Studer SFC-16
A box that solved the problem nobody wanted to admit digital audio had: too many sampling rates and no way to move signal between them.
Overview
The SFC-16 is a two-channel, purely digital sampling frequency converter — a piece of infrastructure gear that existed because the professional audio world of the early 1980s could not agree on a single clock rate. Studer built it to accept arbitrary sampling frequencies from 30 to 554 kHz and translate between them, so that digital programs could move between recorders and systems running on conflicting clocks. It is the kind of product that never appears on a wish list and yet becomes indispensable the moment a studio owns more than one digital machine.
The architecture is unusual enough to be worth describing. The converter operates strictly under the control of the incoming clock signals, which means it requires no programming at all — no menus, no setup routines, no reference to a sample-rate table the user has to consult. It simply locks and converts. DB Magazine (April 1983, p.12) describes the filtering as a four-filter cascade mathematically equivalent to a fixed linear-phase FIR filter of roughly 1.18 million taps, an enormous computational structure for the period. DB Magazine (April 1983, p.12) characterized the noise performance as "quite good," noting an output noise floor measured on a pure sinewave, and described the conversion as performed at an effective rate of approximately 1.57 GHz — a figure that speaks to how hard the hardware was working to interpolate between unrelated clocks.
Physically, the two-channel version fits into a 19-inch rack or cabinet of 11-inch height. DB Magazine (April 1983, p.11) reports that the stereo unit is built around eight digital circuit boards: one for interfacing and filter self-testing, one for clock generation and processing, and further boards handling the processing circuitry and its self-test functions. The design supports both up-conversion and down-conversion — DB Magazine (April 1983, p.11) described the scaling relationships in the UP and DOWN modes, with a relative bandwidth of roughly half the Nyquist frequency, which works out to about 0 to 21 kHz at a 48 kHz sampling frequency and about 0 to 19.3 kHz at the lower rates.
What the SFC-16 was for, in practice, was the messy middle ground of early digital audio. Studer positioned its applications as ranging from format conversion to mastering for the Digital Audio Disc, and to digital transmission of audio programs. The company's own product literature from the period is candid about the standards chaos driving the design: Studer ReVox simultaneously called for a worldwide agreement in professional digital audio, proposing 32 kHz for broadcast transmission, 44.1 kHz for Digital Audio Disc production, and 48 kHz for digital recording and digital video compatibility. The SFC-16 was the hardware answer to the fact that no such agreement existed yet.
The converter also turns up in Studer's own system documentation as a problem-solver for varispeed work. In the description of the D820X-2 digital two-track, the varispeed control allows a deviation of ±12.5%, and the digital output signals vary accordingly — but an SFC-16 can be used to provide automatic correction during transfer to another digital recorder. That is the sort of application that made the box quietly valuable: any session where a digital machine was running off-speed, or where two machines at different rates needed to talk, had an answer.
A prototype SFC-16 appeared alongside a prototype Studer digital 24-track on the cover of Recording Engineer/Producer, with the complete Digital Recording System slated for demonstration at the AES Show in Montreux. The unit also appears in BM/E's coverage of the standards debate, photographed beneath a 3M multitrack digital recorder as an example of the new conversion hardware arriving to bridge the format wars. Contemporary commentary framed it plainly: the SFC-16 was described as the first of what would undoubtedly be many digital audio standards converters, with the architecture in place for a digital audio matrix.
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