Electric Bass Signals

Walter Harley, November 1999 (last revised: April 2000)

In designing various electric bass signal processors (some of them on these pages), I've had to learn a lot about the characteristics of an electric bass signal. They're different than I initially expected. Here's some of what I've found:

Signal Strength

To measure peak strength, I played the basses through a single JFET opamp stage with 10M input impedance and unity gain, powered by +/-15V supplies, into an opamp-based peak detector. The following numbers should be a fairly good representation of true peaks; these are much higher than the RMS signal strength because of the unusual waveform (see below).

My Keith Roscoe five-string, with active 18V Bartolini electronics, puts out as much as 6V peak when I bang on all the strings with both hands as loud as I possibly can. I imagine it would put out more if it had bigger batteries! In realistic but aggressive playing, slapping and plucking as hard as I can, it puts out peaks of 2V at most. In more normal (but still aggressive) playing, peaks are about 1V. That's with the tone controls flat; boosting the tone controls boosts the peaks correspondingly (the Bartolini EQ is +/-18dB).

My Fender acoustic/electric bass, with Lane Poor P-style pickups connected directly to the output jack and no electronics at all (no tone or volume control, no preamp) puts out somewhat less: peaks are 2V making the most noise I can without breaking the instrument, 1V playing aggressively, and about 0.7V peaks playing normally. (In the past I've measured the output of the piezo pickup in this bass, and into a 10M impedance it's comparable to the Keith Roscoe.)

In both instruments, negative peaks tended to be slightly higher than positive. I imagine this could be because I pluck in one direction, but I'm not sure.

Note that the signal from even the passive pickups is almost strong enough to drive a power amp directly: what a preamp needs to do is provide impedance matching, tonal coloration, and only a small amount of voltage gain. It's different than an electric guitar and very different than a microphone.

Waveform

To measure waveforms, I again played the basses through a unity gain opamp stage with high input impedance and no filtering. This time I observed the results on an oscilloscope. I also recorded some of the results into a DAT deck and then transferred the results into my computer to generate the WAV files and images below and to do Fourier analysis.

Even when playing fairly gently, the waveform has extremely high harmonic content. The wave shape is generally somewhere between a sawtooth and an almost heartbeat-like shape. Here's an excerpt of plucking my E string (and here's the WAV file I excerpted it from):

I expected plucking a harmonic to give a more pure sine wave. Surprisingly, it wasn't much different. Here's an excerpt of a harmonic, and again the WAV file it's from:

Spectrum analysis

Obviously from the above, the wave is far from a pure sine wave. In fact, upper harmonics dominate; here's the Fourier transform of the plucked E string:

Note that the second and third harmonics (80Hz and 120Hz) are both slightly higher than the fundamental, the fourth is about the same, and even by the 11th we're within 18dB of the fundamental. There are harmonics well above the noise floor up to about 1kHz, around the 25th harmonic, and that's on the unfretted low E string with a fairly smooth plucked sound. A slap or pop would probably have much more high frequency content, although much of it would not be harmonic in nature.

What's the significance of that? Partly, it's just a demonstration that I'm using active pickups, stainless steel strings, and a very good bass. An old Fender P-Bass would have much less high frequency content (for better or worse depending on one's purpose - no holy wars here, please).

The data lead me to conclude that the easiest way to alter the sound of a bass is by tonal shaping rather than by distortion. Because the waveform is basically a pulse followed by a period of silence, clipping that pulse doesn't have much effect on the harmonic content: the signal is in a sense already highly distorted. Put differently, what distortion does is add upper harmonics, and those harmonics appear to already be there in spades, just waiting to be colored by filtering. Wah effects, envelope followers, and comb filters (aka flangers and phasers) make sense; distortion doesn't, based on this one datum. More research is clearly called for.

Another conclusion one could draw is that the sound of an un-EQ'd bass should be more appropriate in some forms of music than others. The harmonics correspond to other musical notes, with the higher harmonics tending to be more dissonant notes. For instance, suppose I plucked that E as the root of an Em chord. The guitarist is also playing an Em barre chord, two octaves up. My fourth harmonic is his root; good. But my fifth harmonic is approximately a G#, in the same octave as the guitarist's G natural. Depending on what I'm playing, I might want to roll off my highs to avoid that conflict.

Still to come: Transients

It seems that a lot of the liveness of the sound may come from transient response. Here's the envelope of the harmonic I was plucking earlier, and a few neighbors. Note the big initial spikes:

In this WAV file I've taken one of these harmonic plucks, duplicated it and digitally edited that initial spike in the second copy only, to reduce the volume to the same level as the following peaks. So, what you hear is the harmonic as played, followed by the modified version. To my ears, the modified version is smoother, more processed sounding, and less lively. I'd suggest setting up your media player to autorepeat and listen for a while, paying attention to the initial attack.

The peak is only a single half of a cycle; the actual attack, from zero to top of the peak, lasts 1msec. I conclude that transient response - the ability of an amplification system to go from zero to peak in very short time - is important in capturing the liveness of the bass sound. If you want that liveness, you probably want to make sure your gear is able to deal with those transients, and if you use a compressor you want to set the initial attack time slow enough not to affect them. Or the opposite, if you want the smoothness of the modified wave.

So, is reproducing those transients really a hard thing? How does commercially available gear fare at it? What characteristics of an amp or preamp design are important to preserve transient response? More on that later.

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