Lexicon PCM 91
A digital reverberator with a front panel that tells you what every button does - if you know to hold it down.
Overview
The display is a two-line fluorescent type, and the manual makes a point of its viewing angle. It's readable from any position, whether the room is bright or dark. Two separate knobs handle SELECT and ADJUST, so loading and editing programs takes a pair of controls instead of a menu crawl. The unusual part is an Info mode. Press and hold any button and the unit explains what that button does, or reports status such as the name of the running effect or the current tempo rate. It sounds like a gimmick until you are three menus deep at 2 a.m. and have forgotten what a parameter does.
The connectivity shows the PCM 91 was built for professional use. The rear panel carries combined three-pole XLR and 1/4-inch jacks for the balanced inputs, electronically balanced, with input impedance of 50kΩ unbalanced and 100kΩ balanced, accepting levels from -22dBu to +20dBu. Balanced outputs run at 105Ω with up to +18dBu maximum full scale, and both 1/4-inch phone connectors and XLRs are provided. Digital I/O covers AES/EBU on XLR and S/PDIF on phono. The manual notes that both S/PDIF and AES outputs are active at all times - only the input side forces you to choose one or the other. A two-position Input Level switch adds 20dB of gain in the In position for unbalanced sources, or 0dB in the Out position for balanced ones, and the analog output level is selectable between +4 dBu and -10 dBu. As Sound on Sound (September 1998, p.136) noted when Lexicon introduced the successor PCM 81 and 91, the internal signal path is 24-bit, and a 24-bit signal can be taken into the unit via the digital I/O if required. The new models also added a pair of AES/EBU balanced XLR digital connectors alongside the S/PDIF phono connectors already fitted to the earlier PCM 80 and 90.
Two footswitch jacks finish off the back. One is a 1/4-inch Tip/Ring/Sleeve jack for two independent momentary footswitches. The second TRS jack takes a footpedal in the 10kΩ to 100Ω impedance range. Front-panel patching and MIDI both provide system control beyond the plug-and-play layer, and the manual points anyone wanting the full picture to the user guide.
Know this before you patch anything in: when the PCM 91 loses lock, it mutes the digital input rather than passing garbage downstream. The unit logs errors continuously - Validity, Confidence, SlipSample, CRC, Parity, and Biphase - and you can scroll the list from the Dig In Status display. Each error is reported by name with a running count, up to 9999 instances before the display switches to ">9999". The manual is blunt about the usual culprit. Parity, Biphase, and Confidence errors most often trace to inappropriate cabling, and it specifies 75Ω video-grade cable kept as short as possible. Standard audio cable will not work reliably. A Confidence error means excessive jitter or noise on the line without corrupted data. A SlipSample error flags a single sample misaligned with the window defined by the input source, which can happen when an external master changes sample rate or is powering up. The log clears on power up, whenever the unit relocks with Auto Lock On, or when you reselect Dig from the Control Mode Input Source display.
Power behavior is documented in the troubleshooting chapter. In a low-voltage or brown-out condition the PCM 91 freezes in its current state - no control responds - and when voltage returns to normal it resets itself as though freshly powered on. If it does not reset on its own, cycling the power resumes normal operation. On a normal startup the unit runs a series of self tests, displays the PCM 91 copyright notice, then loads and displays the last loaded effect. If that sequence does not appear, the manual directs owners to Lexicon Customer Service.
Setting levels takes some attention because the signal path offers both analog and digital connections. The manual lays out three separate gain stages. The rear-panel Input Level button conditions the analog front end to avoid overloading the DSP input. The front-panel INPUT knob sets level into the analog-to-digital converter to optimize noise and avoid overload. The InLvl parameter inside each algorithm sets level into the digital signal processor. Analog and digital sources are selected in Control mode, with choices of 44.1, 48, Ext: XLR, and Ext: Coax.
One housekeeping note from the manual, for anyone shipping one of these: save the packing materials, because Lexicon's own instructions assume you may need to ship the unit again. The unit uses a 3-wire grounding plug, and the manual warns against installing it in an unventilated rack or directly above heat-producing equipment such as power amplifiers.
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