Lexicon PCM 81

A 1U rack processor with 24-bit internal paths, a true-stereo signal path, and balanced analog I/O. Working engineers reached for it when they needed reverb and pitch shifting in the same box.

Overview

The PCM 81 arrived as Lexicon's replacement for the PCM 80 and PCM 90, and the company made no secret of the lineage. Sound on Sound (September 1998, p.136) noted that the PCM 80 and 90 had been superseded by "virtually identical-looking models that still cost less than their forebears did a year ago". Lexicon kept the chassis and rebuilt the inside. The PCM 81 uses one EIA-standard rack space, measuring 19 inches wide, 1.75 inches high, and 12 inches deep, at 6.4 pounds net. It can sit on any level surface or mount in a standard 19-inch rack. Lexicon's manual advises supporting the rear of the chassis in a rack or road case to prevent damage from mechanical shock and vibration.

The processing architecture is what separates the PCM 81 from the 80 and 90. 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. Sound on Sound (September 1998, p.136) flagged that as a new addition over the earlier models. Internally, conversion runs at 20 bits while the DSP operates at 20 to 24 bits. The unit supports sample rates of 44.1 kHz and 48 kHz, with word clock selectable internally or locked externally. The manual walks through the lock procedure in detail. If the receiving unit cannot find the incoming clock, it displays an error, and the operator must check the source and cabling before the unit will sync.

The converter specs are worth reading closely. The A/D section quotes frequency response of 10 Hz to 20 kHz, ±0.5 dB, with crosstalk held to less than -65 dB across that bandwidth. Signal-to-noise ratio is greater than 102 dB over a 20 kHz bandwidth, THD is less than 0.003% maximum from 10 Hz to 20 kHz, and dynamic range exceeds 102 dB. The D/A side is nearly as good: frequency response is identical at ±0.5 dB, crosstalk is below -80 dB, S/N is above 98 dB, THD is under 0.005%, and dynamic range is above 98 dB. The analog-to-analog path is what you hear when the signal goes in one jack and comes out another. It holds greater than 96 dB S/N and dynamic range with THD below 0.005%. Those A/A figures are the honest ones, and they are serious numbers for the era. Latency is documented at both supported sample rates. The A/D contributes 24 samples of delay, 0.54 milliseconds at 44.1 kHz and 0.50 milliseconds at 48 kHz. The D/A adds 50 samples, or 1.13 milliseconds at 44.1 kHz and 1.04 milliseconds at 48 kHz. Neither figure will trouble anyone tracking through the box.

Around back, the PCM 81 has combined 3-pole XLR and 1/4-inch TRS phone jacks for the two audio inputs, electronically balanced. Input impedance is 50kΩ unbalanced and 100kΩ balanced, and the inputs accept levels from -22 dBu to +20 dBu. A rear-panel two-position switch selects between 0 dB gain (Out position) and +20 dB gain (In position) to match input gain to the source. Either position may be used for balanced or unbalanced sources. In the 0 dB/BAL switch position, CMRR is 50 dB minimum from 10 Hz to 20 kHz. Output connectors are 1/4-inch TRS phone jacks and balanced XLRs with pin 2 high. Output impedance is 100Ω balanced, and levels go up to +18 dBm full scale at the +4 dBu setting, or +4 dBm at the -10 dBu setting. Relays mute the outputs during power on/off.

The digital I/O is a big part of the PCM 81's appeal. Coaxial RCA connectors handle S/PDIF (IEC-958 consumer standard), while balanced XLR connectors handle AES/EBU (AES3-1992 professional standard). Both S/PDIF and AES outputs are active at all times, though only one input option may be selected. Sound on Sound (September 1998, p.136) specifically called out the pair of AES/EBU balanced XLR digital connectors as augmenting the S/PDIF phono connectors already fitted to the PCM 80. The manual notes that analog and the selected digital source are mixed at the input to the effects processors, with separate Analog Lvl and Digital Lvl parameters controlling each stream. Setting both to 50% blends them equally. Pushing the combined mix past 100% can cause overload.

The front panel is built around a headroom display that shows both headroom and overload information from a variety of measurement points. The meters display the sum of both analog and digital input data. To examine either stream alone, set the Level parameter of that data stream to 100% and the other to 0%. The manual includes a chart illustrating the adjustment range for both balanced and unbalanced operation. It advises operating at the higher amplitude end of the recommended range when a choice can be made, to optimize noise performance. With the input switch in the +20 dB position, overload occurs above 0 dBu, acceptable levels range from 0 dBu to -22 dBu, and signals below -22 dBu are too low and noisy. In the 0 dB position, overload occurs above +20 dBu, acceptable levels range from +20 dBu to -2 dBu, and signals below -2 dBu are too low.

Gain staging is a three-stage process, and the manual is unusually explicit about it. The rear-panel input gain switch conditions the signal first. Then the front-panel INPUT knob fine-tunes it into the A/D converter, and finally each algorithm's InLvl parameter handles the level into the DSP itself. Proper setting of both the switch and knob is important for best performance of the A/D converter. The headroom display's 0 dB overload indicators light when the signal exceeds the acceptable range, and the manual walks through the diagnostic conditions that trigger them.

The PCM 81's effects engine is built on what Lexicon called a unique dual-DSP platform, which lets the unit combine reverb with additional effects in a true-stereo signal path. Mix (September 1999, p.17) described the architecture as embodying "decades of research" in Lexicon's PCM algorithms and presets, with each effect offering an uncompromised stereo reverb with several voices of additional effects. A full complement of pitch shifters provides doubling and quadruple-tracking. That kind of processing made the PCM 81 a staple in post-production and sound design as well as music tracking. The manual is direct about the input mixing: analog and digital sources blend at the input to the effects processors, and the unit assumes you are running both a console send and a digital source into the same box.

Control connectivity is thorough. Five-pin DIN connectors provide MIDI IN, THRU, and OUT. A 1/4-inch TRS phone jack accepts two independent momentary footswitches, with the system detecting normal-open or normal-closed on power up. A separate 1/4-inch TRS jack accepts a footpedal with 100Ω minimum to 10kΩ maximum impedance. The rear panel also has a standard 3-pin IEC power connector with detachable cord. The PCM 81 operates from 100 to 240 volts AC, 50-60 Hz, with automatic switching to the correct voltage range, and draws 35 W. The unit conforms to FCC Class B for RFI/ESD.

External memory comes via a PCMCIA Type I card slot - 68 pins, conforming to PCMCIA 2.0 / JEIDA 4.0, supporting up to 1 MB SRAM (attribute memory not required). The manual notes that a 1 MB card can store as many as 2,350 PCM 81 registers organized into 47 banks of 50. Registers stored on a properly formatted card are recognized whenever the card is inserted and can be accessed via the front-panel Register Banks button exactly as internal registers. Cards can also store "setups", meaning the system configuration as set in Control mode, with as many as five PCM 81 setups per card. That allows complete PCM 81 environments to be transported between units. Cards also provide storage for additional program maps and effect chains.

The PCM 81's diagnostic system is unusually thorough for a rack processor. Power On Diagnostic Tests run at startup, and errors are stored in an error log file in SRAM for future analysis. When a failure occurs, the red 0 dB LED lights and a binary test/error code is displayed on the headroom LEDs. It is a 4-bit binary number with the -24 dB LED as the LSB and the yellow -6 dB as the MSB. An E and the test ID number also appear on the front-panel display. The manual documents the operator's options. Power cycle the unit and see if diagnostics pass on restart. If the failure occurred during CPU, ROM, or SRAM tests, press Down to enter a special diagnostic test loop walking 1s and 0s through the address and data bus, or press Load to execute the next test. Power cycling and then pressing Control and Tempo simultaneously initiates Functional Diagnostic Tests.

The theory of operation section of the service documentation describes a fixture powered by an external ELPAK WM063 or equivalent supply providing ±12V at 180 mA each and ±5V at 380 mA, fed via a 5-pin DIN connector. The fixture uses half of a 4558 op amp as a low-frequency oscillator with R4 setting the rate, routed through a 1kΩ resistor to a switch that normally connects to the output. When connected to the PCM 81, +5V is fed from the ring of the controller input being tested to the ring on the fixture's connector, and sent to the base of a transistor through a resistor. If voltage drops below 4V, the transistor turns on and biases two others on, lighting an LED and muting the fixture output. That verifies both that +5V is present at the ring and the value of the resistor feeding it.

System clock verification requires a frequency counter and oscilloscope, with the scope's ground lead connected to the PCM 81 chassis away from the power supply. Looking down on the unit with the top cover off, the DSP board sits on standoffs just to the right of the power supply area. Near the center is Y1, a crystal running the Lexichip, which should measure 25.8 MHz. Removing the DSP board and powering on displays Error E7. Y3, the 16 MHz clock running the microprocessor, is located to the left of the microprocessor and is measured at U40 pin 1. Crystals Y1 (12.28 MHz) and Y2 (11.29 MHz) sit toward the rear. Y1 generates the 48 kHz sample rate clock and is measured at U21 pin 10. Y2 generates the 44.1 kHz sample rate and is measured at U22 pin 10.

Analog audio performance verification is documented in detail. A low-distortion oscillator with less than 100Ω output impedance and less than 0.005% THD feeds a 1 kHz sinewave at +20 dBu (7.75 Vrms balanced) into the left input. After setting the input level just below clipping, the left output should measure between +18.5 and +21.5 dBu. Switching the output level to -10 dBu should produce a reading between 7.5 and 4.5 dBu. Frequency response sweeps from 10 Hz to 20 kHz should vary less than ±0.5 dB referenced to the 1 kHz output level. THD+Noise should measure below 0.008%, and signal-to-noise ratio should exceed 90 dB. The S/PDIF circuit is verified with an on-board wraparound test in Extended Diagnostics. Connect an RCA cable between S/PDIF IN and OUT, then hold Control and Tempo while powering up until the display reads "PCM 81 Diagnostics." The AES/EBU ports require a second PCM 81 or another device transmitting and receiving AES/EBU formatted signals, with XLR cables between the AES/EBU inputs and outputs of both units.

Mix (May 1998, p.27) ran Lexicon's advertising with the tagline "You'll run out of ideas before you run out of effects", attributed to sound designer Scott Martin Gershin. Mix (July 1998, p.158) announced the PCM 81 alongside another Lexicon processor as a new rack-mount signal processor. Sound on Sound (September 1998, p.136) framed the PCM 81 as part of Lexicon's move to replace the acclaimed PCM 80 and 90, noting that end-of-line 80s and 90s were being offered at reduced prices as the new models arrived. The PCM 81 retained the familiar chassis and control layout while upgrading the internal path to 24-bit and adding AES/EBU connectivity.

The PCM 81 is not the flagship PCM 90 or the later PCM 91, but it offers the Lexicon reverb algorithms and pitch-shifting capabilities that made the PCM series a studio standard, in a package that fits a single rack space and weighs under seven pounds. The PCMCIA card slot, MIDI implementation, and comprehensive diagnostic system make it serviceable in a way that many contemporary processors are not. The manual's detailed performance verification procedures mean a technician can confirm the unit is operating to spec rather than guessing. The 24-bit internal path and dual-format digital I/O keep it relevant in setups where a purely analog processor would be a bottleneck.

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Lexicon PCM 81

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