Behringer Ultra-Curve PRO DSP-8024
A 24-bit digital equalizer-analyzer that packs an RTA, a 31-band graphic EQ, and a feedback destroyer into a two-rack-space box for the price of one analog graphic EQ.
Overview
The Ultra-Curve PRO DSP-8024 is Behringer's attempt to fit an entire rack of outboard processing into one box. According to the period review in Audio Express (February 2003, p.54), the unit is a 24-bit digital audio processor. It combines a real-time analyzer with an Auto-Q function for automatic room measurement and correction, a 31-band graphic equalizer, and a three-band parametric equalizer. That's a lot for a single 3.5-inch-tall chassis, and the list keeps going. The same review notes a peak limiter, an adjustable digital delay, a noise gate, and a feedback destroyer feature, all in the same unit.
The package is rack-friendly. Audio Express (February 2003, p.54) lists the dimensions at 3.5 inches high by 19 inches wide by 12 inches deep, with a net weight of 11 pounds. The review unit carried a list price of $230 U.S. and a one-year warranty, with service available through Behringer USA in Edmonds, Washington. The price is the big story here. The review in Audio Express (February 2003, p.54) frames the DSP-8024 as part of a broader shift in the market. What once took a rack full of separate processors - a graphic EQ, an RTA, an auto feedback controller, a limiter/noise gate, and a delay line for loudspeaker management - could now be had in a single piece of gear for less than any one of those separates might have cost only ten years earlier.
Connectivity is split between analog and digital. Audio Express (February 2003, p.54) reports two analog line inputs and one microphone input, provided through XLR and stereo phone jacks. An optional AES8024 feature adds AES/EBU digital inputs and outputs to the DSP-8024, though the review unit did not include this option. On the rear panel, per Audio Express (February 2003, p.55), the layout includes an IEC power receptacle with the power switch just above it, a line fuse in a drawer within the IEC receptacle, and a blanking plate covering the cutouts for the AES/EBU digital jacks on units not so equipped. Below that plate sit MIDI out, through, and input DIN connectors. Next come stereo pairs of analog output phone jacks and gold male XLR jacks, then stereo pairs of analog input phone jacks and gold female XLR jacks, and finally a gold female XLR microphone connector that also provides +15V phantom power. The third pin of the AC receptacle is connected to the chassis.
Inside, the DSP-8024 is a serious piece of digital hardware. Audio Express (February 2003, p.55) describes the chassis as black painted steel with a 3mm thick aluminum front panel. It's rack-mountable, with removable rack supports for shelf mounting, and four adhesive rubber feet are included for that purpose. The review notes that "BEHRINGER" is spelled out in the cooling louvers on the top cover. The front panel centers on a 240 x 64 backlit LCD, with four softkeys whose functions change depending on operating mode, plus a key to access menus.
The digital signal path is built around two Alliance 1MB x 4 DRAM chips and a Windbond 32K x 8 SRAM chip for memory. A lithium backup battery maintains RAM contents when power is off. A Dallas DS1210 nonvolatile controller monitors battery status and write-protects the RAM, so the unit always returns to its last-used functions when powered back on. Audio Express (February 2003, p.55) reports that processed analog signals are restored by a Cirrus Logic CS4390 24-bit, 48kHz stereo digital-to-analog converter. The DSP-8024 has adjustable system sampling rates including 32kHz (AES/EBU only), 44.1kHz, and 48kHz, with 48kHz as the default analog signal sampling rate. Digital de-emphasis is provided for all three rates.
The measurements are where the review gets interesting. Audio Express (February 2003, p.55) found the input impedance measured 24k9 unbalanced and 49k5 balanced, independent of volume setting, against a manufacturer's rating of 25k unbalanced and 50k balanced. Output impedances measured 48 ohms unbalanced and 98 ohms balanced at 1kHz, against a rated 30 ohms unbalanced and 60 ohms balanced. The reviewer noted no change in gain going from balanced to unbalanced inputs or outputs. Normally one would expect 6dB more gain through balanced connections, but the servo-balanced output stage detects the signal type and adjusts gain accordingly. Frequency response measured +0/-0.2dB from 20Hz to 20kHz, slightly better than the manufacturer's +0/-0.5dB rating. With a 600-ohm load, response was down 1.4dB, with slightly more droop at the low end, possibly due to coupling capacitors in the analog output stages. The microphone input showed the same flat response.
The DSP-8024 provides a master gain range of ±16dB, and Audio Express (February 2003, p.55) reports volume control tracking between channels was perfect. Crosstalk was described as impeccable, near the noise floor until 20kHz, where it increased to -90dB. Hum and noise at +16dB gain with input shorted measured 0.18mV, with a DC offset of 0.27mV. Relays mute the output during startup and shutdown, and bypass the entire electronics section when OUT mode is selected. When switched out of the loop, THD+N dropped from 0.0042% to 0.0035%. The test set's own residual THD level is 0.0025%, so the bypass path is essentially transparent.
Distortion is where the DSP-8024 asks some care from its user. Audio Express (February 2003, p.55) found the unit had a somewhat limited range of input/output level combinations where distortion stayed below 0.01%. The input voltage clipping point was 8.7V RMS (+21dBu), beyond which the unit goes into hard "brick wall" clipping. For any combination of volume setting and input signal level, the output clipping point was always limited to 4.64V RMS, against a manufacturer's rating of +16dBu balanced/unbalanced. The reviewer's advice is direct: take maximum advantage of the input level overhead to keep distortion low, because low-level analog sources or microphones with low output will tend to produce more output distortion. The least distortion occurs just below where the red digital overload indicator activates. Operating at the -10dBV consumer reference level will limit dynamic range much more than the pro-audio +4dBu reference level. The DIG-MAX reference represents the true maximum output level (0dB peak, -3dB RMS). In all cases the red overload LED signals the start of digital clipping, which the reviewer says should be avoided at all times because it is not harmonic distortion but harsh and objectionable noise.
The review also noted some less flattering details. Audio Express (February 2003, p.57) reports that the distortion residual signal showed a bit of high-frequency noise. The reviewer considered it insignificant since THD+N at that point was just 0.0039%. As input level was reduced, this noise increased rapidly. The reviewer observed that since data is theoretically processed at 24-bit 48kHz, this suggests either the input ADC or, more likely, the output DAC may not have true 24-bit performance. Square-wave testing revealed Gibbs phenomenon ringing associated with the steep digital filters used in the DSP section, and a 10kHz square wave was rounded over into nearly a sine-wave shape. A 40Hz square wave had very little tilt at 100k load, and only slightly more tilt with a 600-ohm load. During sine testing, as output level increased beyond the initial clipping point, there was an increased amount of this ringing.
The 36-page user's manual earned praise from the reviewer, who described it as extremely well written, with useful illustrations, examples, and connection diagrams for every application. Audio Express (February 2003, p.54) also notes that users can copy, compare, add, or subtract EQ and analyzer curves, and that 100 user memory locations are available. Three level meter references are provided: dBu (dB in reference to 1mW at 600 ohms, equal to 0.775V RMS), dBV (dB relative to 1V peak-to-peak), and DIG-MAX referenced to maximum output level. If overload occurs, either from excessive analog signal levels or due to digital signal processing limitations, the green LED above the IN/OUT switch turns red.
The EQ section is the main event. Audio Express (February 2003, p.54) reports that the graphic EQ does its job exceedingly well, far better than analog designs, because its digital filters reduce overlap between EQ bands to negligible levels. The reviewer found the EQ very natural sounding, adding no texture of its own to the sonic palette. The claimed S/N ratio is 103dB unweighted, and the reviewer found no reason to doubt that spec. Even with high frequencies boosted to the max, there was no self-noise added, unlike analog circuitry. In addition to the dual 1/3-octave EQs, the DSP-8024 offers a pair of three-band shelving filters with user-adjustable corner frequencies, and a pair of three-band parametric filters. The parametric EQ section provides ±16dB to -48dB in 0.5dB steps with bandwidth from 1/60 to 2 octaves, while the graphic equalizer offers ±16dB to -16dB in 0.5dB steps. The feedback destroyer operates down to -48dB with bandwidth from 1/60 to 12/60 octaves. The digital delay ranges from 0.1ms to 2.5s per channel. The noise gate threshold is adjustable from -44dB to -96dB in 1dB steps, and the limiter threshold from 0dB to -36dB in 1dB steps with a 500 to 5000ms release.
Behringer's own response to the review, printed in Audio Express (February 2003, p.57), describes the DSP-8024 as representing "digital mainframe technology at its best," built around a 24-bit dual-DSP architecture using two high-speed DSPs and 24-bit A/D and D/A converters from either Cirrus Logic or Crystal. The company pointed to the relay-controlled hard bypass with auto bypass function, which guarantees uninterrupted operation in case of power failure. It also noted that the unit has an open architecture for easy software updates, with free Windows editor software and firmware v1.3 downloadable from the Behringer website. The manufacturer also cited monitor engineer Scott Schenk, who worked with Dwight Yoakam, Smashmouth, and Third Eye Blind, as a user who valued the DSP-8024 for its parametric and third-octave equalization. The limiter and delay let him line up musicians' in-ears with monitor speakers.
The DSP-8024 was a loudspeaker management system - an LMS - before that term became common. Audio Express (February 2003, p.54) notes that while it was intended primarily as a tool for professional sound reinforcement and studio duty, its capabilities may well make it a worthwhile addition to most home stereo systems as well. The RTA and EQ features of the two-rack-space box can correct the sonic deficiencies inherent in practically any room/speaker combination. The reviewer, who had owned a DSP-8024 for about a year before writing the piece, described his experience as almost totally positive and said he wouldn't hesitate to recommend it to anyone looking to upgrade their sound system.
Power requirements are listed at 30W maximum, and the lithium backup battery is rated for approximately 3 years typical life. The manufacturer's rated THD+N is 0.004% at 1kHz +4dBu N/S, with the measured result coming in at 0.0039%. IMD (CCIF 19+20kHz) is rated below -103dB, 22-22kHz, with the measured result below the noise floor. CMRR is rated 40dB typical, greater than 55dB at 1kHz. Maximum analog input level is rated +21dBu balanced/unbalanced, and the measured clipping point matched at 8.7V RMS (+21dBu). Maximum output level is rated +16dBu balanced/unbalanced, with measured output clipping at 4.64V RMS (+15.5dBu).
The 100 user memories, the MIDI connectivity, the optional AES/EBU interface, and the open-architecture firmware updates all point to a unit built to stay useful. The 1/3-octave RTA with Auto-Q is the headline feature, with automatic room measurement and correction in a box that cost $230 at list. The caveats are real: the limited headroom window for low-distortion operation, the non-true-24-bit behavior at the output stage, and the steep digital filter ringing on square waves.
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