SPL Transient Designer 4

The SPL Transient Designer 4 is a unique rackmount dynamics processor that can significantly improve audio signals by changing their attack and sustain characteristics.

FIG.1: The Transient Designer 4 uses VCAs controlled by envelope generatorsto process dynamics independent of levels.FIG.2: With four XLR inputs, four XLR outputs, a ground-lift switch, and anIEC connector, the TD4's rear panel reflects its overallsimplicity.FEATURES4.0EASE OFUSE4.5AUDIOQUALITY5.0VALUE5.0RATING PRODUCTS FROM 1 TO5PROS: Awesomesounds that no other processor can duplicate. Ridiculously easy tolearn and operate. Useful on a variety of sources. Saves time in thestudio. High-quality signal path. Outstanding value.CONS: No I/O levelcontrols. Rudimentary metering. No unbalanced I/O connectors. Difficultto disconnect Switchcraft XLRs. No sensitivity switch.Manufacturer
tel. (805) 241-5140
e-mail info@spl-usa.com
Web www.spl-usa.comTransient Designer 4SpecificationsInputs(4)balanced, cross-coupled XLR (2 per channel)Outputs(4) balanced,cross-coupled XLR (2 per channel)Nominal InputLevel+6dBuMaximum InputLevel+24dBuMaximumOutput Level+22.4dBuMinimumLoad600žFrequencyResponse20 Hz-100 kHz(-3 dB down at 100 kHz)CCMR (CommonMode Rejection)-87 dBu @ 100Hz
-80 dBu @ 1 kHz
-75 dBu @ 10 kHz
-70 dBu @ 20 kHzTHD +Noise0.004% @ 1kHzS/N CCIR468-3-89dBuS/NA-weighted-105dBuDimensions1U × 9.33"(D)Weight7.5lb.

The Transient Designer 4 is an analog dynamics processor like noother (except its 2-channel sibling, the Transient Designer 2). Unlikeconventional dynamics processors that treat only signals that exceed ordrop below a specified threshold, the Transient Designer 4 can shapesounds independent of their levels. Imagine increasing the attacktransients of quiet instrumental passages, or abbreviating loud soundsthat would normally bully a gate to stay open. Such tasks are child'splay for the Transient Designer 4. While that might not seemearth-shattering at first blush, trust me, it is. The TransientDesigner 4 (or the TD4, as I'll call it from here on) is one of themost revolutionary products I have ever worked with, and the sounds itcreates are nothing short of astounding.


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The TD4's spartan front-panel layout points to the 1U rackmountunit's simple and intuitive operation but belies the power of thisamazing processor. Four independent channels each feature single Attackand Sustain knobs, an On button, and a solitary signal-present LED (seeFig. 1).

To increase the level of an input signal's attack (transient)portion, simply turn the TD4's Attack knob clockwise from the noonposition. To soften the attack, turn the Attack knob counterclockwisefrom the noon position. Similarly, turning the TD4's Sustain knobincreases or decreases the sustained portion of the sound with,respectively, a clockwise or counterclockwise adjustment from its noonposition.

When a channel's On button is depressed, a red status LED inset inthe button lights to indicate that the channel's dynamics processing isactive. When the On button is switched out, a hard-bypass relay circuitis engaged. (The bypass circuit also automatically engages when theunit loses power for any reason, preventing embarrassing silences inlive-performance applications.) A channel's signal-present LED lightswhen its input signal's level exceeds -40 dBu. Unfortunately, neitherinput- and output-level controls nor clip indicators are provided.Those omissions would be more grievous if the unit were a digitaldevice, which it isn't.

In addition to the aforementioned controls, a Link switch (also withan inset red status LED) is provided for each pair of channels. Whenthe Link switch for channels 1 and 2, for example, is depressed,channel 1's controls (including active or bypassed status) governsettings for both channels. Depending on the status of its two linkswitches, the TD4 can process four mono, two stereo, or one stereo andtwo mono channels at the same time. The TD4 also provides arocker-style power switch (with an inset lamp) on the unit's frontpanel.


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The TD4's rear panel provides a locking, balanced XLR connector foreach channel's input and output (eight I/O connectors in total; seeFig. 2). Disconnecting my I/O cables (fitted with SwitchcraftXLRs) from the review unit was difficult because the TD4's lockingmechanisms would not readily release them. The TD4's nominal inputlevels are +6 dBu (a standard widely used in Europe, where SPL'sheadquarters are located). Pin 2 is hot for all I/O. If you plan to usethe TD4 with unbalanced signals, you must shunt pin 1 to pin 3. Thebalanced, transformerless I/O stages include high-quality,laser-trimmed resistors with a tolerance of 0.01 percent, resulting inexcellent common-mode (hum and noise) rejection.


Transient Designer 4
dynamics processor

I wish that the TD4 provided XLR/TRS combo jacks (which exist onlyfor female connectors, anyway) and unbalanced ¼-inch jacks, butthat undoubtedly would have increased its size and price considerably.SPL says that the unit handles a wide range of input levels, and itperformed almost equally well when I fed it -10 dBV unbalanced signalsand +4 dBu balanced audio. (You'll want to use balanced lines, however,for the quietest operation.) You can connect the TD4 to your mixer'sinserts or in series with line-level (preferably +4 dBu nominal)outboard gear.


As mentioned earlier, the TD4's dynamics processing worksindependent of signal levels. Consequently, you shouldn't expect tofind any threshold or ratio functions, because there are none. The unituses SPL's Differential Envelope Technology to process both low- andhigh-level signals without distinction.

The process produces two envelopes for each Attack and Sustaincontrol (hence, four envelopes per channel). The first envelope, whichis generated by an envelope follower, tracks the shape of the inputsignal's curve. The second envelope is a fixed curve that responds moreslowly than the first envelope. The TD4 derives a difference signalfrom the two envelopes (subtracting the second envelope's amplitudevalues from those of the first) and uses it to vary the control voltageof a single THAT 2181-VCA (one VCA per channel). The VCA rides thechannel's gain up or down during both the attack and sustain portionsof the audio signal to the degree that you boost or attenuate theirrespective controls.

The TD4's attack and sustain circuits work in parallel, giving youindependent control over the attack and sustain characteristics of eachchannel's processed audio. You can boost or attenuate a sound's attackportion as much as 15 dB, whereas you can adjust the sustain componentover a range of 24 dB up or down. That's enough rope to hang yourself,so be sure to watch for overloaded mixer-insert returns or downstreamgear and adjust your gain staging as needed. Drastic settings cansometimes cause unflattering pumping. Nonetheless, I'd much rather usea box that lets me get wild and crazy than one that constrains me tosome engineering department's idea of safe sex.

The Transient Designer won't take the place of a compressor orlimiter, as it cannot, for example, selectively reign in peaks;lowering the TD4's attack control lowers all peaks regardless oftheir level. Nor will the Transient Designer take the place of a gatefor all applications, as it will not selectively weed out low-levelsignals below a certain threshold (as noted before, the TD4 has noThreshold control). Think of the TD4 as a 2-stage (attack and sustain)envelope generator for audio signals, with one caveat: because musicalphrases have a finite duration, the Transient Designer can'telongate the sustain portion of a signal; it can only boost thelevel of the signal while it is still present. You can shorten theduration of audio signals, however, by dramatically lowering the TD4'ssustain control.


The TD4 is so easy to use, you'd have to be in a coma to get lost.Simply turn the Attack and Release knobs until you like what you hear— that's it! Throughout the course of several recording andmixdown sessions, the TD4 delivered unique and remarkable sounds thatwere impossible to achieve by any other means, and it did it in afraction of the time I would have spent fruitlessly tweaking gates andcompressors.

The TD4 consistently worked better than my high-end gates inreshaping drum sounds, with results that were more musical andpredictable. Using the TD4 to shorten the decays of individual trapdrums, I never heard any threshold-related chatter. I was also able toeffectively mute downbeats to create driving rhythmic effects byshaping the envelopes of drum sounds.

What's more, I used less EQ on mixes in which I processed drumtracks with the TD4, as there was far less ringing of drums clutteringup the bass and low-mid spectrums. The beauty of the TD4 is that itcorrects temporal problems in a way that often reduces and sometimesprecludes the need to use static EQ and multiband compressors onindividual tracks. That said, SPL does not recommend using theTransient Designer on entire mixes for mastering applications.

Feel free to use the TD4 in live performance, too. In DigitalPerformer, I compared the waveforms of processed tracks to theiroriginals and found no latency beyond that caused by a round-tripthrough my MOTU PCI-324 card and converters.


I love this incredible box. The TD4 has saved my butt on more thanone occasion. For example, a client brought me drum tracks he hadrecorded in his living-room studio, and the completely undamped kickdrum rang like a timpani. With the TD4's Sustain control turned downmost of the way and the Attack control up slightly, the kick's sustainand boominess were greatly decreased and the track grooved somuch better. That benefited the entire mix, creating more punch andclarity.

On another project, increasing the TD4's Attack and lowering itsSustain control on a snare-drum track yielded two simultaneousbenefits: it lent a gunshotlike crack to the snare drum, and itlessened hi-hat bleed. I could further whittle the sound down to ashort, sharp attack by cranking the TD4's Attack control to the max andturning the Sustain knob to its minimum setting; combining thatexplosive burst with the original snare-drum sound created a slammin'track that rocked harder than dynamite. Conversely, I got a trashy rocksound by increasing the snare-drum track's sustain. The resultsounded like I had a room mic set up over the drums —awesome!

On tom tracks, increasing the TD4's attack nicely enhanced the stickhits. Decreasing the sustain tightened up ringing shells without havingto muffle the toms' heads with toilet paper and duct tape (which Ivirtually never do, because it sucks the life out of a kit).

In the course of processing a drum kit with the TD4, I becameconvinced that the 4-channel version has a practical advantage over theTransient Designer 2. For example, adding attack to the kick and snaresounds resulted in a slight enhancement of the toms' attack, becausethe Transient Designer also processed the toms' bleed (into the kickand snare mics). Having four channels of processing at my immediatedisposal allowed me to counteract the effect by slightly backing offthe attack on the tom tracks using the TD4's two remaining channels.Adjusting the degree of processing for four drum tracks at the sametime lent greater balance to the sound of the overall kit, and it savedme from having to guess how the drums would otherwise sound with twopasses of 2-channel processing.

Another consideration when comparing the TD4 and the TD2 is that thelatter offers only unbalanced I/O that operates at 0 dBu nominallevels. In its favor, the 2-channel version costs approximately half asmuch as the TD4 and represents a highly cost-effective entry point foraccess to this groundbreaking technology.


Turning to a mixdown session, the TD4 also rescued an electric bassguitar track playing on a samba tune. The bassist's performance was abit too relaxed (legato). I turned the TD4's Sustain knob down to thepoint where bass notes sustained no longer than the duration of aquarter note, which was a tad less than on the untreated track. Now thebassist's performance had clear breaks on downbeats, producing a morestaccato performance that really propelled the rhythm section forward.I was dumbfounded by the realization that the TD4 had changed not justthe bass track's sound but also the player's performance. Without theTD4, I would have had to track the entire performance over again to getthe syncopated feel and tight groove that SPL's box produced with thetwist of one friggin' knob! On another song, decreasing the TD4'sattack by 7 dB gave the bass a wonderfully pillowy sound.

Next up was an electric guitar solo, playing blazing eighth-notetriplets for bars on end. It was a very challenging part, and theguitarist understandably sounded just a tad sticky-fingered (staccato).No problem — simply increasing the TD4's sustain roughly 3 to 6dB made the solo sound smooth and fluid, and it took less than 30seconds to dial in the sound.

Finally, I tried the TD4 on a stereo pair of acoustic guitar tracks,in which the guitar played arpeggios. The track was so well performedand recorded that the TD4 could not improve it, yet I was happy to notethat the unit's Link function successfully kept the stereo imagesteady.


The Transient Designer 4 is one of the most exciting products I'vehad the pleasure to review over the past 15 years. It is clean andquiet, incredibly fast to set up, and intuitive to use. Its uniqueprocessing produces sounds that no other signal processor can currentlycreate. At just over $300 per channel, this box is an outrightsteal.

I only wish that SPL would offer a software plug-in version of theTD4 in multiple formats, but the company informed me that — asidefrom a plug-in already available for use with Creamware's Scopeplatform — they do not intend to do so.

Try it for just a few minutes, and you, too, will be hooked. TheTransient Designer 4 will rock your world.

EMcontributing editorMichael Cooperis theowner of Michael Cooper Recording, located in beautiful Sisters,Oregon.