A critically important characteristic of the technique was that it was inherently monotonic even for the quietest signal levels. The H910 used an advanced version of a technique developed by Richard Factor for the 1745 delay lines.
With each new Eventide product through the 70’s and 80’s we painstakingly improved our designs. Prior to the development and commercial availability of over-sampling ADCs in 1989 however, pro-audio quality ADC design was a true art. D to As are rather simple – that’s why the 1st consumer digital audio products – which didn’t appear until 1982! – were CD players. One of the things that set Eventide apart, all those years ago, was our skill in designing great sounding A to D converters. In fact, the glitch was embraced as a musical effect by some (thank you Laurie Anderson!). On the third hand, most of the time, the two summed signals were somewhat out of phase and the resulting soft glitching was less than awful. On the other hand, if the signals were 100% in phase, the glitch would disappear. This improved matters dramatically, although ‘soft’ glitches still resulted because if the time-shifted signals were 100% out of phase, they would cancel and one would still hear a drop out. The result was analogous to tape editing resulting in a splice, a crossfade, rather than a hard glitch or a drop out. Two multiplying DACs were wired in parallel but fed from different points in the circular delay. The innovation that tamed the glitch involved using a pair of offset delays, fading one in and the other out at the splice point. Plus, clicks would preclude feedback which was a key design feature to make arpeggiation and repeats possible.
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Since the audio on each side of the discontinuity cannot be controlled or determined, these glitches can be quite nasty, full scale clicks or complete drop outs.įull scale clicks or drop outs wouldn’t cut it for a piece of pro-audio gear. As the pitch ratio moves away from unison, the glitches occur more and more frequently. For pitch ratios close to unison (pitch ratio ~1.00), the delay changes slowly and glitches occur infrequently. What does the glitch sound like? How noticeable is it? Well, that depends on the pitch ratio and the audio source. Of course, the instantaneous jump results in a “discontinuity” in the audio signal. Use a ‘circular delay buffer’ of some length (typically 20-30 msec) with the delay abruptly jumping from zero to max in the case of increasing pitch (decreasing delay) and from max to zero in the case of decreasing pitch (increasing delay). The ‘solution’ is both simple and imperfect. In fact, for real-time performance, it’s best to limit maximum delay to no greater than ~20 msec. Delay can’t decrease past zero (that would require a magical digital “advance” line). It helps to think of this as increasing pitch by decreasing delay and decreasing pitch by increasing delay. If you continuously read from memory slower than you write to it, the delay will grow until you’ve completely filled up the memory. If you increase pitch by continuously reading from memory faster than you write to it, you’ll run out of data. The challenge for a real-time pitch changer that does not change tempo is rather obvious.
Of course, as with tape, the audio plays back at a faster or slower rate. This is the equivalent of recording to tape at one speed and playing back at another speed. Random Access Memory ICs became commercially available and pitch change was made possible by reading the audio from memory (playing back) at a different rate than writing (recording).
The Challenge: Pitch Change Without Speed Changeīy 1975, IC technology had sufficiently advanced to the point that it became practical to design a digital pitch change effects box - the H910. However, the interface was not designed to easily dial in pitch-related effects, and there was a technical challenge to overcome. Eventide’s DDL1745M had an optional pitch change module and a handful of studios began to discover digital effects. Prior to its introduction, studios had adopted digital delay as a utilitarian tool to replace the bother of using an expensive tape machine (and salaried tape op) for double tracking and plate reverb pre-delay.
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The H910 was arguably the first pro audio digital effects product.
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