SEKD Prodif 96

One software package that fully capitalises on the Prodif 96 is Samplitude 2496, also from SEKD. This provides a comprehensive multitrack audio recording environment.

If you want more of everything digital, why not try 24‑bit recording at a 96kHz sample rate? Martin Walker is floored as his dynamics increase in leaps and bounds.

As regular SOS readers will be aware, it is becoming increasingly common to carry out digital recording, processing and mixing at greater resolution and a higher sample rate than the 16‑bit, 44.1kHz standard of commercial CDs. This has led some people to believe that recording at 16 bits and 44.1kHz cannot produce recordings of sufficient quality for commercial release.

One of the problems is that while the final product can use the full 96dB dynamic range of 16 bits, digital audio recordings must always be made with some headroom to avoid the possibility of digital clipping. So, in an effort to use as much of this dynamic range as possible, many people's eyes stay glued to the meters to get a hot signal level. To cope with unexpected peaks when recording live music, however, a typical safety measure is to work at nominal levels that are 12dB or more below digital clipping. (That's in addition to using a limiter as an emergency 'brick wall'.) Clearly, under these conditions, the dynamic range is reduced to 84dB and only uses 14 of out the 16 available bits.

A second problem is that because so many software processes are now being used to modify digital audio data after recording, the quality can suffer during the application of the many arithmetic processes involved.

The answer is to start with a higher bit resolution, which instantly gives a much greater dynamic range. You can then operate with a higher headroom, without worrying so much about compromising audio quality, and you can also keep your eyes fixed on the performance rather than the level meters. The current goal seems to be 24‑bit, and to cater for future commercial formats, having the option of a maximum sample rate of 96kHz is also advisable, although this only extends the frequency response to about 40kHz, and has no effect on dynamic range.

Cards On The Table

These are suitable buffer settings for trouble‑free operation in Wavelab at 24‑bit resolution. The Get Play position from audio driver option is also checked, which ensures that the cursor and level meter will be in sync with the audio signal.

Enter then the SEKD Prodif 96 soundcard. This is a PCI card providing extensive stereo digital I/O and a D‑A converter for analogue monitoring. The digital I/O handles 16, 20 and 24 bit depths (but not 8‑bit), and supports sample rates of 32, 44.1, 48, 64, 88.2, and 96kHz (but not 22.05kHz). All digital inputs and outputs are stereo, and are provided in optical (Toslink) and coaxial (phono and XLR) form. Input and output format can be either S/PDIF (consumer) or AES/EBU (professional). I/O is transformer‑coupled for hum free operation.

The optical sockets are mounted on the backplate, but the phono and XLR sockets are at the far end of a supplied nine‑inch flying cable, which attaches to the soundcard via a 9‑pin 'D'‑type connector. There is also an additional internal digital input on the card for the direct connection of a suitable CD‑ROM player. A single error LED is also provided on the backplate, and this stays lit until a valid digital signal is detected at any digital input.

For monitoring purposes, there is a 20‑bit D‑A converter, which again supports sample rates of up to 96kHz. This line output is from a quarter‑inch stereo jack socket. This option is better than the usual 3.5mm stereo jack socket, although you may have to make up a special splitter cable to separate the two output channels at the other end of the cable (none is supplied). There is also a switch for line output level between +6/‑6dBu — an unusual pair of values, but still in the right ball park for typical +4/‑10dBu mixer inputs. SEKD say that since the impedance is low, you can use this output not only to feed a mixing desk, but also for headphones.

To achieve the required data transfer rates, most soundcards use software drivers to control their many features directly, an approach which can place a considerable load on the CPU. The Prodif 96 uses a high speed XILINX 4006 programmable FGPA (Field‑Programmable Gate Array) chip which the driver software is only called on to configure and thereafter leaves well alone. Essentially, the software is running inside this hardware chip (see the review of the Digital Wings soundcard in SOS February '98 for more details of FGPA technology). With drivers available for Windows 95, Windows NT and, soon, for MacOS, this spec looks ideal for recording high quality audio. But, as always, the proof of the pudding...

Installation

All the operational switches are found in the Properties page. The sample frequency appears as soon as a valid signal has been detected at the selected input.

The PCI card is just short of five inches long, and so should fit into virtually any available slot on your motherboard. I only had a single slot left on mine, and initially had some conflicts with existing hardware. This was quickly resolved after I removed my Event Gina card during the review (although it may well be possible to get both cards running side by side with a bit more tweaking).

The Prodif 96 hardware was recognised on the next boot. I inserted the driver floppy disk and looked for the Windows 95 folder mentioned in the manual. Unfortunately, there was no such folder, but it didn't take long to contact SEKD distributors SCV London to discover which of the five sets of drivers was the one to use. I was soon back to my Windows 95 desktop with the new driver in place, along with an extra desktop shortcut to the Control Panel/System window (which, as I explain later, you are likely to need on a regular basis).

Actually, the manual needs updating, since apart from the problem with the driver software, it also apologises for a hardware limitation that has now been solved, and has an uncredited screenshot of the Wavelab Preferences page showing suitable soundcard settings for 24‑bit operation. I use Wavelab, so I recognised it, but anyone without this software might be very confused.

Analogue Audition

As I already had some 24‑bit WAV files on my hard drive, I started by auditioning the D‑A converters. These are 20‑bit devices from AKM, and have a claimed dynamic range of >94dB. I could just hear 1kHz tones below ‑100dB, which is pretty good going. In general, playback quality was excellent, with very low noise. I also plugged in a pair of 60Ω headphones, and got a healthy level with the output level switch at its higher, +6dBu setting. For anyone mainly using external converters, this would be a useful extra function.

When used at the maximum 96kHz sample rate, your bandwidth will be much higher than normal. Although SEKD don't quote a figure, AKM claim that the frequency response extends to 40kHz at +/‑0.5dB, and +/‑0.1dB at 20kHz. I was able to try out some audio at 88.2kHz, and this certainly sounded very good. Mind you, despite taking twice as much space on your hard drive as a 44.1kHz signal, the actual improvement will be comparatively small — such is the stuff of high‑end audio.

Incidentally, you may be wondering why SEKD don't provide a 24‑bit D‑A converter. Well, I suspect that they wisely decided that 20 bits is quite sufficient for general purpose monitoring. After all, it can be argued that 24 bits are needed more at the recording end of the chain. A 24‑bit D‑A device would not only be significantly more expensive, but would probably be better placed in an external rack unit (see the Suitable Partners Box) as the insides of computers are not renowned as being pristine audio environments.

A Few Bits More

On the digital side, the Properties page provides a host of useful options and information. Unlike analogue signals, it is sometimes difficult to work out why a digital one is not working, but here you can click on any of the four inputs (Optical, Coaxial, XLR, and the internal one on the card) and, as soon as a valid input signal is detected, the Sample Frequency is shown. This is updated twice a second, and makes fault‑finding a lot easier. If you are round the back of the computer, plugging in a valid digital signal causes the red error LED to go out.

The Outputs have three selectable modes. Automatic is the normal one, where the input signal reaches the output only during recording. However, if you are using a digital mixing desk, you can change this to Play Only, to avoid getting feedback when you start the recording software. The third option is Input, and this lets you continuously monitor any signal appearing at the input, without having to run other software in Record Ready mode.

With AutoSync activated, you can start recording on the fly, as the card is constantly searching for a valid input signal. This is clever, since in the absence of a valid input, the card uses its internal quartz clock, and as soon as the PLL (Phase Locked Loop) circuitry on the card can generate a clock from an input signal, it switches over automatically. You can also use this feature to lock several cards together during playback, by chaining each output to the unused input of the next. The input of the first card would still be available for recording. AutoSync can be disabled if you are connecting the inputs and outputs of a DAT machine for example, where it would lead to a local feedback loop.

The output format is switchable between Consumer and Professional, and rather than just passing any status information from input or output, the Prodif 96 creates a new header. This has been optimised to be widely compatible with other hardware — the appropriate bits are set according to the selected format, and the copy bit is set for No Copyright/Copying Permitted. This should ensure that even budget DAT recorders with optical or coax S/PDIF inputs accept the signals. Finally, since the incoming status bits have been discarded, ticking the Emphasis box allows you to set this bit for recordings that need it, and you can immediately hear the corrected frequency response through the D‑A converter. I had no problem connecting a variety of digital devices, and even my sometimes truculent Sony DAT recorders seemed quite happy to send and receive data.

Summary

SEKD manufacture a wide range of PCI soundcards, but the Prodif 96 is among the most expensive. This is hardly surprising (24‑bit/96kHz capable chips don't come cheap), although at £599, the price has come down a bit since the card appeared in the Buyer's Guide in SOS March '98. According to the current manual, initial models were only capable of digital transfers at up to 50kHz, but the digital I/O chip has recently been upgraded to the 96kHz capable version (the Crystal CS8414 chip was only released in February '98).

I didn't really have the Prodif 96 long enough to extensively explore the improvements inherent with a 96kHz sample rate, but 24‑bit resolution certainly made a significant difference to the noise floor (as you might expect), and allowed me to concentrate more on recording the best performance to disk by leaving more headroom, rather than having to worry about compromising dynamic range. The onboard D‑A converter also acquitted itself well. Obviously I cannot comment on recording quality, since there are no analogue inputs, but the digital recording options are comprehensive and well thought out, and should resolve most of those niggling incompatibility problems when interfacing consumer and professional gear. The Prodif 96 should provide any user with a fairly future‑proof interface for high quality hard disk recording.

One multitrack package that does fully capitalise on the capabilities of the Prodif 96 is Samplitude 2496, from (you guessed it) SEKD, which also retails at £999. This provides comprehensive recording facilities, and not only supports third party DirectX plug‑ins, but also has facilities to burn a CD directly from within the same environment.