Since there are four effects "units" in the DP/4, any combination of four of the above algorithms can be used simultaneously. (Doesn't that come out to over 1,600 different combinations?) Furthermore, you'll notice that many of the algorithms contain two and three effects. So, it is possible to get quite a few effects going at once, and the four 24-bit DSP chips in the unit provide ample processing power to cleanly handle the task.
The other three algorithms require a little more processing power than one effect unit can provide. Two of them are "2 Unit" algorithms. They are "Pitch Shift 2U" and "3.3 Second DDL 2U." By the way, DDL refers to digital delay, and the algorithm above does indeed provide 3.3 seconds of delay. In fact, the DP/4 can be configured to provide a maximum delay time of 6.4 seconds using all four effects units.
The final algorithm requires four processing units. This 4-Unit algorithm is the Vocoder, and yes, it works and sounds like the real thing. Hooking up the vocoder is simple. Input a mike into input 1 on the rear (or front) panel and a synthesizer into input 2, then select the vocoder algorithm. The unit analyzes the signal from the mike and uses this analysis to control the sounds from the synthesizer. There are several parameters available to fine tune the characteristics of the vocoder, and it worked well for us, not only with a synthesizer plugged into channel 2, but with an electric guitar as well. In fact, we used a CD of sweeper effects in place of the synth and guitar audio and got some very interesting vocal effects.
The 2-Unit algorithms mentioned above are stored in "2-Unit Presets." The 2-Unit Presets of the DP/4 account for another 100 of the 400 presets, most of which are a combination of two of the 1-Unit algorithms. There are also 100 "4-Unit Presets" which consist of four separate "1-Unit" algorithms placed in series and/or parallel with each other.
So...you have all these algorithms and presets to choose from, and four effects boxes you can plug them into. And, you can connect these four effects boxes together to process a single input! You can use just two of them, or you can use just three. You can use the DP/4 as four separate mono effects processors, or you can use it as two completely independent stereo processors. You can configure the unit as two mono processors and one stereo processor to process three separate source signals. These various configurations are achieved with an internal "patch bay." This is accessed with the Config button which selects Config Presets. These Config(uration) Presets account for the last 100 of the 400 presets in the DP/4 and contain the previously mentioned 88 presets that are "hidden" by the factory. (The Vocoder algorithm uses all four effects units and needs a specific input/output configuration, so it is also stored in a Config Preset.) It is these Config Presets that determine how the signals are routed -- how many input channels are used and how the effects units are connected to each other. If you plug three separate signals into three inputs, a specific Config Preset must be used to accommodate those three sources. Likewise, if you want to use all four effects units to process just one input, you have to tell the machine that is how you want things done.