Lexicon PCM 90

A 1U digital reverberator with 18-bit converters, a 100 dB signal-to-noise ratio, and a front-panel headroom display that shows how hard you're pushing it.

Overview

The PCM 90 is a single-rack-space digital reverberator, 19 inches wide, 1.75 inches high, and 12 inches deep, weighing in at 6.4 pounds net. It's a 1U box with a front panel built around a headroom display and a control layout that assumes you'll actually read the manual — and the manual rewards that assumption, because the PCM 90 is a machine that tells you what it's doing at every stage of the signal path.

Start with the converters, because that's where the numbers get interesting. The A/D and D/A paths are 18-bit, with internal DSP running 18 to 24 bits. The analog-to-analog path covers the full trip from input jack through converters and DSP and back out to the output. It quotes a frequency response of 10 Hz to 20 kHz, ±0.5 dB, with a signal-to-noise ratio of 90 dB minimum and total harmonic distortion of 0.008% maximum across that same bandwidth. The A/D side alone is tighter: 100 dB minimum signal-to-noise, 0.004% maximum THD, and 100 dB minimum dynamic range over a 20 kHz bandwidth. The D/A side lands at 100 dB minimum S/N, 0.006% maximum THD, and a dynamic range of 92 dB minimum with 96 dB typical. Crosstalk is -65 dB maximum on the A/D side, -80 dB maximum on the D/A side, and -55 dB maximum analog-to-analog. Those are professional numbers for the era, and the A/A figure is the honest one, since it describes what you get when you patch the thing in.

Around back you get combined 3-pole XLR and 1/4-inch TRS phone jacks for the two audio inputs, with a rear-panel switch selecting between 0 dB/balanced and -20 dB/unbalanced operation. In the 0 dB/BAL position the input impedance is 100k ohms balanced, and the input accepts a minimum of -2 dBu for full scale up to a maximum of +20 dBu, with common-mode rejection of at least 40 dB from 10 Hz to 20 kHz. Switch to -20 dB/UNBAL and the impedance drops to 50k ohms, the minimum for full scale falls to -22 dBu, and the maximum input is 0 dBu. The manual is explicit that proper setting of both the rear switch and the front-panel INPUT knob matters for getting the best out of the A/D converter. The switch gain-conditions the signal, the knob fine-tunes it into the converter, and each algorithm's InLvI parameter handles the level into the DSP itself. That's three stages of gain staging, and the manual walks through all three.

The headroom display makes the PCM 90 less of a black box than most reverbs of its generation. It shows headroom and overload information from a variety of measurement points, displaying either analog or digital input data depending on which Audio Input Source is selected in Control mode. The manual includes an adjustment-range chart for both balanced and unbalanced operation, and the guidance is plain: when a choice can be made, operate at the higher amplitude end of the recommended range to optimize noise performance. The 0 dB overload indicators light under three conditions: A/D overload, overload at any point in effects processing, or input level within 1 dB of maximum. The middle condition is the one to know about. Level buildup from certain reverberation modes can push the DSP into overload even when the input A/D or digital receiver data stream isn't at full scale, usually from a combination of extreme parameter settings. Adjusting parameters or levels clears it. So the PCM 90 will tell you when a reverb algorithm is building level internally, which is more than most boxes do.

On the output side, the PCM 90 offers two 1/4-inch TRS phone jacks and two balanced XLRs with pin 2 high, at an impedance of 125 ohms per side balanced. Output level is set in Control mode and has two positions: +4 dBu for devices that can handle peak levels of 18 dBu, and -10 dBu for devices that can't handle peaks above +4 dBu. Full-scale output is +18 dBm in the +4 dBu setting and +4 dBm in the -10 dBu setting. Relays mute the outputs during power on and off, which spares your monitors a power-cycle thump. The manual's calibration procedure is specific. Apply a 1 kHz sine at +20 dBu (7.75 Vrms) for balanced input, turn the front-panel Input Level knob up until the signal clips on a scope, then back it off to just below clipping and note that setting. You'll be reminded to return to it in the following tests. With the output level at +4 dBu, acceptable left output measures between +18.5 and +21.5 dBu. Switch to -10 dBu and the reading should land between 7.5 and 4.5 dBu. Frequency response is verified by sweeping 10 Hz to 20 kHz and confirming the output varies less than ±0.5 dB referenced to 1 kHz. THD+noise should measure under 0.008%. The manual expects you to have a distortion analyzer and an oscilloscope, and it tells you what to do with them.

Digital I/O is coaxial RCA conforming to S/PDIF CP-340 Type II and IEC-958 consumer standards, running at 44.1 kHz or 48 kHz. The Audio Input Source selection in Control mode offers Digital, Analog 48kHz, and Analog 44.1kHz. Internal delay through the converters is 24 samples on the A/D side - 0.54 milliseconds at 44.1 kHz, 0.50 milliseconds at 48 kHz - and 50 samples on the D/A side, or 1.13 milliseconds at 44.1 kHz and 1.04 milliseconds at 48 kHz. Base memory is two 256k x 18 DRAMs. There's also a PCMCIA Type I card slot, 68 pins, conforming to PCMCIA 2.0 / JEIDA 4.0, that supports up to 1MB SRAM cards, with attribute memory not required.

Control connectivity covers MIDI and foot control. There are 5-pin DIN connectors for MIDI IN, THRU, and OUT. A 1/4-inch TRS jack accepts two independent momentary footswitches, and the system detects normal-open or normal-closed on power up. A separate 1/4-inch TRS jack takes a footpedal with a minimum impedance of 100 ohms and a maximum of 10k ohms. Power requirements are 100-240 VAC, 50-60 Hz, 35 W, through a 3-pin IEC connector. The unit ships at 9.5 pounds.

The manual's compatibility notes spell out where the PCM 90 sits in Lexicon's line, and they're candid about its relationship to the PCM 91. The PCM 91 can receive MIDI data from either a PCM 90 or another PCM 91, and it can transmit certain messages in PCM 90 format, namely bulk data dumps and SysEx automation, with format translations handled automatically. Automation mode sends all MIDI commands in PCM 90 format, so a PCM 90 can control a PCM 91, and a PCM 91 can control both. The catch is banks. The PCM 90 has only five banks, numbered 0 through 4, so references to banks 5-8 are ignored when transmitted to a PCM 90. The PCM 91 can also share registers, maps, and chains with the PCM 90 via RAM cards, though the manual specifically advises against sharing setups between the two. Dumps of single effects, banks, maps, and chains all transmit in PCM 90 format and are recognized by either unit, and you can dump from the front panel of either unit to the other.

Contemporary advertising for the PCM 90 leaned on the same pitch Lexicon used across the line. EQ Magazine (January 1996, p.55) ran Lexicon's copy positioning the PCM 90 and PCM 80 together under the line "so good, you'll need both," describing the PCM 90 as the latest addition to Lexicon's line of effects and highlighting its powerful presets. Studio Sound (March 1996, p.34) carried the same campaign, framing the PCM 90 as a digital reverberator with the sound and control expected from Lexicon. These are marketing placements, not independent reviews, and "you'll need both" is a sales hook. What the period coverage does establish is that the PCM 90 was a current product in early 1996, and that Lexicon was selling it alongside the PCM 80 rather than as a replacement for it.

Maintenance on a PCM 90 is minimal by design. The manual specifies a soft, lint-free cloth slightly dampened with warm water and a mild detergent for the exterior surfaces, and that's the extent of the routine care described. The real servicing is the calibration and diagnostics procedure: the SELECT test, the extended diagnostics, the signal-level checks. All of it assumes access to test equipment and a willingness to work through the manual methodically. For anyone buying one today, the practical checks are the obvious ones. Verify that the headroom display responds correctly to input, confirm that both output level settings produce the expected readings, and check that the PCMCIA slot accepts a card. The output muting relays are worth listening for at power-up, since they're there to protect whatever's downstream.

Six point four pounds, 1U, and a manual that documents the full signal path from the rear-panel gain switch to the DSP input level parameter. The PCM 90 is a machine that assumes you want to know what's happening inside it, and it gives you the tools to find out.

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Lexicon PCM 90

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