DCM Time Frame TF1000 loudspeaker
If a loudspeaker produces an output pulse that is an exact replica of an input pulse, then by definition it must have a perfectly flat frequency response. In practice, as to do so would necessitate the speaker being able to reproduce the DC component of the pulsesomething that only a fan can do, DC implying a constant-velocity stream of airsome modification of the pulse shape is inevitable, equalizing the areas above and below the time axis. But in general with a minimum-phase system, the less the pulse shape is changed, the flatter the frequency response.
Both that early Time Window, according to the original review in TAC, and the Time Frame TF1000 ($1099/pair), according to DCM's literature, are capable of reproducing a pulse with almost no distortion of its shape, implying that the speaker effectively has a flat frequency response. However, the Time Window certainly didn't sound flat when I auditioned it in the late 1970s, suggesting that the impulse test doesn't tell the whole story. In fact, due to the fact that a multiway loudspeaker has more than one source emitting the same sound, the shape of an impulse from that speaker will change according to the height of the measuring microphone, due to the different path lengths from each source to the mike. Therefore, while it may be possible, with difficulty, to obtain a reasonably accurate pulse shape at one point in space with a multiway speaker, that doesn't indicate that the response at every listening position will be flat. Nor does it take into account the response of the speaker off the intended listening axis, which influences the spectrum of the room sound and thus the overall subjective impression; or the effect of resonances and reflections of the treble sound from grille-frames, etc., which will be nonminimum-phase.
In any case, the question of whether absolute time integrity of the signal is subjectively a good thing is moot: no-one has yet shown that loudspeakers with time-coherent behavior always sound better; in fact, there are speakers with quite time-smeared behavior which perform well in the main area where time-coherent designs are supposed to be preeminentthat of stereo imaging. Perhaps only when all the other common problems are fixed will time coherence serve to distinguish the subjective sheep from the objectively identical goats.
DCM's Time Frame TF1000 is a tall, floor-standing loudspeaker that bears a superficial resemblance to a planar speaker. In fact, it is a relatively conventional box loudspeaker, using a shallow but wide six-sided cabinet with a depth that tapers from 7.5" deep at the base to 5" at the top. This asymmetry should minimize internal standing-wave problems by staggering their preferred frequencies. In common with many speakers of US origin, the grille consists of a wraparound knit fabric covering the whole frame, held clear of the baffle by narrow ½"-deep trim strips at the sides. The entire cabinet seems also to be covered in a thin foam covering, presumably to widen the high-frequency dispersion by minimizing diffraction effects. An oak top-plate adds the finishing touch to the aesthetics, while the wooden plinth has hinged feet that swing out to the rear to help stabilize the TF1000 on carpeted floors. The speaker is, however, still a little top-heavy.
The TF1000 is a three-way system, the bass being handled by an 8" woofer loaded with a transmission line, this terminated by a 3" port on the front baffle. Classically correct transmission lines, where the backwave from the rear of the woofer cone is completely absorbed in a long line, are very rare. The DCM, as with many other "transmission-line" systems, has considerable LF output from the rear of the line and thus should be classified as some kind of overdamped reflex. The midrange unit is a small 4" cone positioned at the top of the baffle, while the tweeter is a ¾" dome positioned between the other two units. This is fitted with an "acoustic lens," again to optimize dispersion. As the drive-units are placed asymmetrically, the TF1000s come in handed pairs; in use, the drive-units should be placed on the inside edges.
Details of the crossover points and slopes were not supplied, but the TF1000 is said to feature electrical time delay in the feeds to the woofer and midrange driver to equalize the path lengths from the various drivers to one preferred listening position. Their outputs should thus be time-coherent, enabling the TF1000, in theory at least, to produce an accurately shaped pulse at that position. Electrical connection is via spring-clip terminals; though these will accept relatively thick wire, I must admit that I much prefer good old five-way binding posts for a secure connection.
Though the instruction book contains quite a lot of information on how the user can get the best from the Time Frames, singularly lacking is any mention of where the speakers should be positioned in the room, other than mentioning that speakers and listener should form an equilateral triangle, without nearby obstructions. Are the TF1000s optimized for free-space positioning, or do they work best near a rear wall? I hadn't a clue. After some experimentation, therefore, I ended up placing the speakers about four feet from the rear wall, about seven feet apart, and nine feet from my listening chair. It then proved hard to determine the optimum listening height. The upper mids were rather forward on the tweeter axis, but above that, the lower HF became too prominent. I felt the best midrange balance to be just below the tweeter axis, on a level with the upper half of the woofer.
The next set-up problem concerned whether or not to toe-in the speakers. The upper midrange exhibited rather more "vertical-venetian blinding" than I am used to. (In this phenomenon, relatively small side-to-side head movements produce noticeable and rapid changes in tonal balance. It is generally a sign of uneven dispersion in the midrange and treble.) Again I chose the toe-in that gave the optimum midrange balance, the speaker axes crossing just behind the listening chair. This also gave the best (ie, most stable) stereo imaging.
Laterally, the soundstage presented by the TF1000s was very well-defined, the sound being well separated from the loudspeaker positions. This possibly correlates with the supposedly coherent time performance of the speaker. A task I find to be an acid test of a speaker's ability in this area is to play a recording of a panpotted, centrally placed female singer. With many speakers, it proves impossible to get a stable central image; with the TF1000s, singers hung precisely in the air between the two speakers, with little tendency for their image to bloat at some frequencies.
Depthwise, things were not as good, images being shallow compared with those thrown by the Celestion SL600s. Marni Nixon's voice on the Reference Recordings Gershwin song collection (RR-19) can be heard to be in front of Lincoln Mayorga's piano on the Celestions; the DCMs presented her in the same plane. Though the sound was detailed, enabling reverberation tails to be easily discerned, the recorded ambience didn't quite gel with the direct sounds. On my recording of Anna-Maria Stanczyk performing a Chopin waltz on the Hi-Fi News & Record Review Test CD (footnote 2), I used a Calrec Soundfield mike in crossed coincident figure-8 mode, which should give a coherent image with easily discernible depth. Presented by the TF1000s, the left-to-right delineation of the piano image was accurate, but the instrument sounded too close. Part of this, I am sure, was due to the TF1000's rather forward tonal balance, but mostly I think it was due to the lack of coherence between the recorded reverberation and the direct sound of the instrument.
As indicated above, the midrange did seem a little forward, but not excessively or unmusically so. Coloration levels were reasonably low, though the upper midrange seemed a little "clattery," adding an emphasis to piano-action noise and rendering the oboe tone a little closer to that of the English horn. The lower midrange was a little recessed compared with the rest of the band, cello and male voice sounding a little smaller than usual. Though clean at low levels, it got a little confused when things heated up. I suspect that this was due to sound emitted from the front baffle, which, being a large-dimensioned piece of wood, seemed quite lively in this region.
The treble also seemed cleaner at low levels than at high, the sound coarsening at orchestral climaxes and the contrast between instrumental tone colors becoming less obvious. The tweeter seemed better-behaved than most soft-domes, though there was a touch of wispiness to the mid-treble which slightly emphasized sibilants and tape noise.
Low-frequency extension seemed excellent, organ pedals, kick drum, and bass guitar having a good weight to their sound, the floor even shaking occasionally. This good subjective extension was not achieved at the expense of the upper bass; though it was a little looser than I have heard, the low-frequency sound overall was quite tight.
DCM's Time Frame TF1000 struck me as an extremely competently engineered, well-balanced design at its price point. The purchaser gets excellent low-frequency extension, quite low levels of coloration, an acceptably forward midrange balance, with good presentation of recorded detail, and excellent lateral image definition. However, I found the sound to coarsen at high levels, a factor that, coupled with the lack of image depthsomething very important to memeans that I wasn't as happy with the speaker's overall performance as I might have been. The DCM TF1000 will produce a sound in line with many people's tastes; it just didn't do enough of what I consider important.
Footnote 1: The Audio Critic ceased publication in summer 2015, but some back issues can be found here.
Footnote 2: This test disc, which I produced when at Hi-Fi News & Record Review magazine, is no longer available, but the same Chopin recording can be found on Stereophile's first Test CD.