Now that more computers are equipped with Universal Serial bus connections, musicians have started to demand compatible peripherals. Martin Walker tries out one of the first to arrive, a simple interface dedicated to piping digital audio in and out of USB machines.
There has been a lot of interest in USB (Universal Serial bus) products over the last couple of years, but it has only been comparatively recently that musicians could actually buy any useful ones. I first mentioned USB in the March 1997 issue of SOS, as part of my PC Musician feature about Plug and Play. It then seemed an ideal way (along with the even faster Firewire technology) to expand beyond the limitations of PCI expansion slots — once you ran out of slots you could add further peripherals externally in a daisy chain of up to 127 devices, all powered from the USB port on the back of your PC or Mac. USB also has the benefit of being hot‑swappable, so you don't need to switch off your computer before plugging in a peripheral.
So why has it taken so long for the first USB peripheral to appear in SOS? Well, until Windows 98 was released, USB support wasn't provided as standard in Windows, and partly because of this many PC manufacturers didn't bother to fit USB sockets, despite having the appropriate circuitry on the motherboard. (Incidentally, for those of you with USB support but no sockets, I spotted a suitable backplate with two USB sockets at my local Tandy store for £10.) The launch of Windows 98 encouraged a few more USB products to appear, but the ball really started rolling when the iMac was launched, since this dispensed with serial ports altogether, replacing them with USB connectors. Suddenly any musician buying a new Mac was faced with the problem of how to add a MIDI or digital audio interface (see April's Apple Notes for more details), hence the two main areas currently being targeted for musical USB products — MIDI and digital audio.
Installation
Figure1: DATport control. Selecting a new sample rate for recording purposes is done via this small utility, and involves sending and then verifying new code within the DATport itself.
Without further ado, let's look at the first ever USB peripheral to grace the pages of SOS — the Opcode DATport Digital Interface. The name says it all, really, and the design is simplicity itself: a small, lightweight aluminium case with a single USB socket at one end and a pair of gold‑plated phono sockets (for co‑axial S/PDIF In and Out) and a single green LED, that lights in the presence of a digital input signal, at the other end. A 2‑metre USB cable, which should be long enough for most applications, is also included.
The beauty of USB products is their ease of installation, and anyone buying a DATport should be up and running before the average soundcard is even bolted into the expansion slot.
Although you can hot‑swap USB peripherals, they still each need driver software installed in the host computer to let the operating system know what features they have. The DATport manual provides clear instructions on the installation procedure. Using the provided floppy disk, you first run the Setup.exe program to install the Config utility, and after restarting the PC you can then plug or unplug your DATport any time you like (subject to one condition, which I'll explain later). The first time you do, Windows will detect new hardware and then ask for the Windows 98 CD‑ROM if the USB audio drivers haven't yet been installed. The entire install took me a couple of minutes, and I found a new 'USB Audio Device' entry in the System applet of Control Panel.
At the time of writing, only PC drivers were available for the DATport, as there's apparently a problem with Mac drivers (which is being resolved in conjunction with Apple themselves). This USB stuff is still fairly new‑fangled for audio, and Opcode have already spent a considerable amount of time helping to sort out teething troubles on the PC with Microsoft before finally shipping this product. During the setup procedure an updated Microsoft file named USBAUDIO.SYS is transferred to the user's hard drive. This will solve a previous Windows 98 audio glitch problem. The Windows 98 CD‑ROM recognises that this file is newer than its own, and you should leave the newer file in place to achieve significantly improved USB audio performance.
Setting Up
While the DATport is connected there will be a new icon on the Taskbar for the DATport configuration utility, but after the unit is unplugged this will disappear within a few seconds. The utility, which can be launched by double‑clicking the icon, lets you set the Input Sample Rate (32, 44.1 or 48kHz) and Bit Depth (16 or 24 bits), as well as giving each connected unit a name. After any of these settings have been altered it's necessary to click on the 'Set' button to make them come into force; a small window showing 'Writing DATport' appears, followed by 'Verifying DATport'. The utility is presumably downloading new code to flash ROM in the unit. The entire process took 37 seconds on my PC, and it's then normally necessary to restart the computer.
You can also enable the Instant Playback feature with the same sample rate options. Apparently, this sends a constant stream of digital zeros to the S/PDIF output circuitry, so that as soon as a legitimate signal is started it can take over immediately, without the normal USB startup glitch (of which more in a moment). It also ensures that connected equipment is 'ready to roll' (some DAT recorders won't enter record mode unless a clock signal is detected). The Instant Playback sample rate should be set to that of your playback file, otherwise Windows 98 may decide to perform some sample‑rate conversion without asking.
When I queried the convoluted rebooting procedure that seems to be required after changing settings, Opcode told me that, although they didn't mention it in the manual, you can bypass the rebooting procedure by temporarily unplugging the DATport for at least five seconds and then re‑connecting it. Tis forces the operating system to recognise the new settings. The only proviso (and the reason why the manual is silent on the subject) is that you must disable Instant Playback first, or you could face a nasty system crash.
In Use
Apparently, you can connect up to three DATports simultaneously (to provide three stereo outputs), given enough USB sockets, although only one of them can be in record mode. Since the USB audio drivers are supplied by Microsoft, DirectX support is also available, and this provides much lower latency values than the standard MME drivers.
As soon as I plugged the output of one of my Sony DAT recorders into the DATport the green LED lit up, and I then transferred a track from a DAT tape using Steinberg's Wavelab. I experienced no problems during the transfer, and subsequent playback using my other soundcards sounded fine. I even sent a WAV file to a DAT recorder using my Event Gina audio card's S/PDIF output, transferred it back again using the DATport, and compared the two files. They were bit‑for‑bit identical.
The DATport's S/PDIF output level at first seemed to be exactly 6dB lower than that of the WAV playback file, but this turned out to be because USB audio is provided with a Volume Control (the small speaker icon on the Taskbar launches the familiar Microsoft mixer applet). This initially seemed weird to me, until I spotted the 'Speaker' title on the mixer. For consumer purposes USB audio is likely to be used for digital speakers, and these need a volume control at the PC end. For digital transfer you always need this control at the very top, or your audio will not be making full use of the available bit depth.
Switching on the Instant Playback feature certainly cured a rather nasty 'crack' when starting playback using the USB S/PDIF output, but I still experienced glitches when I played back files. I connected the DATport output to the S/PDIF input of my Gina card and encountered a low‑level random glitch every few seconds. Despite various suggestions by Opcode, of the kind that I have mentioned before in these pages (graphic card buss‑mastering off, setting Read Ahead Optimisation to None, and disabling Write Behind caching), I never managed to completely cure this. I may have been unlucky, but I know of other users who have experienced similar problems (Opcode have helpfully provided a user forum on their web site), and this did leave me feeling rather concerned. However, a Microsoft Service Pack will be shipped shortly (Opcode think within the next couple of months) that "improves the efficiency of USB audio handling", and this may cure such problems once and for all.
Conclusion
USB provides a very useful way to get MIDI and audio in and out of any computer fitted with a suitable port, and for laptop owners without soundcard support it should be a godsend. Some people expected USB to provide a really cheap solution to digital audio I/O, but it is now possible to buy consumer soundcards with digital I/O as standard at less than the DATport's £199. The SB Live! (reviewed in SOS May 1999) is one example. However, the beauty of USB products is their ease of installation, and anyone buying a DATport should be up and running before the average soundcard is even bolted into the expansion slot. In addition, the SB Live! converts all digital signals to 48kHz on its digital output, which defeats the object of a digital transfer for many people.
Many systems work fine with the DATport, but other users complain of operating system glitches that won't go away.
Opcode have obviously worked hard on this, their first USB product, and have already overcome many hurdles, including jitter reduction (see 'USB Audio Design' box). They've also co‑operated with Microsoft to improve the USB audio drivers, something which will benefit all future USB products. However, some doubts remain as to how compatible with existing systems USB audio is. Many systems work fine with the DATport, but other users complain of operating system glitches that won't go away. Opcode are doing their best to help sort out these problems and I suspect that they will be resolved in due course, so I give the DATport a cautious thumbs‑up. However, I do advise you, if at all possible, to check its performance with your system before you part with your cash.
Masters & Slaves
Although many people consider that 'bits is bits', digital audio needs careful handling if you wish to avoid clicks and glitches. When recording from external equipment, such as a DAT machine, the DATport input must act as a slave, so that it can lock to the incoming clock signal. Conversely, when the DATport output is being used to transfer audio to an external recorder it must act as the master clock, to which the external gear slaves. However, it's possible to experience dropped samples and glitches if you attempt to use the DATport input and output simultaneously (for instance, while attempting to use a PC as an effects loop to add DirectShow plug‑ins to an external signal), unless you can lock the external signal to slave to the PC.
As an example, if you were using the DATport with a Yamaha digital mixer you'd need to put the Yamaha into 'External Sync' mode and let the PC be the clock master. As long as the DATport's Instant Playback was left on, the USB audio stream would continue regardless and keep the Yamaha mixer locked on. If you needed to change a setting, such as sample rate or bit‑depth, you'd have to turn down your monitor volume, as Instant Playback would stop as soon as the Config utility was opened, and the result would be high‑level bursts of digital noise. Only when the Config utility is closed does Instant Playback become active again.
USB Audio Design
I whipped off the top cover of the DATport for a peek inside and was surprised to see so much circuitry — a total of 10 integrated circuits, including a Crystal CS8425 digital audio interface transmitter and receiver. There is also provision on the circuit board for audio In and Out sockets to be fitted for Opcode's new SONICport model, which includes 20‑bit A‑D and D‑A converters. A third SONICport Optical model, as its name suggests, swaps the co‑axial digital I/O for a pair of Toslink optical sockets.
One of the reasons for the circuit complexity is apparently that the USB exhibits high levels of jitter. Opcode told me that they were originally experiencing up to 1mS of timing variation from the buss, but their de‑jittering design reduces this to 10nS with 44.1kHz signals, and even lower with 48kHz signals. However, this does mean that all USB audio devices (even MIDI ones) have the potential for sloppy timing — don't automatically
assume that 'bits is bits'. Let's hope that all manufacturers are as careful as Opcode to reduce such effects, and that in future jitter figures are mentioned in specifications.
As a further source of possible confusion, Intel have just announced USB 2.0, which they hope will be ready in about 18 months. This will be 10 times faster than the current USB 1.1 (and therefore comparable in speed to Firewire and SCSI), but will still be fully compatible with it. Many industry experts are not convinced that this can be done, since it is likely that connecting a slower USB peripheral will greatly slow down a simultaneously connected fast one.
Pros
- Provides bit‑for‑bit transfer of digital audio.
- Works with USB‑equipped laptops.
- You don't have to open up the computer to install it!.
Cons
- Some users may experience playback glitches, depending on their system.
- Reconfiguring the input is long‑winded.
- Mac drivers not yet available.
Summary
The DATport is the first USB audio product to be launched, provides the easiest way to get digital audio in and out of a PC or Mac, but may have a few lingering problems in some systems.