The Stereophile Test CD Page 2

Assembling the master tape
Like many CDs, all the editing on this disc was performed electronically in the digital domain. Unlike most CDs, however, Stereophile's Test CD (footnote 1) was edited and assembled on a Macintosh IIx computer. Instead of transferring digital audio data from one tape to another through an editor, as in conventional digital editing, this project was assembled on a computer hard disk attached to my Macintosh. Hard-disk-based editing is creating a revolution in professional audio. These systems may one day be as common in recording studios and mastering facilities as word processors are in offices.

The complete editing system consists of a Macintosh computer, Digidesign's Sound Tools editing system, and a CMS Enhancements PD600 600-megabyte hard disk. Sound Tools is comprised of a digital interface box (DAT I/O), digital signal processing board for the Macintosh (Sound Accelerator), and editing software (Sound Designer II). A large-capacity hard disk is required for digital audio storage because full-bandwidth, 16-bit digital audio consumes about 10.5 megabytes per stereo minute (footnote 2). The entire system is mouse- and menu-driven using the Macintosh graphic user interface. Sound Tools also has a variety of signal-processing functions including parametric and graphic equalization, phase invert, sampling rate conversion, and FFT frequency analysis, all performed exclusively in the digital domain.

The various DAT cassettes, with original digital recordings or digital transfers from analog—the musical selections and spoken announcements in rough form (with count-offs, no fade-ins, etc.)—were played back on the Nakamichi 1000 transport and the data transferred to the hard disk attached to the computer. Once the data were on the hard disk, the music waveforms, both channels at a time, could be made visible on the monitor. The cursor was then moved over the waveform with the mouse, allowing the signal to be both seen and heard (the data being fed to, and decoded by, the Nakamichi's playback DACs), in order to select edit points. Direction and speed of the cursor are controlled by direction and speed of the mouse, allowing the editor to "rock'n'roll the tape" in order to zero in on the exact edit point, the combination of visual cues and hearing the signal combining synergistically, allowing precise selection.

Once the edit-in and edit-out points were selected, the section between points was highlighted with the mouse and saved. After each desired segment had been defined, a playlist was created, putting the pieces in the correct order for replay. (The data stay in the same place on the hard disk; only the access points are changed by the program.) A variety of crossfades between the selections is available with selectable transition times. This simulates an angled cut on analog tape, ensuring a smooth transition between sections. Glitches and ticks can be removed by expanding the waveform visually on the screen until the tick can be seen. A pencil icon is then grabbed by the mouse and the waveform redrawn without the tick.

For the edits in the Schumann Romance that switches between versions transferred through different A/D converters, JA followed the musical score as we listened to the music and watched the waveform. At an appropriate note, we stopped playback, zoomed in on the waveform, listened and watched the cursor move over the waveform, and defined the edit point. After this was done for each portion of the piece, and the playlist assembled, the system switched seamlessly between the two versions. (Time flies when you're involved in this procedure, the four minutes of music taking three hours to be assembled.)



Footnote 1: The Stereophile Test CD costs just $6.95 plus postage and handling. It can be ordered from the secure Recordings page on this website.

Footnote 2: This figure comes from multiplying the sampling rate (44,100) by the length of the quantization word (16), by the number of seconds in a minute (3600), by two (for two audio channels), divided by 8 (8 bits to a byte).—RH

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