Lexicon T-102
A stereo digital delay from the early digital era, built to get as much fidelity as possible out of a signal path that most of the industry was still figuring out.
Overview
The Lexicon T-102 is a stereo digital delay that came out of the Delta-T 102 series, a line Lexicon developed with Gotham Audio. It started four years before the 102 series with the Delta-T Model 101, which Lexicon and Gotham introduced as the industry's first all-electronic audio delay system. The 102 series was the second generation, built on practical experience and user feedback gathered since the 101 shipped. DB Magazine (April 1974, p.17) covered the announcement and noted that the new series offered a level of performance and flexibility previously unavailable to the industry.
The 102 series used Lexicon's proprietary floating-point digital encoding, which gave a 90 dB dynamic range. That was a big number when many competing delay lines, digital and analog, struggled with the noise and distortion that come with quantization. The gritty, signal-dependent noise that plagued weaker digital designs showed up 90 dB down here, well into inaudibility. The 102-S specification called for less than 0.2% total noise and distortion at 1 kHz maximum level. The unit also held distortion to less than 0.3% at levels 34 dB down from maximum, a critical spec that many manufacturers conveniently left out of their own documentation.
The T-102 has two independent delay lines in a single chassis. They can be clocked together via crystal or VCO, or run completely independently with different signals, which is handy when a session calls for a delayed echo and doubling at the same time. The VCO module, designated VCO 102, does the interesting work. It handles signal processing in the time domain, which gives you natural double tracking, flanging, vibrato, time delay panning, extreme pitch modulation, and signal transformation for special effects. The front panel controls modulation frequency from 0.2 Hz to 2 kHz, with amplitude and delay peak-to-peak adjustable alongside. A toggle switch selects between crystal, for normal operation with a fixed time base, and manual control, for delay adjustment over a 1X to 2X range.
System specifications include 192 ms of delay per channel with 3 ms taps, dual five-position LED headroom indicators, and computer-quality components throughout. The modular construction was deliberate. A studio could start with a single channel and expand to full stereo later by installing modules in the field. Delay or memory could also be expanded in 48 ms increments. For facilities that would otherwise buy two separate delay units, adding a second delay this way saved more than $1,000. As Recording Engineer (June 1977, p.65) observed, a VCO combined with a two-channel system proved extremely powerful, and the modular approach gave users both economic value and a hedge against obsolescence.
The VCO also opens up performer-controlled uses that go well beyond studio processing. A foot pedal connected to the VCO lets a musician control time delay over a 2:1 range during performance. For flanging, that means the spacey sound can be changed at will, faster or slower, lingering at a particular comb setting, with the performer hearing the effect on phones or speakers. A percussionist with a foot pedal can tune percussive sounds up and down over an octave range in real time. A vocalist working with a basic 20 ms delay can vary doubling across a 20 to 40 ms range, pushing into slight echo territory. The pitch shift while moving the pedal can also bend notes briefly, which interests keyboard players stuck with instruments that don't allow slurs or bending. The VCO also accepts an external 0 to 5 volt control source, so a synthesizer, lab function generator, or homebrew circuit can drive the system clock over its 2:1 range.
The frequency response design got serious engineering attention. The 102-S uses Butterworth, maximally flat filters at input and output. That gives smooth, flat response out to the edge of the passband, and it rolls off with good margin well before the Nyquist half-sampling frequency to prevent aliasing. This is hard to do in analog delay designs, particularly bucket brigade devices. Keeping the response smooth and extended while the clock varies to change delay is extremely difficult, and noise rises steadily as a function of delay time multiplied by frequency response. The digital design of the 102-S reduces these problems to nonaudibility at the A-D conversion stage. After that they stay independent of delay time across the unit's frequency range.
The system supports 1 to 50 outputs and delay times from 2 ms to 3.2 seconds across the 102 series. Three basic models let buyers pick frequency response and delay interval for sound reinforcement, studio, or laboratory use. All models have a five-position 40 dB headroom indicator for setting levels correctly. The 102 series was priced competitively with contemporary 10 and 12 bit units, even though it delivered the dynamic range of a 15 bit system.
For live recording in concert halls and other reverberant spaces, the 102-S has a particular advantage. Recording engineers have long dealt with a conflict between the close pickup needed to highlight or balance certain instruments and the more remote pickup needed to capture the overall orchestra embedded in room sound. A versatile delay unit like the 102-S lets the engineer blend close and ambient pickup without the phase issues that plague simpler approaches.
Lexicon offered a complimentary application note with a companion demonstration record. Interested engineers could get it by writing to the company on letterhead and requesting the AN-3 Application note. The record contained several of the effects described in period coverage, so potential users could hear the unit before buying. Delivery was quoted at 45 days.
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