Virtual Pickups Part 4

A few years ago, I wrote a three part article series about virtual pickups: emulating guitar pickups in software. Now it is 2020 and after a long journey, I am continuing with parts 4, 5 and 6 of the Virtual Pickups series, in anticipation of the release of the Ascend VPU plugin — the first of a series of plugins designed for multichannel processing.

Go ahead and check out the Ascend VPU User’s Manual, or review the original Virtual Pickups articles (Part 1, Part2, and Part 3) if you haven’t done so yet, or a refresher if you already read those.

Recap

The basic premise is that you only need one multichannel pickup (e.g. The Nu Multi) near the bridge. You can emulate other pickups positioned anywhere from the bridge to the nut, using DSP building blocks comprised of comb filters and resonant low-pass filters. The comb filters simulate pickup placement, while the resonant low-pass filters simulate the electrical characteristics of the coils in the pickups.

I wasn’t really aiming for exact emulation of specific guitars. I wanted to use emulation only as a starting point so the user will have something familiar to start with. But, as we shall see later, it is indeed possible to get something really close to, or even indistinguishable from the real thing, if the modelling is done right. We’ll see about that later.

So… everything beyond this point will require reading the original Virtual Pickups articles (Part 1, Part2, and Part 3), unless you don’t really care how this thingamajig works, and so you can probably just skip this whole article. I am always just proud to share the inner workings of a fine-tuned engine, down to the chrome plated screws.

Single Coils

To recap, here’s the graph of the computed frequency response of the neck pickup: E string, 82 Hz, 25.5-inch scale, pickup located 16.2 cm (6.375 inches from) the bridge:

The Nu Multi (or any other multichannel pickup, for that matter) has its own (non-virtual) set of comb-filter notches that imparts its own color, depending on placement. Here’s the actual Nu Multi’s spectrum when placed 3.21 cm. from the bridge:

Two things to take note of:

Due to this natural comb-filter effect, you can never have a flat frequency response regardless if the pickup itself has a flat response (e.g. the Nu), and regardless where you place the pickup. But the closer the pickup is placed adjacent to the bridge, the higher the frequency of the first peak will be.
There’s a low frequency roll off from before the first peak down to DC. This roll of gives pickups placed near the bridge its characteristic thin sound and it gets thinner the more you move the pickup towards the bridge.

We want a preprocessor that can somehow negate this inherent comb effect.

Inverse Comb

As mentioned in Part 3 of the Virtual Pickups articles, a feedback comb filter, has a spectrum which is the inverse of that of the feedforward comb filter. We can use a feedback comb as a pre-filter to negate the actual pickup’s natural comb-filter effect.

Here’s the spectrum of the feedback comb filter with the same parameters as that of the Nu Multi’s natural comb-filter above (i.e. 3.21 cm. from the bridge), and with a feedback gain of 0.7:

The amount of signal being fed back should be less than 100% for this to be stable, otherwise, you will get infinite repeats. The higher the feedback, the narrower the peaks, but anything above 0.9 results in a metallic sound, similar to a flanger with feedback set too high.

The comb filter notches nulls certain frequencies. Such frequencies can only be somehow recovered to a certain extent. There is no way to recover frequencies that are not there to begin with, by processing alone (i.e. without synthesizing). We will only attempt to recover just enough to more or less flatten the curve. Therefore, after lots of listening tests, I find that a feedback gain of 0.7 is good enough. Additionally, we can enhance the bass frequencies below 50Hz with a 6dB boost.

Here now is the final result after preprocessing. Not perfect, but certainly a lot better than the original raw signal. Note: This is the best I can get to a flat frequency response without resorting to expensive FIR filters. If you have a better suggestion, I’d love to hear it!

Sonically, those spiky notches do not seem to matter at all. The result, after preprocessing alone, is very rich, sweet and pleasing —very much like an acoustic guitar. You get all the full sonic range. No bass roll of. No deep notches. The quality of the result is very useful in and of itself.

Here’s an audio clip of the raw (unprocessed sound):

Now compare that with the preprocessed sound:

Next Stop

That’s all for now. In the next installment of this series, I will write about pickup modeling using the Ascend VPU plugin, with emulation of various pickups as well as exploring what’s doable beyond what can be done with physical pickups.

Ascend-VPU vs. Actual Guitar

For now, if you’ve read this far, here are teaser sound clips demonstrating the emulation of a modified Stratocaster’s middle and neck pickups. The Strat is equipped with Dimarzio Injector at the neck and bridge positions and Dimarzio Area ‘67 at the middle position.

To help in A/B (split) testing, each sound clip is a two part mix.

The first part starting at 0:00 is the recording of the emulated pickup.
The second part starting at 0:03 is the recording of the actual Stratocaster guitar.

These are direct to DAW recordings with the Ascend-VPU plugin inserted in each of the multichannel guitar input, no other effects, no EQ. The actual Strat recordings are direct with no other effects and no EQ.

Strat Mid + Neck Pickups:

Strat Neck Pickup:

OK, there you go… cheers! See you next time.