Among its many tools, the E-MU Emulator 4 features a compressor, expander and limiter for processing audio samples. The manual explains their usage, although not in the most straightforward way. It also includes some rather exotic settings for the threshold in three modes: Above, Center, and Below. The best way to understand how they work is to see them in practice. The explanation in the manual describes their behavior as follows:

• Above: Only signal levels above the threshold % will be affected by the compressor.
• Center: Signal levels above and below the threshold % will be affected by the compressor.
• Below: Only signal levels below the threshold % will be affected by the compressor.
• The % determines the threshold level as a percentage of 100% of 16-bits.

My understanding is that only in the position labeled Above does the compressor behave in the way we’re accustomed to. In the Below configuration, it acts as an inverse(?) compressor, which can be a bit hard to grasp, while in the Center configuration, it seems to function as a combination of the two. I know it’s confusing, hence let’s move to the graphics instead.

The explanation from the User manual might satisfy some, but it still leaves a few questions unanswered, especially when we factor the Expander into the equation. Instead of trying to explain this in detail, the best approach is to display the results graphically. Each row in the table below shows the settings that were used and the resulting output. Although the source signal is the same, it’s displayed alongside the processed result in every row for clarity, which is why it repeats. In the first series of tests, white noise was used at 0 dB FS, and in the second series, a sine wave was used at -1 dB FS. In all tests, a 10 second long waveform was used.

The Compressor
Let’s explain what’s shown in the image below. There are three columns. On the left, we have the Emulator 4 screen display with the compressor (or expander) settings. In the middle (shown in green), we see the source waveform, which starts at -96 dB, fades in all the way to 0 dB, and then fades out back to -96 dB during the period of 10 seconds. It’s literally a heavy zoomed out waveform display of a simple fade-in and fade-out waveform designed to reveal the envelope of the compressor (and expander). In the right column (shown in green), we have the processed waveform, again zoomed out heavily. Layer below it, in gray, is the outline of the input waveform, making it easier to see how the compressor’s envelope affects the signal. Since we have three threshold types (Center, Above, Below) we have three rows.

Threshold type: Above
Let’s first look at the middle row, which represents the threshold type setting: Above. This setting is how your average (normal) dynamic compressor works. Let’s now focus on the settings that were used. With the threshold set to 50%, it seems to activate at -6 dB, which I guess is correct. Still, it’s a pity they didn’t use a dB scale, as working with percentages is very tricky. It essentially involves trying to manipulate a logarithmic scale using a linear tool. Unfortunately, it is what it is, and we can’t change that.

The ratio was set to 5:1, and we can see the gain reduction falling somewhere around -5 dB. This seems reasonable, as for every 5 dB input, we should get a 1 dB increase. Using this calculation, it’s approximately 6:5, or around 1.2 above 6 dB. Apologies for the image being small (there were simply too many tests / images involved to place at the same page). Looking at the vertical axis on the right side of the graph: the top tick is 0 dB, the second tick is -3 dB, then -6 dB, -12 dB, and so on. So far, everything seems correct.

However, examining the envelope shape, it seems that something is “off”. Either there’s an error in design (which I doubt, as E-MU was kingpin back then), or the compressor was modeled after a very specific type of design. Unfortunately, the manual doesn’t provide enough detail to clarify. We observe a nonsymmetric response during the descending (fade-out) stage, which shouldn’t happen. The signal should be compressed symmetrically in both directions, but instead, on the descending slope, it rapidly loses volume, then pauses in amplitude briefly before almost morphing into expansion.

This behavior is indeed strange, and I’m trying to understand what this design might be modeled after. It’s certainly not a classic opto compressor—it must be something else entirely. Keep in mind that this test uses a 10-second long sweep. Just imagine such a drastic change of volume on short sounds like kicks and snares. We also need to note how the compression seems to continue even after the waveform has reached its peak and passed below the threshold. In fact, compression occurs throughout most of the sample, from the middle to the end, at what appears to be a 5:1 ratio. This is easily noticeable, even on this small image—just observe the angle of the descending slope of the processed signal and compare it to the original, which is shown grayed out in the background. All in all, it’s a rather intriguing and unconventional design!

Threshold type: Center
Let’s now look at the top row, which represents the threshold type setting: Center. This appears to be a compressor that works with the average signal value, seemingly combining the behavior of the second and third rows, which represent the Above and Below threshold settings, respectively. In this configuration, it seems to automatically gain the volume below the threshold while compressing the volume above it. This results in a rather interesting type of compression.

Threshold type: Below
Next, let’s examine the bottom row, which represents the “Below” threshold setting. As expected, it behaves as described—it leaves the area above the threshold untouched while compressing and auto-gaining the area below the threshold. This is quite an exotic and unique compressor design.

Such functionality could be particularly useful, as it maintains the integrity of transients while adding gain to the quieter portions below the threshold. Honestly I’ve never encountered a compressor like this before—it must be incredible for drum and percussive sounds!

Gentle compression
In the image below, we have another example of compression—this time with a mild 2:1 ratio set at 50%. The numbers appear to be correct, although the output should not exceed -3 dB. Between -6 dB and 0 dB, there should only be a gain increase of 3 dB, resulting in an endpoint exactly at -3 dB, rather than the observed -2.8 dB (hi res picture shows peak at around -2.8dB). However, let’s not forget, this slight discrepancy could be a feature of the particular (analog modelling) design being emulated here.

Conclusion
It seems that E-MU has provided us with three distinct compressors hidden under the hood, each offering completely different results and envelope shapes. Some are excellent for “rounding” the waveforms, while others are entirely exotic types of compression. In the next episode, we will explore the similarly intriguing behavior of the expander and limiter, providing tests with normalized data, among other things, and determine which compression technique produces the loudest sound—a trend that seems to be popular these days (though not among the author).