Introduced in 1978, the Publison DHM 89 B2 Stereo Digital Audio Computer was an early digital studio processor combining independently adjustable delay, echo, pitch-shifting, reverse playback and sound-memory manipulation within a single rackmount system. Developed and manufactured in France, the Publison DHM 89 B2 Stereo Digital Audio Computer used specialised digital processing rather than adapting general-purpose computer hardware, giving recording studios access to techniques that had previously required tape machines, mechanical editing and several separate effects units.
Digital audio architecture
Publison described the Publison DHM 89 B2 Stereo Digital Audio Computer as a minicomputer specialised in audio signal processing. Its design concentrated upon the particular requirements of professional recording equipment, including wide bandwidth, low noise, low distortion and sufficient dynamic range for studio operation.
The input conversion stage used a quasi-16-bit flying-comma converter. This architecture was intended to maintain consistent relative conversion accuracy across a wide range of signal levels, rather than providing maximum resolution only near the upper end of the available level range. Publison quoted a dynamic range of 95dB in delay mode and 90dB in pitch-shifting mode at a pitch ratio of 1.
The conversion process took 18.9 microseconds, corresponding to a sampling frequency of 52.91kHz. This allowed the Publison DHM 89 B2 Stereo Digital Audio Computer to provide a maximum frequency response extending from 20Hz to 20kHz when configured for its widest bandwidth.
Converted audio was stored in 210kbits of random access memory. The memory could operate as a conventional digital delay line, but it could also be manipulated for pitch-shifting, reverse playback, repeating sound fragments, automatic arpeggiation and other effects based upon changing the position and direction from which the stored audio was read.
Selectable frequency response
The Publison DHM 89 B2 Stereo Digital Audio Computer provided three selectable frequency response settings: 20Hz to 5kHz, 20Hz to 10kHz and 20Hz to 20kHz. Each setting was specified within +0/-3dB.
Selecting a narrower frequency response increased the maximum available delay time because fewer samples were required to represent each second of audio. The 5kHz setting therefore provided the longest delays, while the 20kHz setting used the available memory to preserve the widest audio bandwidth.
The selected frequency response also interacted with the stereo operating mode. True stereo operation divided the available memory between two independent channels, while quasi-stereo operation allowed the memory to be used more extensively for a single input signal.
Pre-emphasis and complementary de-emphasis circuits were used during delay operation to increase dynamic range and reduce distortion. These circuits were bypassed in pitch-shifting mode because changing the pitch ratio would otherwise move the emphasis curve and produce a non-flat frequency response.
True stereo operation
In true stereo mode, the Publison DHM 89 B2 Stereo Digital Audio Computer processed the left and right inputs as two independent channels. Each channel had its own input, output, digital delay path, pitch ratio, delay setting and crosspoint controls.
At the 5kHz frequency response setting, true stereo operation provided up to approximately 600ms of delay for each channel. At the 10kHz setting, the maximum delay was approximately 300ms per channel.
The independent channels allowed two separate mono signals to be processed simultaneously or a stereo source to retain separate left and right processing paths. Different delay times or pitch ratios could be applied to each side, allowing the Publison DHM 89 B2 Stereo Digital Audio Computer to create stereo movement, widening, double tracking and two-part pitch effects.
Quasi-stereo operation
Quasi-stereo mode used a single input signal to feed both processing channels. The Publison DHM 89 B2 Stereo Digital Audio Computer could then generate two independently processed outputs from the same source.
Because the memory was not divided between two separate input streams, quasi-stereo operation offered longer delay times and access to the full 20kHz frequency response setting. Maximum delay was approximately 1,200ms at 5kHz, 600ms at 10kHz or 300ms at 20kHz.
The delay, pitch and crosspoint controls for the two outputs could still be adjusted independently. A mono voice or instrument could therefore be converted into a stereo effect using different delays or pitch ratios on each side.
The Publison DHM 89 B2 Stereo Digital Audio Computer also provided a short-delay operating range. This divided the normal delay scale by ten, allowing more precise adjustment of the very short delay times required for double tracking, comb filtering, flanging and related effects.
Digital delay
The Publison DHM 89 B2 Stereo Digital Audio Computer incorporated an independently adjustable digital delay for each processing channel. Delay time was controlled from the front panel and shown on the central numerical display.
The available delay depended upon the selected frequency response and whether the processor was operating in true stereo or quasi-stereo mode. The shorter delay range provided finer control when precise timing differences were more important than maximum delay duration.
The digital delay could be used for conventional single repeats, stereo offsets, artificial double tracking and spatial positioning. Very short settings produced phase-related effects, while the longer settings available at reduced bandwidth supported more distinct rhythmic delays.
Because both channels could be adjusted independently, the Publison DHM 89 B2 Stereo Digital Audio Computer could produce two different delay times from one source or apply separate corrections to the two sides of a stereo signal.
Echo and recirculation
Echo effects were created by recirculating processed audio through the delay system. The repeated signal could pass through the pitch-processing section, allowing successive echoes to remain at the original pitch or move progressively upwards or downwards.
This combination of delay, feedback and pitch change extended the Publison DHM 89 B2 Stereo Digital Audio Computer beyond a conventional digital delay. Repeating patterns could be constructed in which every recurrence appeared at a different pitch, forming rising, falling or alternating sequences.
The available memory could also be latched so that a selected section repeated continuously. Pitch change and reverse playback remained available while the memory was latched, allowing a captured sound fragment to be transformed while it repeated.
Pitch-shifting
The Publison DHM 89 B2 Stereo Digital Audio Computer produced pitch translation by continuously varying the position from which audio was read within the digital delay memory. Increasing the delay lowered the output pitch, while decreasing the delay raised it.
Because the delay line had finite capacity, Publison used two adjustable crosspoints to define the part of the memory over which the pitch-shifting process operated. When the moving read position reached one crosspoint, it returned to the other and continued travelling through the selected memory region.
The crosspoint system allowed the Publison DHM 89 B2 Stereo Digital Audio Computer to perform continuous pitch-shifting while controlling the point at which the internal read position was transferred. Adjusting the crosspoints altered the length and behaviour of the pitch-processing cycle.
Pitch ratio could be controlled independently for the left and right channels. The normal control range extended from a ratio of approximately 0.25 to 2, corresponding to two octaves below the original pitch through to one octave above it.
Values below a ratio of 0.25 could be produced through external frequency control, although the reduced effective sampling frequency could become audible. Publison therefore treated 0.25 as the practical lower limit for normal operation.
At a pitch ratio of 1, the output remained at the original pitch while still passing through the pitch-processing path. This operating condition was used for the published pitch-mode noise, dynamic range and distortion measurements.
Crosspoint control
Crosspoint 1 and Crosspoint 2 established the boundaries used by the pitch, reverse and memory-latch functions. Their settings determined which portion of the digital memory was read and how the system behaved when the read position reached the end of the selected region.
The crosspoints could be adjusted from the front panel or controlled externally through the rear remote connector. External voltage control allowed the selected memory region to be changed dynamically.
Because crosspoint position affected the transition between sections of stored audio, its adjustment could influence both the timing and character of pitch-shifting. Different crosspoint settings could be chosen according to the programme material and the type of effect being produced.
Reverse playback
Reverse playback was produced by setting Crosspoint 1 higher than Crosspoint 2. This inverted the direction in which the selected part of the memory was read, creating an effect comparable to magnetic tape running backwards.
The length of the reversed section was determined by the two crosspoint positions. Short settings could reverse individual attacks or fragments, while longer settings allowed a more extended part of the stored sound to play backwards.
For rhythmically synchronised reverse effects, the Publison DHM 89 B2 Stereo Digital Audio Computer could be triggered through its remote-control facilities. The optional Publison KB 2000 keyboard included a Reverse Synchro function for coordinating reverse playback with an external performance or production.
Memory-latch operation
Pressing the Memory Latch control captured the contents of the memory at that moment and repeated the selected section continuously. Crosspoint 2 selected the beginning of the repeating sequence, while Crosspoint 1 selected its end.
Once latched, the sound could continue repeating without requiring the original input to remain present. Pitch-shifting and reverse playback could then be applied to the stored material.
The memory-latch function allowed the Publison DHM 89 B2 Stereo Digital Audio Computer to operate as an early sound-memory processor. Although its recording time was limited by the available RAM and selected frequency response, it could capture, loop, reverse and transpose short fragments of recorded sound.
Automatic arpeggiation
The Publison DHM 89 B2 Stereo Digital Audio Computer could combine pitch change, delay and recirculation to produce automatic arpeggio effects. A single incoming note or sound could generate a repeating sequence in which each recurrence appeared at a new pitch.
The direction and interval of the sequence were determined by the pitch ratio and feedback arrangement. Ratios above 1 produced rising patterns, while ratios below 1 produced descending patterns.
This function was particularly distinctive at a time when programmable digital sequencing and sampling systems were not yet widely available. The effect was created within the audio processor itself rather than by sending a sequence of notes from a separate digital instrument.
External frequency control
Each pitch-processing channel could accept an external frequency signal through the rear remote-control connector. Connecting an external frequency automatically disconnected the internal pitch-ratio setting for that channel and replaced it with the externally supplied control frequency.
The input accepted a TTL square wave. A frequency of 105.82kHz corresponded to a pitch ratio of 1, while lower and higher control frequencies produced proportionally lower and higher pitch ratios.
Holding the control input at a high logic level latched the operation and suppressed the sound output. This provided a simple method of preventing unwanted sound when no key was being played from an external control keyboard.
The external frequency input allowed the Publison DHM 89 B2 Stereo Digital Audio Computer to be connected to the Publison KB 2000 or other suitably configured control equipment. Pitch could therefore be selected from a keyboard rather than adjusted only from the front panel.
Voltage control and remote operation
Delay time and both crosspoint positions could be controlled by external voltages. Once the relevant internal front-panel control was disconnected through the remote interface, a control voltage from 0 to +5V could be applied to the corresponding pin.
This allowed delay, memory position and pitch-processing behaviour to be automated from external equipment. The Publison DHM 89 B2 Stereo Digital Audio Computer could therefore be integrated with keyboard controllers, sequencers and purpose-built studio control systems.
The rear remote-control connection used a 24-pin Amphenol connector. It provided access to pitch frequency control, memory latch, crosspoints, pitch-mode selection, insertion points, channel inputs, bandwidth switching, quasi-stereo selection and internal power supplies for compatible external control equipment.
Separate insertion inputs and outputs were provided for the left and right channels through the remote connector. These insertion points allowed external circuitry or compatible accessories to be placed within the processing paths.
Analogue connections
The Publison DHM 89 B2 Stereo Digital Audio Computer used electronically balanced inputs with an impedance of 20kΩ. Nominal input level was 0dB, with input saturation specified at +20dB.
Input gain could be adjusted from complete attenuation to +20dB. This wide adjustment range allowed the processor to accommodate different studio operating levels and optimise the signal presented to the digital converter.
The standard outputs had an impedance of 50Ω and were asymmetrical. Optional output transformers provided symmetrical 50Ω outputs. Maximum output level was specified at +20dB.
Performance
In delay mode, harmonic distortion was specified between 0.08% and 0.1%. In pitch-shifting mode at a ratio of 1, harmonic distortion was between 0.15% and 0.2%. Output noise was measured across a bandwidth of 40Hz to 15kHz. The quoted result was -75dB in delay mode and -70dB in pitch-shifting mode at a ratio of 1.
Dynamic range was specified at 95dB in delay mode and 90dB in pitch-shifting mode at a ratio of 1. The difference reflected the use of pre-emphasis and de-emphasis in delay mode and their removal during pitch-shifting.
Historical importance
The Publison DHM 89 B2 Stereo Digital Audio Computer appeared when digital studio processors were still highly specialised and costly systems. Digital memory capacity was limited, and many familiar recording effects continued to be created with tape loops, variable-speed tape machines and multiple analogue delay devices.
By combining stereo digital delay, independent pitch-shifting, reverse playback, repeating sound memory and external control, the Publison DHM 89 B2 Stereo Digital Audio Computer brought several emerging digital techniques into one studio processor.
The modular relationship between the main processor and the optional Publison KB 2000 also anticipated later systems in which digital sound processing could be controlled from a musical keyboard. The Publison DHM 89 B2 Stereo Digital Audio Computer was not a conventional sampler in the later sense, but its memory-latch, reverse and pitch functions allowed short recorded sounds to be captured and performed in ways that pointed towards later digital sampling instruments.
The Publison DHM 89 B2 Stereo Digital Audio Computer also established processing concepts that Publison expanded in the Infernal Machine 90. The later system offered greater memory, more extensive programmability, reverberation, MIDI control and additional storage options, but retained the company’s emphasis on pitch manipulation, sound memory and flexible digital audio processing.
Features
- Two independently adjustable digital processing channels
- True stereo and quasi-stereo operating modes
- Selectable 5kHz, 10kHz and 20kHz frequency response settings
- Digital delay with standard and short-range adjustment
- Independent pitch-shifting for the left and right channels
- Reverse playback using adjustable memory crosspoints
- Memory-latch function for continuously repeating captured sounds
- Automatic arpeggio effects using pitch change and recirculating delay
- External frequency and voltage control through a 24-pin remote connector
- Compatibility with the optional Publison KB 2000 control keyboard