Lexicon PCM80

Paul White tries out a new hardware solution for dodgy vocalists.

Lexicon's PCM80 is an extraordinarily powerful effects processor, but it doesn't come with an intelligent pitch‑shifter as standard, so Lexicon instead decided to develop a dedicated algorithm that could be loaded from a PCMCIA card for the specific purpose of correcting vocal pitching problems. The result is the Vocal Fix ROM card, which may be used with any PCM80 version 1.10 or above.

Dealing With The Card

The easiest way to load the algorithm is to switch on the PCM80 with the card in place, at which point the new algorithm loads automatically and is available in addition to any existing programs. The card may be removed from the PCM80 and the algorithm will continue to work until the unit is powered down. However, the card is required for loading the algorithm each time the machine is powered up. You can also insert the card into a running machine and load the algorithm from the front‑panel controls. It should be noted that only one algorithm card can be loaded at a time — if you've already loaded up algorithms from another card, the second card will refuse to load.

Vocal Fix is designed to help patch up vocal tracks that aren't as perfectly pitched as they might be (unlike other PCM80 effects cards, this one contains just a single algorithm, combining a dedicated pitch‑shifter with a general‑purpose chamber reverb). The algorithm is true stereo, allowing stereo sources to be processed, but as the pitch‑detection circuit requires a monophonic input you have to treat only one vocal part at a time. Figure 1 (below) shows the signal flow of the algorithm.

The main reason the Vocal Fix requires a monophonic input is that it must first track the pitch of the incoming signal in order to work out how far off‑pitch it is. In addition, treating monophonic vocal lines means that it's also possible for the pitch‑shifter to constantly adapt its splicing routine to fit in with the vocal frequency, resulting in far smoother pitch‑shifting. All conventional pitch‑shifters work by breaking the input signal into a stream of very short audio samples, which are then either sped up or slowed down, looped, and then spliced back together. If this process occurs with no regard for the input frequency, the sound can become very glitchy or lumpy, but if the splice points coincide with individual cycles (or multiples of cycles) of the processed waveform, they become much less obvious.

At around half the price of a typical software plug‑in, the Vocal Fix card combines good value with an extremely simple user interface.

Using the algorithm is fairly straightforward, and tweaking just a few parameters optimises the process for different vocal styles. (There are also a couple of special effect modes that will be described later.) When a vocal signal is fed into the PCM80, the display shows the pitch to the nearest chromatic note, plus the number of cents deviation. Pitch correction is best applied to notes identified as being audibly wrong — this isn't one of those systems that tries to correct every sung note to the nearest semitone, as such an approach is fraught with technical problems and is, in any case, rarely artistically desirable. You can try it, if you want to, by applying correction all the way through a track, but this isn't something you'd normally do.

Correction is carried out manually, either using the Adjust knob (or you could use a switch) on the PCM80 itself to activate the Correction process, or via a MIDI keyboard. When 'Correct' is activated, the pitch is pulled towards the nearest semitone by an amount determined by the Correction percentage parameter. If you're using a MIDI keyboard, the pitch‑bend wheel may be used to directly sharpen or flatten the pitch of the processed sound over a fairly narrow range (useful for fine‑tuning sharp or flat notes) or any sung note can be forced to a completely different pitch by keying the desired note on the keyboard. Pitch correction only occurs when a key is pressed, so you don't have to play along with the whole melody — only the wrong notes. There's no formant correction, so notes shifted too far from their original pitch will suffer from Mickey Mouse syndrome, but in reality most shifts will be fairly small, so the effect of formant shifting should be inaudible. Because most operations require only pitch correction, the default reverb setting is zero, but this is easily turned up if required.

Parameters

Compared with most PCM80 effects, there are relatively few parameters to worry about with the Vocal Fix algorithm.

• The first parameter is 'Correction', which allows you to set the percentage of pitch correction to be applied, from none at all to 100%. If set to 100%, the algorithm will attempt to 'pitch quantise' the incoming vocal as closely as possible to the nearest note, whereas a lower value means it will be shifted in the right direction, but not so far as to become perfectly accurate.
• 'GldResp' (Glide Response) sets the shifter's response time to the detected pitch source, and also to any internally routed controllers that may be in use to further modify the sound. High settings result in close tracking, while low settings mean that the pitch change will lag behind the detected pitch. The manual suggests a setting of around 50% for the most natural results on most material.
• 'Tracking' relates to how precisely and quickly the tracking algorithm follows pitch changes in the original source. At its fastest, tracking will follow fairly fast vibrato or pitch bending, while slower settings will tend to average these out more. Fastest is usually the best setting for natural‑sounding pitch‑shifting; the slower settings can be used to create special effects. There's also a Hold setting that freezes the output pitch at the last detected pitch — ideal for renditions of the One Note Samba.
• 'Low Pitch' and 'High Pitch' enable limits to be set on the highest and lowest notes to be detected, which is useful if there's headphone spill from instruments outside the voice's natural range that might cause false triggering during vocal pauses. In such cases I'd also be inclined to gate the input to the PCM80, to give it as clean a feed as possible.
• 'Tuning' enables a tuning reference other than A440 to be used if desired, while 'Splice' adjusts the crossfade between the successive packets of sampled audio that are created in the shifting process. Higher values than the default 8ms produce a smoother sound, but at the expense of transient definition.
• Last, but not least, there's the 'Shift Cents' and 'Shift Semitones' parameters, which allow a fixed pitch‑shift to be set up in coarse steps of one semitone and fine steps of one cent. The range is one octave in semitone steps, but as the cents range covers up to 1200 cents, this can be added on, to provide a maximum of two octaves of shift in either direction.

Using Vocal Fix

Generally, the vocal track to be processed would be routed via the PCM80 and then recorded to a spare track on your multitrack. As with any digital processor, the pitch‑shift algorithm involves a certain amount of time delay, but if you use a digital recording system that allows negative track delays to be set up, these can be compensated for. The amount of delay depends on the pitch of the note being shifted, with the best case being around 30ms and the worst case being a hefty 82ms. Lexicon provide a table of the delay in samples at various pitches, so that you can choose a suitable offset.

For most real‑life situations, Vocal Fix gives you the right tools to do the job as painlessly as possible.

Slight pitch corrections can be made very smoothly, but the success of using a MIDI keyboard to play in completely new notes depends on both your patience in choosing the best settings and on the nature of the incoming vocal line. I found that vocals with heavy vibrato or strong pitch scoops could fool the system, giving rise to the occasional yodel!

The card comes with just 10 presets, numbered 0 to 9, the first of which is a general‑purpose patch using a MIDI keyboard as the controller. Here the MIDI key selects a new note, the pitch‑bend wheel offers fine pitch control, and the sustain pedal turns the Correct mode on and off. Correction may also be turned on and off using the Adjust knob. The next four patches are similar, but with different vocal ranges set for bass, tenor, alto and soprano.

Preset 6 allows a constant offset to be applied to tracks that are consistently sharp, though the keyboard can still be used to fix totally wrong notes. Preset 7 provides a very authentic double‑tracking effect, where the delay and pitch‑shift of the doubled sound can be made to wander randomly via the Adjust knob. Preset 8 uses the Adjust knob to freeze the pitch of any vocal being processed, while preset 9 is designed for demonstration use. In this preset, only the right input is active, and all the pitch parameters are in the PCM80's softkey row for easy access.

Summary

At around half the price of a typical hard disk workstation software plug‑in, the Vocal Fix card combines good value with an extremely simple user interface. The fact that you can switch in correction only when you need it means that vocal tracks stay natural‑sounding, and though you don't get the same scope for fine‑tuning and tweaking as with, say, the Antares Autotune TDM plug‑in software, the results achievable with Vocal Fix are generally good and very easy to obtain — though if you listen to the dry vocal in isolation you can just about hear the tell‑tale modulation that informs you you're listening to a pitch‑shifter. Using the MIDI key shift to change notes takes a little getting used to, and on tricky vocals you may have to vary the parameters relating to pitch‑tracking and shifting, but the quality of shifting is generally smooth and natural. Even so, if the singer uses a lot of pitch‑bends you can find the correction algorithm dithering between two different pitches, and if the material is really difficult to track you may have to resort to using the pitch‑bend wheel to put in the necessary shift manually. However, for most real‑life situations Vocal Fix gives you the right tools to do the job as painlessly as possible.