Sunday, June 1, 2008

Inside the V-Synth, part 2

In Part 1 of this series, we looked at the oscillator types and functions of the V-Synth. Now, let's look at the rest of the voice signal chain.


Recall that the V allows, for a given patch, a choice of one of three "structures", which determines which order the processing elements are placed in. This concept is a carryover from earlier Roland digital synths; given the limitations of the technology used in those synths, it was necessary to use structures in order for the synth to determine the DSP resources needed for a given patch, and to provide for certain cross-element modulation functions such as ring modulation. However, the V is smart enough to allocate DSP resources dynamically, and it provides other means of modulation. So the only thing the structure selection on the V really does is determine where the COSM filters are placed in the chain; everything else is the same in all three structures. Structure 1 places both of the filters in series after the oscillator modulation mixer. Structure 2 places COSM 1 in line after osc 1, ahead of the filter. Structure 3 places both COSM filters ahead of the mixer, one each in line with each of the two oscillators.  The diagram below illustrates them in the same configuration as the structure selection buttons on the panel (for some reason, I haven't been able to take a good picture of the actual buttons):

The Oscillator Modulation Mixer

The oscillator modulation mixer (usually referred to as the "mod" for short) combines the outputs of the two oscillators (one or both of which may have been processed by a COSM filter at this point, depending on the chosen structure). The mod takes care of all forms of modulation (except cross-modulation) in which one oscillator effects the other. The mod operates in one of four modes:

  1. Add mode. This simply mixes the two signals.
  2. Ring modulation mode. This allows osc 2 to amplitude-modulate osc 1 in any amount up to full balanced, or ring, modulation. (Recall that ring modulation is simply AM at 100% modulation.)
  3. FM mode. This mode allows osc 2 to frequency-modulate osc 1. In Yamaha terms, this is only 2-op FM, so it may sound lame. However, recall that the canonical Yamaha FM system uses only sine waves, while the V allows any waveform to be used as either the carrier or the modulation. So very complex and bizarre results are easily (a bit too easily!) obtainable.
  4. Oscillator sync mode. In this mode, osc 1 will sync to osc 2. Osc 2 must be a virtual analog waveform.

The COSM Filters

The "COSM" acronym is a Roland trademark which stands for Composite Object Sound Modeling. Basically, it's a set of software algorithms that implement all of the familiar analog filter types (high pass, low pass, bandpass, and notch), as well as a number of more complex types, physically modeled resonators and some other algorithms that either aren't actually filters or do things other than just filtering (such as compressors and guitar amp simulators). The complete list:

  1. TVF -- "time-variant filter", a long-standing Roland-ism for the digital equivalent of your basic VCF. Lowpass, bandpass, highpass, notch, and "peak" (which I presume is a high-Q bandpass) are available, and you can select 1-, 2-, or 4-pole response.
  2. Dynamic TVF, a filter whose cutoff frequency tracks the envelope of the input signal. Same options as above.
  3. Dual filter. This allows you to put a lowpass and a highpass in series or in parallel, or put two bandpass filters in parallel.
  4. TB Filter, which simulates the VCF of the famous/notorious TB-303 bass machine.
  5. Sideband filter. This is basically a lowpass and a highpass arranged in a "back to back" configuration so that they create two passbands about a center notch. Useful for adding a sense of pitch to unpitched noises, and for picking out certain harmonic bands from an FM-generated signal. There are two of these, a first-order filter and a second-order filter.
  6. A comb filter. This is the type of filtering produced by flangers.  It can also be used to creat formants.
  7. A resonance simulation. This simulates the resonance of a hollow-bodied guitar. It can also simulate a banjo or resonator guitar (e.g., a Dobro) body.
  8. Amp and speaker simulations.
  9. Distortion and overdrive.
  10. A wave shaper, which can alter and distort wave shapes in various ways.
  11. A "lo-fi processor", which is actually a bit crusher (reduces the resolution of the samples), and downsampler (interpolates the samples down to a lower sampling rate).
  12. A compressor and a limiter. These are interesting because they are actually part of the voice; like the other filter algorithms, they process each note separately. These can be very useful problem solvers. A typical problem when setting up patches with drastic filter settings is that some notes play much louder than others. If COSM 1 is used as a drastic filter in a patch, setting up COSM 2 as a compressor or limiter will smooth out the variations in loudness between different notes.
  13. A frequency shifter. This duplicates an effect that was originally developed in analog form in the 1950s, but was seldom used because the necessary circuitry was expensive and difficult to keep in calibration. Unlike the more common pitch shifter, the frequency shifter does not maintain the harmonic relationships between overtones. Needless to say, it can produce some very strange effects.

Each COSM algorithm has its own set of tweakable parameters, which are denoted in the detailed descriptions in the manual as parameters #1 through #4 (or fewer if the filter type doesn't have that many). The parameters labeled #1 and #2 are always controllable via the "cutoff" and "resonance" knobs on the panel respectively. Generally, these will be, if not actually cutoff and resonance, something analagous. However, in the case of things like the distortion and overdrive types, it's not always obvious; you have to read the manual. All four of the numbered parameters can also be assigned to the control matrix, which means they can be controlled using any of the synth's performance controls and/or MIDI Continuous Controller messages; this will all be described in a future installment. As in the case of the oscillators, each COSM has its own LFO which can be assigned to almost any parameter, and most paramters have their own envelope generators.

The final voice component in the chain (in all three structures) is the TVA. This has its own envelope generator, which has sliders on the panel (of the dozens of envelope generators in the voice architecture, it's the only one that does). It's somewhat surprising that the envelope is a conventional ADSR, given that the JD-800 and some of the JX series synths had more sophisticated multi-segment envelopes. The TVA does have, like everything else, its own LFO.

In the next installment, we'll look at the V-Synth's onboard effects.

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