Lexicon PCM 80

A 1U rack processor with 18-bit converters and a 24-sample A/D delay. Working engineers reached for it first.

Overview

A PCM 80 is a plain box: 19 inches wide, 1.75 inches high, 12 inches deep, and 6.4 pounds net. It is a single-rack-space unit, standard 1U, and it does not advertise itself. Still, 6.4 pounds in a 1U chassis is not empty air. Inside sits a DSP engine running 18 to 24 bits, fed by an 18-bit conversion stage, with base memory built around two 256k x 18 DRAMs. Two SIMM sockets are provided for expansion, accepting 1 Meg x 9, 4 Meg x 9, or 16 Meg x 9 modules at 70 nanoseconds. Lexicon built that expansion path in from the start.

The manual is direct about what the PCM 80 is for. It describes analog and digital sources being mixed at the input to the effects processors. Set Analog LvI and Digital LvI both to 50% and the two streams blend equally on their way in. Push that combined mix past 100% and the unit will overload. The design assumes you are running both a console send and a digital source into the same box.

The A/D section quotes frequency response of 10 Hz to 20 kHz, ±0.5 dB, with crosstalk held to -65 dB maximum, signal-to-noise of 100 dB minimum across a 20 kHz bandwidth, and THD of 0.004% maximum from 10 Hz to 20 kHz. Dynamic range is 100 dB minimum on the same bandwidth. The D/A side is nearly as strong: frequency response identical at ±0.5 dB, crosstalk -80 dB maximum, S/N 100 dB minimum, THD 0.006% maximum, and dynamic range 92 dB minimum with 96 dB typical. The analog-to-analog path, which is what you hear when the signal goes in one jack and out another, holds 90 dB minimum S/N and dynamic range with THD of 0.008% maximum. Those A/A figures are the honest ones, and they are still serious numbers.

Latency is worth a mention because the manual bothers to specify it. The A/D contributes 24 samples of delay — 0.54 milliseconds at 44.1 kHz, 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, and the fact that Lexicon documented them at both supported sample rates suggests the PCM 80 was expected to sit in signal chains where that mattered.

The digital interface conforms to S/PDIF CP-340 Type II and IEC-958 consumer standards over coaxial RCA connectors, and the unit runs at 44.1 kHz or 48 kHz. Word clock can be set 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 holds the last value it read until you nudge the ADJUST knob to the new rate. In practice, the PCM 80 will lock to 44.1 kHz automatically once it sees a valid source. The external memory card slot accepts PCMCIA Type I cards on a 68-pin connector, conforming to PCMCIA 2.0 and JEIDA 4.0, and supports up to 1MB of SRAM with no attribute memory required. The card slot is how you move your own creations between units or archive them.

Around back, the audio I/O is on 1/4-inch T/R/S phone jacks, two in and two out. Input impedance is 100k ohms balanced in the 0 dB/BAL switch position, or 50k ohms unbalanced in the -20 dB/UNBAL position. The rear-panel input gain switch has two positions: In adds 20 dB of gain for unbalanced sources, Out provides unity gain for balanced or unbalanced sources. Full scale arrives at -2 dBu minimum in the 0 dB/BAL position with +20 dBu maximum, or -22 dBu minimum and 0 dBu maximum in the -20 dB/UNBAL position. Common-mode rejection is 40 dB minimum from 10 Hz to 20 kHz in the balanced position, which helps when you patch into a console with long cable runs. Output impedance is 125 ohms per side, balanced, with +18 dBm maximum full scale in the +4 dBu setting or +4 dBm in the -10 dBu setting. Relays mute the outputs during power on and off, which saves monitors.

Setting levels on a PCM 80 is a two-stage job, and the manual is emphatic that both stages matter. The rear-panel switch conditions the signal first, then the front-panel INPUT knob trims it into the A/D converter. The headroom display reads the sum of both analog and digital input data, so to examine one stream alone you have to set the other stream's Level parameter to zero. The overload indicators light on A/D overload or on internal peaking anywhere in the effects chain. High-Q filters and certain reverberation modes can trip them even when the input is nowhere near full scale. The manual is candid about a trap here: an incorrect rear-panel input switch setting can cause analog overload that never lights the indicator at all.

The output level parameter has two positions for a reason. Devices that can handle peaks up to +18 dBu want the +4 dBu setting. Anything that cannot take more than +4 dBu wants the -10 dBu setting.

MIDI runs on standard 5-pin DIN connectors for IN, THRU, and OUT. A 1/4-inch T/R/S jack accepts two independent momentary footswitches, and the system detects normal-open or normal-closed contacts at power-up. A separate foot controller jack takes a pedal with a minimum impedance of 10k ohms and a maximum of 100k ohms. The manual's own test procedure walks through a Pedal Swell program where the pedal sweeps a parameter smoothly from Off through -85 dB and -2 dB to 0 dB.

Mix (July 1995, p.56) called the PCM 80 the new effects standard for audio engineers worldwide, crediting its unique DSP engine and the room it left for user-created spatial and reverb effects. EQ Magazine (May 1995, p.63) echoed that assessment almost word for word, noting the same DSP engine and the same creative headroom. Lexicon's own advertising in EQ Magazine (January 1996, p.55) framed the PCM 80 alongside the PCM 90 with the line that the two were "so good, you'll need both". That is marketing copy, not a review verdict. Studio Sound (March 1996, p.34) ran the same campaign, describing the PCM 80 as offering all the sound and control you expect from Lexicon. Sound on Sound (March 1996, p.101) carried the identical advertisement copy. That makes four publications in fourteen months.

The PCM 80 asks three things of an owner. The rear-panel switch and the front-panel knob have to be set together, or the noise floor suffers or the converters clip. The output level setting has to match the gear downstream. The digital input stays muted until it sees a valid clock, so a silent S/PDIF feed may just be waiting on one. Get any of these wrong and the unit will let you know, sometimes with an indicator and sometimes without one.

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Lexicon PCM 80

Overview

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