The Compressor/EQ Question: Who’s On First?
by Craig Anderton
Compression and EQ are two of the most common audio processors, which is probably why they're included in Vegas, Sound Forge, and ACID Pro software. However, there's some debate about how to insert these effects when you're using both: Does EQ go before compression, or vice-versa? As with many aspects of audio, the answer is an unequivocal "it depends."
Compression Before EQ
This routing tames the signal's dynamics first, then EQ alters the overall tonal "color." However, if the compressor is there to tame peaks and thus avoid overloads, and the EQ is boosting certain frequency ranges, then the level increase in those ranges might be high enough to overload the output anyway. (If the EQ is serving to reduce response in some frequencies, then the compressor's output control may need to be increased to make up for any signal loss caused by going through the EQ.)
When EQ is used mostly to boost certain frequencies, a common solution is to use compression both before and after the EQ. The pre-EQ compressor does most of the work; the post-EQ compressor typically has a high threshold and ratio, so that it acts more like a limiter. This allows the second compressor to "trap" any excessive transients or potential overloads, but mostly leaves the signal alone.
Fig. 1 shows a typical patch to accomplish this, based on the plug-in chainer in Sound Forge 8 (note that the chainer displays only the selected effect, so the other windows have been added using a paint program). The ExpressFX Dynamics is doing general compression, with a threshold of -4 and a ratio of 2.3:1. However, look at the Paragraphic EQ: There's a significant boost at 1,818Hz to make a sax solo prominent, but every now and then, the sax's harmonics fall right into that range and add a major peak (the treble boost increases this somewhat as well).
Now consider what the Wave Hammer plug-in is doing. It has a high threshold so that only the highest peaks are affected, and when they are, they're flattened due to the infinitely high compression ratio. Even though some of the peaks caused by the Paragraphic EQ have exceeded the Wave Hammer's headroom (note the red peaks above its meters), the Wave Hammer has clamped these peaks so they don't affect the final output (note the lack of red above the output playback meters in the lower right).
EQ Before Compression
The problem—and advantage—of this option is that the compression "undoes" some of the effects of the EQ. For example, if the EQ is set to boost a range of frequencies, compression will tend to bring that boost back down again. Conversely, if the EQ is being used to cut, compression will bring that range up a bit.
Where EQ before compression really works well is for situations where the EQ might add a significant, unpredictable resonant peak, as might happen with certain instruments if a note falls within the range of the boost. Although the EQ will change the signal's tonal character, that one note will really stick out—but the compression will cut it down to size. This can also really help tame resonant peaks with synthesizer filters.
The bottom line: Analyze what you're trying to accomplish based on the above, and set up your effects appropriately—or just try both and see which sounds better!
Craig Anderton is editor in chief of Harmony-Central.com, and executive editor of EQ magazine. He has lectured on technology and the arts in 10 countries, 37 states, and in three different languages.