With the increase in popularity of 'slotless' computers, such as laptops and iMacs, several manufacturers of traditional soundcards and audio interfaces are extending their ranges to include external units connected by USB or Firewire. For multiple simultaneous inputs and outputs, Firewire is the clear winner because of its greater bandwidth, although USB 2 will level the playing field somewhat when it becomes more widely supported. In situations where the I/O requirements are more modest, however, USB works perfectly well. Packaged as a small, silver desktop unit, the M Audio Audiophile USB works via a standard USB connection and draws its power from an included 9V AC adaptor to provide two channels of analogue I/O, stereo S/PDIF digital I/O and set of MIDI In and Out ports. Bit depths of up to 24-bit are supported with sample rates from 8kHz to 96kHz, though there are some I/O limitations when working at high sample rates or high bit depths. Interestingly, the S/PDIF outputs are also capable of carrying multi-channel surround formats such as Dolby or DTS, which means the Audiophile USB also has applications in computer-based home theatre.
The analogue inputs are on unbalanced quarter-inch jacks and phonos; using the jacks overrides the phones. The analogue outs are on phonos only while the S/PDIF ins and outs follow the common co-axial (phono) format. No adaptor cables are needed for MIDI as the sockets are the usual 5-pin DINs. Additionally, there's a quarter-inch headphone out with level control, plus an output level control for the analogue phono outs, which means you could easily run this interface with active monitors while retaining physical control over the output level. A standard USB connector is used to link to the host computer and a suitable USB cable is included.
The Audiophile USB runs under Windows 98SE, 2000, Me and XP, and also supports Mac OS 9 and OS X. Any reasonably specified Pentium II 266MHz or Mac G3 or above with USB connectivity can be used with the Audiophile USB, though Mac OS 9 users will need to run Opcode's OMS to use the MIDI port. ASIO drivers are included for Mac OS 9 and Windows users, with separate support software provided for Mac OS X users.
Once the software has been installed from the included driver CD-ROM, the Audiophile USB is set up via its own control panel, the default setting being 16-bit. Ticking the box for the S/PDIF input activates it and allows the Audiophile USB to clock to the incoming digital signal, but problems will arise if this box is ticked when no digital source is connected as the unit will be in external clock mode with no clock signal to lock to.
Because of the bandwidth limitations of USB, not all combinations of I/O configuration, sample rate and bit depth are available, and full four-in, four-out operation is only possible at 16-bit up to 48kHz. At 24-bit, the number of active stereo ports is reduced from four to three, while 24-bit/96kHz operation is only possible with one stereo input or output activated. The control panel also includes a choice of latency settings to accommodate older, slower computers, though on my G3 600MHz machine, no problems were encountered. Indeed, in normal operation, the latency was insignificant and could be ignored.
The manual also mentions DD/DTS Pass Thru, a control panel button that allows surround-encoded digital audio to be output from a suitable source such as a DVD. This setting shuts off the analogue output for bandwidth reasons but conventional stereo digital signals can still be output normally in this mode. A suitable surround decoder is required to make use of the DD or DTS signal at the digital output and many commercial decoders have optical digital inputs, so you may need to use an optional co-axial-to-optical converter box. However, the DD/DTS Pass Thru button didn't appear when I installed under Mac OS X so it may be that this feature is only available to Windows users.
For this test I installed the Audiophile USB on a Mac G3 iBook running Mac OS 10.2.6. Installation of the software was painless, after which all I had to do was select the Audiophile USB as the designated audio device within Logic Audio 6.0, the audio program I chose for testing. I/O settings are easily configured from the control panel, though you don't set the sample rate there. Instead, you get a readout of the maximum available sample rate depending on bit depth and number of I/O ports activated, after which the sample rate is set in the host program.
Once installed, the Audiophile USB did exactly as claimed with no evidence of glitching or otherwise struggling. The MIDI I/O worked straight off with no other configuration necessary and the MIDI timing felt adequately tight, even though it is probably less good in this respect than a MIDI interface that uses intelligent pre-buffering, such as the Steinberg Midex 3.
The limitations on I/O aren't handled as elegantly as on the Emagic EMI range of audio interfaces, where the output bit depth automatically drops to 16-bit for overdubbing when the bandwidth limitations of the configuration so dictates, but for 'real world' sample rates, the restrictions are not serious. It's more of a problem if you want to work at 96kHz because then you can only use one port at a time, so monitoring while recording isn't possible. While this may be fine for editing, it means that overdubbing multitrack parts at 96kHz isn't possible. Then again, I'm one of those people who sees little point in using 96kHz for multitracking at all unless you have a world-class studio with superb acoustics and converters that cost around the same as a small car, in which case you probably wouldn't choose the Audiophile USB anyway!
Other than not getting the option to switch to surround pass-thru mode under Mac OS X, the Audiophile USB performed exactly as claimed, delivering sound quality that compared favourably with good consumer hi-fi equipment and with no glitching or other foibles. Nevertheless, tales abound of pieces of USB gear that work perfectly on one system and are nothing but trouble on others, so you need to know that the USB support provided by your computer is up to scratch and also that you can connect the Audiophile USB directly to its own USB port and not via a hub.
The MIDI In and Out worked first time with no obvious timing problems, so the only significant criticism of the unit is its inability to work in full-duplex mode at 96kHz, though being able to power the unit directly from a USB port would also have been useful in mobile applications as not many beaches in the Bahamas have mains power points! As it is, you have to use the AC power adaptor.
Despite its limitations, most of which are concessions to the bandwidth of USB, the Audiophile USB is ideal for simple audio applications where you only need limited I/O and you are working at sample rates up to 48kHz. It's less useful at 96kHz but it still works fine in this mode for editing or playback. The sound quality is good, with a specified analogue dynamic range of 107dBA. Having an inbuilt MIDI interface that works so transparently (and without sacrificing the ability to use the digital I/O at the same time) is also a huge plus for laptop users as is the headphone monitoring, so on the whole, the Audiophile USB delivers a practical and well-behaved solution for adding audio to slotless computers.