dbx Model 700 Digital Audio Processor
A professional delta-modulation processor that put broadcast-grade digital audio within reach of studios that couldn't afford a full digital chain.
Overview
The dbx Model 700 is a digital audio processor built around dbx's proprietary CPDM - Companded Predictive Delta Modulation - technology. It encodes the *change* in signal level between successive samples rather than the absolute sample values. Because delta-modulation processors work from level differences instead of absolute amplitude, they can get useful resolution at lower sampling rates than a straight PCM converter of the era. High Fidelity (August 1984, p.46) walked readers through this principle, noting that a prediction filter in the delta-modulation architecture adds an additional 10 dB to the signal-to-noise ratio. The same piece put the Model 700's price at $4,600 - roughly one-fourth the cost of the competing professional digital systems it was measured against.
That price is the whole story of the machine. In the early 1980s, digital audio in a professional facility meant a big capital commitment. dbx built the 700 to sit in that gap: a processor that could encode and decode with enough fidelity for broadcast and studio work, at a fraction of what a full digital recorder chain cost.
The Model 700 was not a consumer curiosity. The professional press documented it as a working tool. Recording Engineer (October 1982, p.148) ran a two-part feature on it - design parameters and systems implementation from dbx senior project engineer Robert W. Adams, paired with an operational assessment from Crescendo Recorders in Atlanta. A follow-up piece in the same publication a year later, in October 1983 (p.24), carried a progress report by dbx vice president of engineering Leslie B. Tyler, under the heading "One Year Later." So the manufacturer's own engineering leadership wrote the technical documentation.
The broadcast world took notice. DB Magazine (April 1984, p.38) recounted how Bill Busiek, engineer-in-charge for WGBH 89.7 FM in Boston, initiated an unannounced test of the dbx 700 from the Symphony Hall recording booth. The station ran a blind test with one Model 700 encoding from Symphony Hall and a second unit at the studios encoding studio-originated programming. The response reportedly prompted one of WGBH's severest critics to call the station and declare it the best Boston Symphony broadcast he'd heard in twenty years. That's an anecdote, not a measurement.
By 1984 the 700 was appearing in facility equipment lists. Recording Engineer (April 1984, p.177) noted that the processor, using dbx's CPDM technology, was being adopted by radio stations for broadcast applications as well as by recording studios. A later facility update in the same publication recorded Chase Productions in Hollywood installing a *second* dbx Model 700 alongside a Sony PCM-1610. The Sony was the reference digital processor of the period, and the dbx was the one they bought twice.
The Model 700 is not plug-and-play vintage gear. It's a professional processor from a transitional moment, when delta modulation was a live alternative to PCM and the format wars were still open. By the mid-1980s dbx itself was gearing the technology toward professional recordists while the broader delta-modulation effort faced an uncertain future, as High Fidelity (August 1984, p.46) observed in its discussion of where the technology was heading.
Similar items on eBay
No exact dbx MODEL-700 listings are live right now — these searches show current dbx and similar vintage audio gear on eBay:
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.