What is an envelope filter or a wah pedal?
The shortest answer is that they are effects used mostly in funk music for a "quack" or "bow wow" sound on the bass, guitar, or keyboards. Here's the longer answer:
A filter is a circuit that adjusts the "shape" (relative levels) of all frequencies that pass through it. A high-pass filter allows the highs to pass, cutting the lows. A low-pass filter allows the lows through and cuts the highs. In both cases, the cutoff point is not a sharp drop, but a curved slope. A band-pass filter allows a hump-shaped range of frequencies in the middle to pass through, and it cuts off frequencies above and below that range.
Any EQ is a filter, often several filters connected together. But usually when people talk about filters they mean effects like a wah or an envelope filter. Those have much stronger tone changes than just a regular EQ, often using resonance to emphasize certain frequencies, for sounds that can be described as "fat", "vowel-like", "screaming", "quacky", or "wet". Resonance is actually a type of feedback that causes certain frequencies to amplify themselves much more than other nearby frequencies.
The "envelope" is the shape (the changing "hump" of levels) of each note you play, from the initial spike when you pluck or pick, through the drop in level as each note trails off. The time from the moment you trigger the filter, to the maximum amount it will peak at, is called the attack. The time from that max peak, sloping down to the minimum level, is called the release. The circuit that "reads" the changing levels of your signal is called an envelope follower, and it adjusts some quality of an effect in direct proportional response to those changes. It's not always a filter--there are some phasers, flangers, delays, and other effects that may be controlled by an envelope follower. Some of these devices are designed to exactly follow the envelope of your instrument signal; some are preset to change at some other rate; and some have knob controls over those time settings. These timing differences (whether inherent or controllable) are a big part of what makes one filter suit your personal style better than another.
The sensitivity of a filter is the degree to which it will react to your input. You will almost always have to adjust the pedal's sensitivity and adjust the levels of the signal you feed into it, in order to get the best results. We speak of a filter "opening" and "closing" as a way of describing how it widens or narrows the range of frequencies allowed through; and a filter may open "up" or "down". Up means it is normally closed, opening (letting more frequencies through) when the envelope is triggered; down means it's normally open, and then closes when triggered. A wah-wah pedal, on the other hand, does not open and close--instead, you use the foot treadle to change the peaking resonant frequency of the filter. The term "auto-wah" is used by different people to mean two different things: either an envelope filter, or a filter that constantly opens and closes in a repeating cycle (controlled by an "LFO", a low-frequency oscillator). So be aware of that double meaning when asking about auto-wahs, or when reading the label on a pedal. A "touch wah" is just an envelope filter.
Because envelope filters are so responsive to your signal, you can really only get good results if you learn to play the filter itself, instead of just playing your instrument. In other words, you will have to adjust your playing technique in order to get the filter results you want--it's not like a flanger or distortion, where you just step on it and the effect happens regardless of how you play. Additionally, the sensitivity of the filter has to be matched up with the output range of your instrument, and unfortunately a lot of instruments have output that is too low or too high to ideally trigger any one specific filter, no matter how you set its sensitivity knob. The solution is to raise or lower the level of your instrument, using its onboard volume control or an external pedal that can boost or cut levels, before the input of the filter. To be clear, if you are getting bad results from a filter, odds are that you are not using it correctly.
People often complain about a "volume jump" when using a resonant filter. The thing to understand about that is that it's not a boost to the volume of your whole signal, it's only a boost to the resonant frequency range. If you turn down the volume on the pedal, you can bring the resonant peak down to a more normal level, but then the highs and lows above and below the resonant peak have been reduced way too far. For this reason, some people prefer pedals with less intense resonance, as that allows more natural levels and tone. For people that want the extreme gloopy effects of Bootsy Collins however, you need big resonant peaks to get that intense fat wet sound. Those people need a peak limiter after the filter--this will keep the volume spikes of the resonant filter in check, without dropping the levels of the rest of your frequencies. Unfortunately there are no "perfect" limiter pedals out there. Check my reviews and my "top picks" page for examples of pedals that do a decent job of peak limiting.
Bootsy has used such an intensely complex rig to achieve his sounds, that no one pedal can get you his sound exactly. His bass has several pickups, with a different output jack for each pickup, and each output goes to its own separate effects chain and amplification. Each effect chain has a different filter with different settings. It's nuts. To further complicate things, he endorses a lot of products that may not help you at all. The "Bootzilla" wah pedal by Snarling Dogs, for example, is a flimsy piece of crap that doesn't sound like P-Funk to me. Some people find that a vintage Mutron III is closest thing to a single-pedal Bootsy solution, and there are a few modern clones of that circuit, such as the 3Leaf Proton. Certainly there are other pedals that can also bring the funk; it's just up to you how badly you want to get that coveted sound, versus how much equipment you are willing to carry around (and pay for).
There are a few filter effects that use two or more filter circuits, in order to get a more complex tone and action. These dual/triple filter stages may be set with different envelopes from each other, different resonant frequencies, or different sensitivity (or any combination of those). You may see references to "2 pole" or "4 pole" filters; this describes how many stages of filtering is in the circuit, and the number of stages can affect the intensity or flavor of the effect. However there's no standard for what a pole/stage sounds like from one filter to the next, so the number of poles is really only meaningful if you have one specific filter that can switch between 2 and 4, for example. There are also some filters that have an effects loop built in; a loop is an insert point, where you can plug in other effect pedals. This loop is after the follower circuit, but before the filter circuit; what this means is the filter's envelope will be triggered by your clean input signal, but then the sound of any effects you put in the loop will go through the filter effect. This is especially useful because many effects will change (or even flatten) the dynamic range of your signal, so they reduce your ability to trigger the filter properly.
Analog versus digital... Honestly I am biased in favor of analog filters, because so far I haven't heard a digital filter with the same subtlety of envelope response and organic fatness as a traditional analog circuit. However the digital modeling quality is improving all the time, so my bias may become obsolete very soon. In fact it may already be obsolete, but I am still clinging to the old-school circuits I know and love. In part it's because their quirks and flaws are inherently funky!
A special note about the DOD FX25: This is my favorite filter pedal ever, and it is also famously used by Flea and Bill Laswell. However it is a finicky little troublemaker, and many people try it out and end up thinking it sucks. There are several reasons for this:
--There have been six or seven versions made over the years, and they do not all sound identical. They are very similar, but their circuits are not quite the same, in particular the range of their sensitivity to your input. The very best ones are the oldest two-knob ones in a pale avocado or medium green color, not sparkly/metallic.
--They have a big resonant volume spike, so you need a limiter after the filter.
--It is a bandpass filter, so you may need to adjust your signal path, or use a parallel blender, to retain your low end during part of the filter sweep.
--The bypass circuit (footswitch) is poor quality, so it needs to be modded or put in an external bypass loop.
--It is prone to a specific failure over time where the volume drops by about half. The culprit is the "sensitivity" potentiometer, so just replace it with a new 500K linear pot. As far as I can guess, DOD must have bought a large shipment of bad-quality 500K pots.
--The thing I mentioned earlier about matching your instrument's level to the threshold of the envelope follower, plus the part about having to really "play the filter", is very excruciatingly true with the FX25. If you don't do the work of making sure you interact with it "just right", it will probably not work well for you, and may even sound broken.
If you're thinking that is an awful lot of trouble to go to for one cheap old filter effect, you're right. For me though it's worth it, because no other filter does the job for my tastes better than the FX25, and every other filter has its own set of compromises as well.