SPL Machine Head

Tape saturation is the latest thing to get the physical modelling treatment. Can two high‑powered DSPs, however, really achieve the same effect as a strip of rust being dragged over a magnet? Paul White investigates.

It seems that the greater the advances made in digital recording technology, the more people look to retro technologies to add some kind of mystique back to the sound. The valve versus solid‑state debate goes back to the '60s, when solid‑state guitar amplifiers first appeared; in recent years, we've seen renewed interest in valve microphones, valve preamps, and valve outboard gear such as compressors and equalisers. There's little doubt that well‑designed valve equipment does change sound in an artistically desirable way, but now that hard disk recorders, DAT machines and digital multitrack tapes have taken over from open‑reel analogue machines, the latest candidate for nostalgia is the analogue tape sound itself.

For a few hundred pounds, you could buy a good used mastering machine, such as a Revox B77, and bounce your DAT tapes off that to add the necessary magic — but along with that magic come tape noise, wow and flutter, and an inability to control the effect very precisely.

Enter modern technology and SPL, who, in conjunction with some very talented software designers (the same ones responsible for the most excellent Steinberg WaveLab optional plug‑ins), have built a DSP‑based hardware box that models the desirable attributes of tape saturation entirely in the digital domain. Being able to do this digitally is particularly important when you're mastering, as recordings can be processed without ever leaving the digital realm. To ensure a certain amount of future‑proofing, signals of between 16 and 24 bits can be accommodated.

In keeping with the SPL philosophy, the Machine Head has very simple controls, but what goes on inside the box is actually rather clever. Tape‑saturation effects are not as simple to model as you might think, not least because audio signals are pre‑emphasised before recording as a means of optimising the signal‑to‑noise ratio. The process is, of course, undone on playback to restore the original signal characteristics, but any tape‑saturation effects due to the non‑linearity of the recording medium affect the signal between these two stages. Among other things, this means that high frequencies (those that are pre‑emphasised) are subjected to more tape compression than lower frequencies. Furthermore, the compression isn't the same as the type you'd expect from a conventional compressor — these simply adjust the overall envelope of the signal, whereas tape compression modifies the shape of each individual wave being recorded. The higher the level, the more non‑linear the recording process; in practical terms, this means that not only are high‑level peaks reduced in level, but harmonic distortion also takes place. This combination of frequency‑selective dynamic cycle‑by‑cycle compression and harmonic distortion is largely responsible for the warmth and power of analogue recordings, and SPL's Machine Head models many of these factors very precisely.

If that's all the Machine Head did there would only be one knob, but the designers have added a little more creative potential by making the high‑frequency saturation characteristics variable to such an extent that you can actually reverse the high‑frequency damping effect of tape and turn it into high‑frequency enhancement. You can also select the speed of your imaginary tape recorder — though, in the interests of preserving sanity, no varispeed control is fitted!

The Red Box

The Machine Head comes as a very deep 1U rack unit and is based around a pair of Motorola 56002 DSP chips clocked at 66MHz. There are no analogue ins or outs, but both AES/EBU and S/PDIF digital options are supported; in the event of a power failure, the input is routed directly to the output via a hard relay bypass. The processing delay is around 5ms, and this is compensated for when bypass is operated, by which I assume that the bypassed signal is also delayed by the same amount. Operation is strictly stereo. RS232 and 434 sockets are fitted, so the Machine Head can be connected to a computer: these are important, as future software upgrades can be loaded in directly from a computer (Mac or PC), via a modem cable. For applications in systems sync'ed to digital word clock, the Machine Head has a word clock input and Thru connector with switchable 75Ω termination; there are also MIDI In and Thru sockets enabling stored patch settings to be recalled over MIDI.

The operator has a fairly easy job, with just four knobs and one button to set up the parameters, and five further buttons to deal with the selection and storing of patches, plus bypass. Three sets of stereo PPM meters monitor the input level, the drive level (how hard the virtual tape is being driven into the virtual red), and the output level; to provide a comfortable analogue feel, the four encoder knobs have a smooth rather than a stepped action. The first LED of each input and output meter lights steadily when a compatible digital input is being received.

The Machine Head has very simple controls, but what goes on inside the box is actually rather clever.

A small LCD window provides information on patch numbers, parameter values and various aspects of the system setup, and the four rotary controls are used to adjust the input gain, the drive amount, the HF compression characteristics and the output gain. The Tape Speed button changes the 'speed' of the virtual tape machine from 15ips to 30ips — though the differences are very subtle, it's nice to have the choice. To avoid accidental patch changing, the Apply button must be pressed for at least one second before you use the Up/Down keys, otherwise they're locked out. Holding down Apply again loads the patch, and you can store up to 99 presets. This is fair enough, but the Apply routine can be a little slow, so it would have been nice to be able to disable it in the setup menu, enabling instantaneous patch changes with just the Up/Down buttons.

To save a setting, press Store for one second, then, when the LED above the switch flashes, find the destination patch using the Up/Down buttons. Holding these down causes the presets to skip through in groups of five. When you've found the location you want, depress Store again to complete the operation. Pressing Store and Apply together will bring up a menu showing the details of the digital data stream, such as I/O format, sample frequency, copy prohibit flags (setting or erasing), error flags, emphasis and so on. The Up/Down keys are used to navigate from page to page, and this section also enables the MIDI channel to be set and the serial input (RS232 or RS434) to be selected. MIDI commands can be used to change settings during a mix without causing glitching, and it's even possible to use MIDI volume commands to produce fade‑outs, which is a nice idea.

Machinations

Operating the Machine Head is very straightforward, and is largely a matter of setting the input gain for maximum level without clipping, then adjusting the Drive level until the right amount of effect is being added. Finally, the output level needs to be set for the maximum level without clipping on signal peaks. The HF adjustment is best done by ear, and the tape‑speed adjustment is so subtle that the difference in harmonic structure is inaudible on most material. Of course, the question is: what does analogue tape saturation sound like, and is it a good idea?

Because the tape‑compression algorithm has the effect of holding down the level of signal peaks, there's a subjective increase in average signal level and a consequent increase in perceived loudness; if you go heavy on the processing, this can be as much as 10dB. This is easily verified just by setting the output gain so that the input and output are subjectively the same level, then looking at the actual difference in peak levels as shown by the meters. On a more subjective level, the squashing of transients makes the overall sound a little more comfortable and better integrated, and instruments such as electric guitar sound smoother and more powerful. Vocals also sound thicker and seem to sit better in the mix. If all this sounds like the kind of pseudo‑scientific burble you hear when people are eulogising about valves, that's because the effect of tape saturation is really very similar to that of tube saturation. Interestingly, material that's already been recorded on analogue, passed through a tube device, or compressed hard, doesn't benefit much from the treatment, but a DAT master made without the benefit of compression during mixing really does sound a lot louder and warmer after processing.

Summary

Because the Machine Head is so simple to use, I found the ability to store and recall patches virtually unnecessary; if you want to return to a project later, though, I guess it could be useful. However, you can't name patches, so you have to remember them by number. Once a patch has been called up, all the control parameters are visible on the screen at one time. Adding the MIDI fade utility is obviously useful, but I found myself asking why SPL hadn't put a few more features in this box to help justify what is undeniably a high price tag for a very specific process. For example, I'd have liked to see some form of digital EQ included, and a sample‑rate conversion facility would also be immensely useful in a mastering situation.

Wish lists aside, what the Machine Head does it does extremely well. As far as I can judge, the analogue tape saturation simulation is extremely authentic and very controllable, and the unit is certainly easy to use. In the analogue domain, a good tube preamp or even an SPL Charisma unit will produce a broadly similar result, but in a mastering situation the Machine Head has the advantage that it keeps everything digital, and doesn't place a further burden on an already heavily loaded computer audio workstation. It's expensive, but if you want to do the job properly...