Hum debuggery

 When I started playing electric guitar I was very much into technology. In my mind, single-coil pickups were inferior because they picked up NOISE and HUM, and therefore humbucking pickups were the only way to go. It was a superior technology.

I was about twelve, so what did I know? But I stuck with 'buckers for many years. Later, I started getting into single-coil pickups. First a Tele, then a Strat. I liked the "chimy-ness" and the definition that single coils provided. I was tired of the darkness and muddiness of many humbuckers. But the noise, dear god, the noise. When I started using guitars with P90 pickups, the noise situation got worse as most P90s I've played are like high-gain antennas when compared to Fender-style single coils. But they do sound awesome. (best pickups ever, IMHO - but I again have several humbucker-equipped guitars as well)

Back in the day most of the electrical noise we encountered in bars and clubs was caused by light dimmers, neon lights, and the occasional jukebox - all exacerbated by bad AC wiring. If you plugged into the "right" outlet and stood in the "right" place, you might be able to avoid picking up the hum. These days, it's not so much neon lights but cellphones, computers, and WiFi that cause the bulk of the RF noise that our guitars pick up. We still face bad AC wiring in most clubs, however. Some things never change. 

Old school solution

So how do we get rid of this noise? The most common tool we have is a Noise Gate. This device is basically an automatic volume knob that turns down the volume when the guitar signal is below a pre set threshold, then turns it back up rapidly when the signal is louder. There are many models on the market, I happen to have an old Boss NF-1. They all work pretty much the same way. It is important to understand that the noise and hum is only eliminated when you stop playing. It relies on the idea of sound masking so that while you play, your guitar signal will be louder than the noise signal and you won't hear much of the noise, but it is always there. (Note that the ISP Decimater is a different beast apparently - I don't have one at the moment, so I can't comment. That'll be the subject of a future investigation)

DSP-Based technology

There is a device made by ElectroHarmonix called the Hum Debugger that is based on digital signal processing technology. In short, it is placed at the beginning of your effect chain - it samples the electrical noise in the signal from your guitar and effectively "subtracts" it using a DSP algorithm. The key to this is that it requires an AC transformer (it does not run off a standard 9VDC wall wart) in order to sample the electrical nose and, I presume, sync to the AC line frequency. The advantage is that it continuously removes the hum and noise from the guitar signal, unlike a Noise Gate, and does this even with audio present.

I have an EHX Hum debugger, and I started using it with my P90 guitars.

I stopped using it almost immediately afterward.

The Good

The EHX Hum Debugger does, indeed, remove the hum and noise from the guitar signal. It's kind of amazing to hear. There is virtually no noise in the background of your guitar signal when the EHX Hum Debugger is activated. There is no attack time delay, there is no fade out. No risk of chopping notes off if the threshold is too high and you start playing softly. It's almost transparent. The pedal has two noise elimination settings, activated by a mini-toggle switch - "Normal" for I guess normal amounts of noise, and "Strong" which applies a stronger reduction. The pedal also has a standard footswitch for true bypass.

I say "almost" because the Hum Debugger does alter the tonality of the guitar a bit. Most people, myself included, hear it as adding some high-frequency emphasis to the tone. Not exactly an EQ change, but somehow brighter. It's easy enough to compensate for, however. Certainly, the dramatic reduction in noise seemed at first to outweigh this tone issue.

The Bad

If only the tone change was the singular problem. I noticed pretty quickly that there were some weird artifacts when I played, especially when I was playing lead lines up high on the neck. The anomalies were more noticeable when bending notes, and were absolutely dreadful when using any kind of dirt pedal. For a while, I used the bypass switch to turn the pedal off while I was playing, but this defeated the purpose of having an always-on hum eliminator. I eventually stopped using the pedal and even avoided playing my P90 equipped guitar in certain bars and clubs where the noise was especially bad.

Investigation

I had to figure out what I was hearing. I can best describe it as a sort of warble/phase shifting with some cancellation seeming to take place. It is very disturbing. 

Let's start with some audio samples. the .wav file below is just the noise floor (o signal) out of the HumDebugger, with the input connected to a P90 equipped guitar, with the volume and tone wide open. The signal is boosted to make the noise more audible. There are three samples, first with the pedal bypassed, then active in Normal mode, followed by active in Strong mode, then back to bypassed. You should be able to hear three distinct noise levels. The pedal is clearly doing what they advertised. 

Nasty Noise:

Clean bends

Here is a clip of three bends, first with the Hum Debugger in Bypass, then Normal followed by Strong. If you listen carefully you will hear some of that "phase shifting" kind of sound in both the Normal and Strong samples, but it's more noticeable in Strong, I'll admit, it's not too horrible if you are playing very clean. 

Dirty bends

But admittedly, I play with a bit of dirt - or a lot of it - much of the time. With distortion or overdrive (keep in mind that the EHX pedal is the first in the chain, so it comes before any drive pedals or the preamp of the amplifier) the audible artifact is really noticeable.

Dirty bends:

The changes to the guitar signal caused by the Hum Debugger are clearly visible in the above waveform. Different notes will have this effect to a greater or lesser degree, but it happens all over the fretboard.

Further analysis

I then tested the pedal's frequency response using a swept-sinewave signal, and the result was surprising. Normally, we would expect to see the response as pretty much a flat line from 20Hz to 20kHz, with just a few dB of roll-off at the low and high extremes. But this result showed that it looked more like a comb filter response, where certain frequencies were notched out. what is a comb filter

This pretty much clinched it for me. While single, non-changing notes would pass unscathed (except for the added brightness) any pitch changes such as bends or vibratos would end up being audibly mangled. The above graph is in Normal reduction, the comb filter was more dramatic and occurred at more frequencies in Strong mode.

What about the "brightness"?

I next looked at THD+N, what we refer to as "distortion plus noise" in audio signals. Since this pedal is not a distortion or overdrive box, it should not be adding any distortion to the signal, but here's what I found when I passed a simple 1kHz (1000 hertz) sinewave through the Hum debugger in bypass, Normal, and Strong hum reduction:

Bypass - all we see is the 1kHz tone, and nothing else. This is good.

Normal hum reduction - we see a new peak, just slightly higher than 1kHz. There are also three other peaks surrounding the 1kHz reference tone. This added higher frequency might be what is causing the primary tone to sound brighter. in any case, this is not what we want in an audio device.

Strong hum reduction - there are now two peaks, one above and one below, added to the 1kHz reference tone. Again, the frequencies of these peaks are very close to 1kHz, so we would hear them as distinct tones, but they will adversely affect the sound of the 1kHz tone.

Conclusion

The tests I performed confirmed what my ears were hearing, another win for science! This also confirmed that I won't be adding this pedal back to my pedal board again. For me, and the king of music I play (and the effects that I use) this pedal just won't cut it. It does, however, do what is advertised, and it might work acceptably for your style of playing.