average person a bit of what makes a high end product worth the price of admission.'

NEXT PAGE: The block - and how it sounds >>

The HRT Music Streamer II is an asynchronous-mode USB audio converter, capable of high-end audio reproduction for less than £140.

The block

Externally, the Music Streamer II is a rather innocuous lump of extruded aluminium just over 10cm long, in unassuming red paint. On one end is a Type B USB port (as used on printers). At the other is a pair of gold-plated RCA phono sockets.

A more expensive version, the HRT Music Streamer II+ is also available at £339. The manufacturer's specifications show a measured performance of 0.008% distortion, in place of the standard HRT Music Streamer II unit's 0.01% figure for THD+N (total harmonic distortion-plus-noise); but other than being finished in black and a little longer in case length, neither designer nor distributor would be drawn on what material differences lie within.

To set up the HRT Music Streamer II, just plug into a spare computer USB port, and connect to a music system's line-level inputs. Power comes from the 5V USB bus, so no extra plug-in supplies are required.

Setup in Mac OS X, for instance, just requires selecting the device within the Output tab of Sound, from System Preferences.

In Windows, you'd go to Control Panels, Sound, and then select the device in the Playback tab.

HRT Music Streamer II: Listening in

The HRT Music Streamer II possesses great subtlety in laying out the sound of recorded and streamed digital music.

The Music Streamer II impressed most with its freedom from glassiness, along with its even, 'unshouty' midband. This DAC is a real smoothie, yet one that doesn't over-compensate for digital edginess by smothering the sound in cotton wool. Music is heard relaxed and flowing, with precious little grain to intrude.

A classic opening from the Dave Brubeck Quartet – 'Blue Rondo A la Turk' – illustrates the ease with which this device can open up a seemingly simple arrangement.

The thrum of a single repeated note plucked from double bass had believable presence, showing more fundamental below and with more overtones ringing on above. The layers that each member of the quartet contributed in the song made more musical sense here; particularly the left-hand chord shapes under the right-hand melodies of the central piano.

By point of reference, a Cambridge Audio DacMagic rendered the song with a more muted thud to its bass line, while the now-splashy cymbal was, relatively speaking, more like you'd expect of a lossily compressed MP3 file. And all the while, the upsampling DacMagic remains one of the best sounding outboard converters under £250.

Stereo imaging from the HRT Music Streamer II unit was most noteworthy. With no steeliness to overly accentuate instruments and voices, as you might find from traditional USB digital converters, instruments sat quite naturally in left-to-right panned perspective.

In rhythmic terms, the HRT Music Streamer II was heard to 'time' well; that is, to enable the musical sense of songs to be plainly heard.

In fact, the HRT Music Streamer II brought out the lucid, soaring saxophone lines with such naturalness, we were reminded of top-tier analogue reproduction. This is not the usual sound of digital you might expect of your music collection.

Best of all was the 'easy' quality of audio – film soundtracks, TV sound, as well as music – experienced through the HRT MusicStreamer II.

There's definitely something in this asynchronous mode USB audio. At the highest level, this kind of technology is now in use on some of the world's finest (and most expensive) digital audio converters from British specialist dCS Limited.

At the other end of the price scale though, the HRT Music Streamer II is currently the simplest and by far the most affordable way to hear computer audio in a natural and relaxing new way.

NEXT: our expert verdict >>

HRT Music Streamer II: Specs

  • USB audio digital-to-analoge converter
  • 24-bit/96kHz
  • asynchronous transfer protocol
  • 2 x RCA analogue outputs
  • manufacturer’s specification: 2.25V rms output
  • 20Hz-20kHz -0.5dB frequency response
  • 98dB S/N ratio
  • 0.01% THD+N (1kHz FS 44.1kHz)
  • 200mA current draw from USB bus
  • 104 x 53 x 31mm
  • 114g
  • USB audio digital-to-analoge converter
  • 24-bit/96kHz
  • asynchronous transfer protocol
  • 2 x RCA analogue outputs
  • manufacturer’s specification: 2.25V rms output
  • 20Hz-20kHz -0.5dB frequency response
  • 98dB S/N ratio
  • 0.01% THD+N (1kHz FS 44.1kHz)
  • 200mA current draw from USB bus
  • 104 x 53 x 31mm
  • 114g

OUR VERDICT

The HRT Music Streamer II is a great-sounding digital converter for your PC. It's a costly item for its small size and simple construction, yet a great investment for its big, natural sound, bringing to life the music in your digital library.

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