Soundstream

Digital Tape Recorder

The Digital Tape Recorder was a portable four-channel digital audio processor containing the analog to digital converters, tape-data recovery and clock generation circuits, and the digital to analog converters. External hardware (tape drive, editing system, and digital delay unit) connected to the DTR through connectors on the back panel. The unit measured 20"×18"×10" and weighed 67 pounds.

Analog signals entered the DTR through standard XLR connectors at the rear of the unit. There, a differential input amplifier routed the signal through the front panel attenuation fader to the input low-pass anti-aliasing filter.

The anti-aliasing filter (custom made by TTE Filters) is a passive 11-pole elliptical filter with a −3 dB point of 22.5 kHz. The analog signal path from input XLR connector to the analog-to-digital converter is DC coupled.

The filtered analog signal passed through a custom sample and hold and was digitized by an Analogic MP8016 16-bit analog-to-digital converter operating at a 50 kHz sample rate. A three-bit sync pattern and an even-parity bit were added to each 16-bit sample to form a 20-bit word that was serialized and transmitted by interface electronics to the tape transport where each audio channel's data were written to two separate tape tracks. The two tracks are laterally separated by ~ 11mm on the 1" tape, thus avoiding the same tape dropout from affecting both tracks containing identical channel information.

Encoding of the 16-bit audio at the time of recording inverts every other audio bit in order to minimize DC going to tape (magnetic channel) in quiet passages when the most significant bits were mostly zeros. Another effect of the encoding process was to increase AC in the channel code to aid in clock recovery (bit sync) on playback. The every other bit inversion encoding process is reversed on playback so there is no effect on audio quality.

During tape playback or while recording and monitoring from tape, the redundant tracks of recorded data from the transport were sent to data recovery circuitry. The first stage of data recovery was the data slicer which automatically detected and adjusted to the ideal threshold for detecting the zero crossings of the magnetic channel data. This reduced clock recovery jitter. Because the data recorded on tape are necessarily bandwidth-limited, the data slicer squared-up the data signals into binary logic level signals and then recovered clocks to match the incoming data streams. Additional circuitry located word boundaries and converted the data to parallel format. In the event of tape dropouts, an error-avoidance scheme selected the unaffected track's data.

The selected data were then clocked into an Analogic MP1926A digital-to-analog converter (DAC) at the original crystal-controlled sample rate. The DAC's analog output signal was buffered by an Analogic MP201A distortion suppression amplifier (aka deglitching amplifier). The MP201A suppresses the glitches present during the narrow segment of time that the binary input word to the DAC was transitioning from one sample to another.^[16]

The analog signal from the distortion suppression amplifier was routed to the output low-pass (reconstruction) filter that is identical to the input anti-alias filter and then to NE5534 based output buffer amplifier with a discrete transistor output stage. Voltage and current gain sufficient for +20dBm into 150 ohms was provided by the output-buffer amplifier with the signal then output at the XLR connector on the DTR's rear panel. The output signal path from DAC to output XLR connector was DC coupled.

Honeywell Tape Drives

Soundstream-modified Honeywell 5600e Instrumentation Tape Drives (HTD) used custom high-frequency 18-track record and playback heads. The two outer tracks were reserved for ancillary data – SMPTE time code and the like. The remaining sixteen tracks were used to record up to eight channels of digital audio – two redundant tracks for each audio channel. The two tracks in a redundant track pair were separated as widely as possible to minimize playback errors due to tape defects (dropouts) – audio channel one was recorded on tracks 1 and 9.

At the front of the HTD, Soundstream installed track selector hardware. The left side selector allowed the operator to choose which audio channel pair (1,2; 3,4) was assigned to which tape track pair (1,2; 3,4; 5,6; or 7,8). The right-side selector controlled the application of record circuitry power for each of the eight tracks.

The HTD initially used 1" Ampex 460 reel-to-reel tape at 35 ips. Later, Soundstream switched to Ampex 466 tape which allowed for the slower tape speed of 30 ips.

Digital editing system

Soundstream's digital editing system was the first instance of a computer used to edit commercial recordings. It consisted of a Digital Equipment PDP 11/60 computer running the DAP (Digital Audio Processor) editing software (written by Soundstream employee Robert Ingebretsen), Soundstream's interface (the Digital Audio Interface) to transfer data between its recorder and the computer's disks (a pair of Braegen 14" disk drives), digital-to-analog playback hardware, a text-based video display terminal for entering commands to operate and control the DAP software, and a storage oscilloscope to display the waveforms of the audio being edited or processed.^{[citation needed]} For all intents and purposes, this system was the very first digital audio workstation. In addition to its own facility, Soundstream installed editing systems at Paramount Pictures (Hollywood), RCA (New York), and Bertelsmann (Germany). A system was delivered to the U.S. Department of Justice to aid the analysis of bootleg recordings.

Editing could be performed at sample accuracy (i.e., 1/50,000 of a second); any mixing was performed digitally.

The sound system in the editing room in the Salt Lake facility used a Threshold SL-10 preamp, a Sumo "The Power" amp, and Infinity RS4.5 speakers.

Ancillary equipment