SigTech TF 1120 Time Field Acoustic Correction System

Imagine an electronic magic wand you could wave at your listening room's acoustical problems to make them disappear. The Cambridge Signal Technologies SigTech TF 1120 may be just such an alchemical device. Its efficacy depends on the nature of the acoustical warts in your listening environment. Operating solely within the digital domain, the SigTech device can perform both drastic and subtle frequency-response and time-domain corrections that are beyond the scope of analog-based units. While the TF 1120 is expensive, in some situations it may still be the most cost-effective way of coping with acoustical gremlins. Is it the solution to your problems? Perhaps.

Adaptive filters
The SigTech TF 1120 is one complex piece of gear. While I could devote page after page of intricate propeller-head technospeak code to its inner workings, I'm going to try to keep this account short, sweet, and simple. While originally designed for professional studio use, the TF 1120 is easily adaptable to home listening rooms. It operates not only on frequency-domain anomalies created by the room, but also on some speaker-generated nonlinearities. During the measuring process the TF 1120 can differentiate between room problems and speaker problems because it has the ability to detect differences between direct and delayed sound. When the TF 1120 develops its digital filters, it compensates independently for the room and the speakers.

While analog equalization has been around for many years, it has some fundamental problems that prevent it from being completely effective. Primarily, it depends on steady-state signals for calibration and setup. These steady-state signals do not allow for any differentiation between anomalies caused by transducers and those caused by the room itself. In addition, an analog equalizer is what is called a "minimum-phase" device; it introduces phase shift that is mathematically related to the amplitude-response change. Unfortunately, colorations due to room acoustic problems are not minimum-phase phenomena and cannot be corrected by a minimum-phase device. Analog EQ may ameliorate some room problems, but never with critical accuracy, and always with degradation in detail, phase-response, and S/N performance.

While it's been known for some time that digital-based equalization is a far more accurate way to effect room and speaker corrections, in the past the problem has been that the hardware engine required to do the job right was prohibitively expensive. With the advent of modern DSP chips, the necessary digital horsepower has become practical and affordable. As early as 1984, a basic patent for AEC (acoustical environment correction) was issued to AR, then a division of Teledyne, Inc. This original patent—developed by Ron Genereux, SigTech's Vice President, while he was at AR—is derived from principles developed for radar to cancel spurious reflections through the use of adaptive filters. SigTech contracted to purchase this technology, and has the exclusive license to make commercial products and sublicense the technology to others. SigTech has taken the original AR patent and turned it into a real-world product. [For more detail on SigTech's background, see "I Have Heard the Future," by the later Peter W. Mitchell, Stereophile, October 1992, Vol.15 No.10, p.7, and "Industry Update," November 1992, Vol.15 No.11, p.50.—Ed.]

The way a SigTech unit works is actually rather simple: It sends test signals through the audio system and measures these signals at the listening position with a calibrated microphone. It then produces, on a PC, graphic displays that show the impulse response and the FFT-derived frequency response at the listening position. The SigTech's internal software then analyzes the response data to generate a set of corrective digital filters. Test signals are then run through the system again, this time with the SigTech filter corrections. Again, the signals are picked up by the calibration microphone and graphic displays are created. At this point the SigTech technician can make changes to the filters—such as reducing or increasing the amount of correction within a particular frequency range—and either run the new filters again, or accept the filters and load them into the SigTech TF 1120 stand-alone unit.

This ability to change the amount of correction is vital. Let's say you have a speaker that has no appreciable output below 40Hz. Without a way to prevent the TF 1120 from attempting to make the system flat to 25Hz, the corrective filters could easily overtax the power amp and speakers as they try to produce tones that are beyond their scope. Also, the SigTech can be programmed to produce a gentle top-end rolloff. This feature is useful in situations in which a speaker setup performs double duty as both a music and Home Theater system.

The SigTech TF 1120 has provisions for as many as four banks of filters, thus providing for the use of more than one set of speakers. While this feature was developed for studios, which usually have multiple monitors in their control rooms, it has benefits for home systems as well. You can also use the SigTech in more than one room, using different filter banks for each particular setup—or you can program the SigTech for multiple speaker locations in the same room.

Some Stereophile readers might want more detailed technical explanations of the hows and whys of the SigTech process. You can download their technical white paper #3375 (E-3), "Adaptive Filters for Loudspeakers and Rooms," first presented in 1992 at the 93rd convention of the Audio Engineering Society, here. The type is small and the writing is less than sprightly, but there are enough hard facts in there to set even the most avid propeller-head's beanie a-twirl.

Most home users will be satisfied with having a trained SigTech technician install the system. Only people who constantly experiment with their speaker and listening locations or do a great deal of location recording will find it necessary to purchase the complete system (along with a portable PC) and learn how to set up the TF 1120 themselves. During testing I had Denis Doyle, Director of Field Engineering, do the setups in my rooms. He visited me five times over a five-month period. (Civilian users get one free installation with the initial cost of the unit; subsequent visits are at the customer's expense.)

The first visit was to set up the TF 1120 in my large room with the Dunlavy Signature SC-VI loudspeakers. Denis returned a month later to tweak the setup and try a few different top-end rolloff settings. On his third visit he installed the TF 1120 in my small room, which hosts a pair of Avalon Eclipses. Visit #4 was intended to reinstall the TF 1120 in the large Dunlavy room, but the resulting measurements revealed that both Dunlavy tweeters had subsequently been partially toasted—each was down 3dB from Denis's original measurements—by oscillations from a pair of Jeff Rowland Design Group Model 6 amplifiers (see my August review of the Dunlavy SC-VI for details). After I'd replaced the tweeters, Denis visited my listening room for the fifth and final time.

The setup procedure was the same for each visit. Denis would unpack his Toshiba T4700CS laptop computer with Deskstation IV, set up his ACO Pacific 7052 measurement-grade calibrated microphone on a boom stand, hook up the computer's audio I/O board to a SigTech TF 1120 via a 37-pin cable, and the SigTech TF 1120 to my power amps. After making sure the microphone was set up at ear-level at the center listening position, he would then run the first set of impulse tones to ensure that the microphone was dead-center between the two speakers. Calibration is sufficiently critical to "dial-in" the position to within 1/2". After the mike was positioned, test-tone levels were set for each speaker, followed by the tests themselves. The test tones sounded like a series of clicks.

In many setups, the first step in this process—initial room analysis—can take quite a while. Many times the SigTech tests can help locate far more optimal locations for speakers and listening position than one's initial setup. In my room, we found that moving my listening couch forward about 6" substantially reduced a bass notch in the 60–80Hz region. Denis has done some setups in which more than half the day was devoted to moving speakers and furniture around.

After the test signals had been run, the information was put through the computer program to generate the TF 1120's digital filters. The signals were then run again, this time using the filters, and the results compared to the uncorrected plots. As separate filters are needed for the 44.1kHz and 48kHz sampling rates, a complete battery of tests must be run for each.

Because the SigTech unit operates in the digital domain, it's usually placed between digital sources—like CD transports and DAT machines—and a D/A converter. That was how it was set up in my system. I connected it to my CD transport via AT&T optical connectors, while my DAT machine was hooked up via the S/PDIF RCA coaxial connections. While the digital signal can pass through the TF 1120 without attenuation (as I requested for two of the four channels, in order to facilitate A/B bypass tests), most of the time the signal is attenuated by about 10dB so as not to overload anything downstream. Analog sources were connected to the SigTech via a pair of inputs: one each for XLR and RCA jacks. Analog inputs were then routed though the SigTech's own optional internal A/D converter.

Once the SigTech is set up and connected to a system, it's controlled via a hard-wired remote box measuring 5½" by 12¼" by 1½". This unit is big enough that it's nearly impossible to lose unless you have a monumentally messy listening room. The remote controls input selection, analog input level, and digital filter selection (you can have a maximum of four filters). You can also push a bypass button to hear what things sound like sans processing.

Via a switch on the remote, the TF 1120 can also be set up to shift the "sweet spot" to the left or right of center position. Let's say your cat or dog has just left a moist calling-card right smack in the middle of your listening couch, but you still want to listen to your system right now (that's dedication): Just push this button, sit in a dry spot, and Voil;ga!—sounds just like you're still in the cat-bird seat.

Cambridge Signal Technologies, Inc.
(company no longer trading, 2021)