The FATSO (Full Analog Tape Simulator and Optimizer) is a hardware device designed by Dave Derr’s Empirical Labs, makers of the highly popular Distressor compressor. Its function is to musically integrate frequencies and transients and increase the apparent volume of your source material in the same way classic analog equipment does. It achieves this through an ingenious design and a creative feature set, giving users the ability to impart the wonderfully warm, pleasing sonic characteristics of magnetic recording tape, Class A transformers, and tube circuits. It’s already well known that Universal Audio has partnered with Empirical Labs to re-create Dave’s FATSO design as a UAD Powered Plug-In. In preparation for this imminent event, below are some condensed, slightly modified highlights from the original FATSO manual.

EL7 FATSO Jr.
The Empirical Labs FATSO Jr.

Overview of FATSO's Four Processing Types

The FATSO was designed to integrate frequencies in a musical manner and provide foolproof, vintage-sounding compression. Generally, it is difficult to make the unit sound unnatural due to its vintage topology. So let's go over the four types of processing the FATSO provides and describe what they do.

Harmonic Generation and Soft Clipper – This is basically a distortion generator associated with the Input knobs. Any time you pass a signal through the FATSO, it passes through this part (except when in bypass). This processing is useful to softly but instantly clip peaks and transients, allowing more average level. Aggressive distortion can be achieved through the same controls.

High Frequency Saturation – Warmth! This circuit simulates the softening of the high end that occurs with analog tape. As the warmth is increased, overly bright signals and transients are quickly attenuated. The time constants are nearly instant, so the high frequencies return very quickly after a loud burst.

Transformer and Tape-Head Emulation – The Tranny circuit simulates the effect of the input and output transformers of older devices and adds the low frequency harmonics that characterize analog tape. This is extremely useful on pure, low-frequency-type tones that don't cut through small speakers. It adds upper "warm" harmonics to frequencies below 150 Hz, especially those even lower, such as 40 Hz (e.g., the low string on a bass guitar), helping them cut through on smaller speakers.

Classic Knee Compression – These are the typical automatic leveling devices used on just about every instrument and vocal track, including the overall buss. Only it’s Empirical Labs compression — smooth and sweet, but in your face!

"The FATSO is a very good answer to what a lot of people loathe about digital recording. It smoothes out the sharp, brittle edges to exactly the extent you choose, and fills in the hairline cracks just right. I use one on almost every mix I do." — George Massenburg

Now let’s delve into the processor details. …

Saturation and Distortion Generator

It’s well known that the triode distortion in tube circuits produces lots of 2nd and 3rd harmonics, in somewhat varying ratios. Analog tape also saturates in this manner. The 3rd harmonic is induced in the FATSO by increasing level through two discrete distortion circuits. It’s usually the result of flattening the tops and bottoms of waveforms. The 2nd harmonic is also added, especially while compressing, in the FATSO. The FATSO’s input clipping will give you the same result. These lower order harmonics form the octave and the octave and a fifth to the fundamental musical tones. They are literally "musical" distortion. Harmonics above the 2nd and 3rd get increasingly harsh and unmusical, and therefore, should be lower in amplitude (<-60 dB) to keep with our line of thinking. The 2nd harmonic is considered the warmest and most "consonant" harmonic distortion.

Distortion indicator lights are provided to indicate some reference operating levels. A 0 VU yellow LED light indicates around 1% THD and the red Pinned LED indicates 5% THD or more. These LEDs are an excellent guide to where the user is in the grunge department. You will find that the harmonic distortion is generally more obvious on overall mixes and complex programs. On individual instruments, sometimes 10% distortion sounds fat, and "analog" and isn't heard as distortion at all.

Warmth Processor

The Warmth circuit is by far the most complex part of the FATSO. It’s a very strange high frequency (HF) gain control circuit, or high frequency limiter. It operates very fast and should be unobtrusive, because it gets in and out of the way quickly. The desired result is akin to the HF saturation that analog tape exhibits when the high frequency amplitude interacts with the tape recorder bias to produce “self erasure” of certain frequencies. We provided a very accurate display of the HF attenuation, with the upper FATSO bar graph showing the gain reduction at 20 kHz. The nature of the filter allows the corner frequency to move as attenuation occurs.

Just one control for Warmth is provided, but there are other ways to control the overall action of this circuit. If you decide to use the compressor, set it up first because it affects the operation of the warmth. There is heavy interaction between the compressor and warmth settings. The Warmth control has eight steps, ranging from no warmth action (no LEDs lit) on up through the highest setting of 7. (Ed: we have found Warmth still affects the overall circuit at extreme settings, even when unlit!) Perhaps the best way to think of the settings is as compressor threshold, with 7 having the lowest threshold, the most warmth, and responding quickly and often to high frequency content. Remember, instead of controlling the overall level, this warmth "compressor" threshold only affects the high frequencies.

The Tranny Processor

“Tranny” is short for transformer. Transformer design and use are art forms, and there will always be tradeoffs. However, it is widely known that a good audio transformer circuit can do wonderful things to an audio signal. This was the goal of our Tranny circuit. We emulated the desirable characteristics of the good old input/output transformers in a consistent, musical way, and in a selectable fashion. Harmonics and peak saturation are added, along with frequency and phase changes on the low frequencies. We found that we could capture the low frequency effects of large, increasingly expensive older output transformers in a weird, internally buffered, switchable design.

To sum up the musical results of our Tranny circuit, there will be a little more edge in the midrange, and the super-low frequencies will be harmonically altered in a way that allows them to sound louder, even though the peaks are less than the original. Playback on small speakers will show an improved audibility of low end as a result of the psychoacoustically-pleasing distortion the Tranny adds.

The Compressor and its Ratios

Now, let’s dig into the compressor section. Each FATSO compressor type simultaneously sets the threshold, the ratio (in the standard sense of the word), the attack, and the decay. This was done to provide an easy-to-set, yet versatile, group of curves. There are essentially four discrete compressors, with the fourth having the ability to be combined with the other three.

Buss – Very gentle 2:1 type ratio with slow attack and fast release. 1-4 dB of compression is common for this compressor type. Very soft knee. 5 or more dB of buss compression is hitting it hard!

G.P. (General Purpose) – A medium attack, slow release type that sounds pretty invisible while able to maintain a consistent RMS level. The slow release will not pull things into your face unnaturally. Both the green and yellow LEDs will be lit.

Tracking Compressor – When both top LEDs are lit, an 1176-type compressor is enabled that is great for tracking instruments and vocals during the recording process, or during mixdown.

Spank – This is a radical limiter-type compressor that was specifically designed to emulate the nice squeeze of the older SSL talkback compressors from the '70s and '80s, but has much higher fidelity, as you will notice. By combining Spank with any of the other three types of compressors, you'll really have seven compressor types (or ratios), although the Spank's aggressive nature will tend to dominate when combined. The release curve of all types is logarithmic, meaning it lets off quickly at first and then slows. This release curve is a big part of the FATSO's compressor sound.

Using the FATSO

Buss use was really the original intent of the FATSO (of course a myriad of other applications exist, too). It is a 2-channel device that can make a stereo digital signal sound analog, and integrate the different frequencies in a musical way. It can be used as a stereo device through which you can mix or transfer to or from a digital medium to make it sound very analog-tape-like.

Mixdown Pre-Processing – By placing the FATSO immediately before a digital mixdown, the recording engineer should be able to get the sweet high end and low frequency fatness that he or she could mixing to an analog tape recorder. Compression should usually be the Buss type. Use Warmth to soften the high end and the Tranny to add some edge and warm harmonics to the low frequencies.

CD Players and Other Digital Recorders – Fatten up pre-recorded audio! Our first goal is to make that CD player have the warmth and softness of the old tape recorders and records. Take the output of the CD player into the FATSO input. If you're going to use compression, set it up first, because it interacts with the other settings. But for this example, let's not use any compression here. First adjust unity gain through the FATSO. Set the input until the 0 VU LED glows on peaks from time to time. This will set up the saturation to emulate some nominal tape-like distortion. Next, if you want the low end to have more bite, the Tranny may be useful, but for most digital recordings, try messing with the Warmth control. Allow 1-5 dB of warmth to occur on peaks. When you A/B it to the original signal, you'll notice a new softness and fatness immediately.

Vocals – There are so many ways to use FATSO on vocals. During tracking, turn on the Tracking Compressor, which illuminates the green and yellow LED in the compressor area. This is a pretty safe 1176-type knee compressor with a pretty quick attack and release. That might be all you need! Using up to 10 dB of compression should be fine with really dynamic vocals. Alternatively, you can try one of the other compressor types, even Spank, to grab occasional peaks and then quickly get out of the way.

Bass – Bass is notoriously hard to round out and clarify sometimes, but the FATSO has many ways to help. First, if a low bass tone is too pure and sine-wave-like, it will "fall off" on small speakers. By this we mean it will not be audible because most of the bass frequencies are below the range of the speakers. The Tranny combined with the saturator circuits is the solution. These will generate upper harmonics that will be musically related. These harmonics will be reproduced on small speakers and the human ear (and mind) will psychoacoustically fill in the lower fundamental! Works great on direct bass.

Electric Guitar – A wide range of settings can be used. To get rid of edgy attacks, use the quick attack and fast release of the tracking compressor. If the guitar is too dynamic, the compressor combined with the saturator and Tranny circuits may be the answer. Sometimes the guitar just needs a touch of fatness without losing attack. Try the buss compressor to maintain transients. Watch that you don’t over-warm guitars with the saturators and the warmth circuits. Crunchy guitars, which are full of harmonics, are notoriously sensitive to tonal changes. Oversaturating or warming can take away the clarity or bite needed to cut through. Other times, it's just what's needed to get the guitar louder in a mix but stay out of the way of the vocals by flattening the upper, spiky harmonics a bit. When you are dealing with an occasionally peaky guitar, where certain chords or notes take your head off, the dynamic action of the Warmth control can be sooooo useful.

Piano/Keys – Acoustic pianos often need less attack and more sustain to fit into a mix, but there are countless exceptions. As far as the compression part, for Bruce Hornsby-ish pianos with medium attack and medium release that achieve a "bite" followed by sustained body: If you find the buss-type compressor a bit gentle, too hot, and distorting some of the peaks, try the Tracking compressor with a fast release. The G.P. type compressor may work, depending on how even the piano part is. If you have a dense mix and really need to make it take up a finite spot in the mix, go for the Spank. This will slap the dynamic range down smoothly and quickly. It won't be an unfamiliar sound. Old Beatles records started using drastic amounts of compression to keep a piano sitting inside a busy arrangement. The saturator, or soft clip part of the FATSO, will also be useful on piano and keys.

Drums – Without any processing activated, the saturator, which is always inline, will pack those peaks down smoothly, giving you 3-6dB more average level. Distortion indicator LEDs, the 0 VU and the Pinned LED give you a good idea of what's going on. On percussion, peaks can light up the Pinned LED without any nasty distortion, if they are short enough. Analog tape cannot handle all the top end and will round out the sound, as will the FATSO's Warmth processor and saturator. Don't use more than 5-10 dB of Warmth on drums though, or you are probably asking for a dull sound! Try putting digitally recorded tambourines or other percussion through the FATSO and listen to the difference! That clacky front edge will become warm and easy to listen to, like the old analog tape and vinyl.

Distortion – The FATSO has very smooth clipping up until about 20%. Folks have already used the FATSO as a distortion box with great effectiveness and musicality. The Warmth provides another way to roll off some harsh distortion grit as well, fattening up the lower harmonics. The Tranny will drastically distort sub-frequencies below the 50Hz range, but adding the 2nd and 3rd harmonics sounds very nice on those low tones. You may even find that plugging a guitar and bass directly in can work.

Effects Warmer – Try using the FATSO as a tape delay simulator by putting it after a digital delay. Turn the compression on Buss compression and the Warmth on a little. Use the Tranny to roll off subs and pump harmonics into the low frequencies.