UREI Model 927 Digital Delay Line
UREI sat out the digital delay market on purpose, then showed up with four isolated outputs, 92 dB of dynamic range, and no companding anywhere in the chain.
Overview
The Model 927 is a four-channel digital delay line. UREI's own trade advertising for it reads less like a spec announcement than a grievance. The copy opens by admitting the company had deliberately stayed out of the DDL market, because the digital delay lines already on sale were marginal performers. UREI said it wouldn't enter the "DDL ballgame" until it had something worth putting in a UREI uniform. The 927, described as the product of more than two years of research and development, was that entry, offered at what the company called a very competitive price. No figure is attached to that claim anywhere in the available documentation.
UREI was attacking the analog front end of contemporary delay lines. The advertising lists the complaints directly: gain pumping and transient overload from companding A/D converters, whistling at the top end, and pre-emphasis/de-emphasis filters that ate 12 dB of high-end headroom just to meet a 90 dB spec. UREI's answer was a conversion scheme it called instantaneous floating point conversion. The company claimed it delivered greater than 90 dB of dynamic range without companding and without high-end pre-emphasis, and without the side effects UREI attributed to other floating-point and delta-modulation designs. The published figure is 92 dB dynamic range, with full-power bandwidth to 12 kHz at all delay settings and distortion typically 0.07% at full rated output. Whether those numbers held up on a bench is not something the sources reviewed here establish. They are UREI's own published claims.
The anti-aliasing job falls to an 8-pole, 6-zero Cauer filter, an ultra-steep design that UREI said put an end to the whistles and beats. One account of the 927's field deployment describes a 150 dB-per-octave filter at 12.5 kHz that effectively looked like a brick wall to any frequencies beyond that. It is credited with eliminating the sampling-rate noise that had plagued other delays of the period. A gentler filter lets aliasing artifacts through, and those are what you hear as grit on top of a horn stack.
The feature that made the 927 unusual is four separate, isolated outputs, each independently adjustable by thumbwheel switch up to 127 milliseconds in 1 ms steps. Most of what the 927 was competing against gave you one delay, or one delay per box. Separate outputs also mean separate amplifier zones can be fed without ground loops or crosstalk between them.
The 927 was announced into a market that was watching for it. Studio Sound (August 1978, p.60) reported from the AES 60th Convention that the new UREI 927 DDL offered four separate outputs with independent delay tuning up to 127 ms on each. By that November, Studio Sound (November 1978, p.54) was noting that UREI would be demonstrating production models of the new Model 927 digital delay line at the AES show, which suggests the August sighting was a preview and the box was shipping by late 1978. DB Magazine (June 1978, p.10) and DB Magazine (October 1978, p.8) both ran Model 927 DDL listings tied to the same "What Took You So Long, UREI?" trade advertising, and Recording Engineer (June 1978, p.79) carried the same headline.
The job the 927 is most famous for shows what it was built to do. In the spring of 1979, Ultra Sound put together one of the first time-corrected concert sound systems built around a conventional stacked multiway array, for a Grateful Dead concert outdoors at Spartan Stadium in San Jose. Dan Healy of the Grateful Dead and Don Pearson of Ultra Sound designed it. The system used four individual columns of speakers per side: 18 bass bins holding 54 E-V M15L woofers, a mid-bass stack of nine cabinets containing 36 JBL K120s, ten mid-range horns on Gauss HP 4000 compression drivers, and 12 ESS HT 600 air-motion tweeters on top. A four-way side-fill system sat behind the main stacks.
Up to that point it was an ordinary large outdoor rig. What set it apart was Pearson's decision to time-align the entire PA using a single UREI Model 927 four-channel digital delay. His reasoning, as he described it, was to delay each frequency band of the PA separately so that a simultaneous pulse into the whole system would combine acoustically in front of the stacks and arrive anywhere in the field as a single wavefront. Ultra Sound had tried this before and been unhappy with the results. The account blames high-end distortion and aliasing inherent in the delay units then available. The 927 solved it: no companding meant no breathing, and the steep filter killed the sampling-rate noise. Pearson wasn't worried about the 12.5 kHz ceiling outdoors, noting that he generally rolled his systems off starting around 4 kHz anyway, down 10 dB at 10 kHz. Four bands took four isolated outputs and four independent delays.
The 927 is not a general-purpose studio delay with a nice display. It's a time-alignment tool for multiway systems, and its reputation rests on a specific, well-documented deployment rather than on a broad base of studio users. Whether it was ever a big seller is not something the available documentation establishes.
The 927 is a late-1970s digital product built around a Cauer filter network, an instantaneous-floating-point converter of UREI's own design, and thumbwheel switches for delay setting. The documentation available here says nothing about service history, common failure points, parts availability, or current market prices. Anyone chasing one should assume the usual risks of a niche 1970s digital device: unobtainable custom parts and no published schematic in the sources reviewed here. The converter and filter topology is one nobody builds anymore. If it works, you get four channels of compander-free delay with a steep anti-aliasing filter, in a box UREI spent two years on before releasing.
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