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Tabor Audio The Box 2

Stereo Sound-stage Meter
Published January 2005
By Hugh Robjohns

This unusual unit provides an exceptionally revealing means of metering stereo signals.

Audio metering is all about compromises. The VU meter hints at the perceived loudness of a signal, but gives no information about peak levels. Quasi-peak meters favoured by broadcasters do the reverse, and even these are insufficiently accurate for digital systems. Stereo metering is even more difficult, as ideally we need to see information about both the relative levels between channels and the relative phases or timing — the key factors in determining the width and quality of the stereo sound stage.Tabor Audio The Box 2

A widely employed partial solution is the dedicated 'phase meter' which indicates the 'coherence' (phase) between two channels, but level differences do not register at all. An alternative approach, favoured by the BBC and other broadcasters, is to meter the difference or Sides signal, which relates both level and timing/phase differences. This works well if the overall signal level is kept up — as it tends to be for broadcasting — but it becomes ineffective with low-level signals.

Far and away the best solution has always been the goniometer, a two-dimensional display where the signal from the left channel is shown on an axis that runs from bottom right to top left at 45 degrees, and the right channel is on an axis running in the opposite direction from bottom left to top right. Thus a perfect mono signal (identical in both channels) produces a thin vertical line and a normal stereo signal produces something resembling a circular ball of wool!Here are three potential displays generated by The Box 2 in response to a reference-tone input. This read-out shows the tone panned centrally.Here are three potential displays generated by The Box 2 in response to a reference-tone input. This read-out shows the tone panned centrally.

A huge amount of information is presented by this kind of display — left and right levels, stereo width, mono compatibility, frequency content (very low-frequency problems are particularly obvious). With practice and experience it is even possible to determine the type of microphone arrangement used in a recording — spaced mics produce a characteristic pattern which is very different from coincident or pan-potted mics, for example.

However, hardware goniometers are still expensive devices and tend to be the exclusive province of mastering houses and very large recording consoles. Fortunately, a more affordable solution is now available in the form of The Box 2.

Readers with long memories may remember the original Stereo Soundstage Monitor designed by the late Frank Fox, or its subsequent re-engineering and re-launch by Philip Stokes and Mike Skeet under the ITZA brand. This latest incarnation — The Box 2 — is slightly smaller and better-engineered than the previous model, and is now produced by Mike Ballance of Tabor Audio in association with Skeet Music.This read-out shows the effect of panning halfway to the left.This read-out shows the effect of panning halfway to the left.

Goniometer Quadrant Display

Housed in a plastic case measuring 150 x 100 x 65mm (hwd), The Box 2 is taller than it is wide, with the meter display on the front and the connections to the rear. Power is via a wall-wart producing 18V DC, and is connected through a coaxial socket on the rear panel. The bottom central green LED in the display illuminates constantly when power is applied. The stereo line-level audio input is unbalanced via a TRS quarter-inch socket, intended for direct connection to the headphone output of a stereo recorder using a standard TRS-TRS cable. A momentary button on the rear panel provides a mono input to the meter to check alignment.This read-out is warning that the left and right channels are out of phase with each other.This read-out is warning that the left and right channels are out of phase with each other.

Construction is to a very high standard, with conventional components mounted on a fibreglass circuit board fixed to the inside of the rear panel. The metering display consists of a diamond grid of 100 intensity-matched LEDs mounted on a second circuit board supported on pillars from the rear panel to sit just behind a clear plastic window in the front of the unit. The extremely elegant circuitry uses a pair of bar-graph meter chips, one driving the left-channel axis of the LED array and the other driving the right-channel axis. The result is a distinctive diamond pattern that conveys the top quadrant of a complete goniometer display — which is sufficient to provide all the required stereo information.

In reponse to a full-width stereo music mix, The Box 2's display should show something along the lines of this image here.In reponse to a full-width stereo music mix, The Box 2's display should show something along the lines of this image here.The LED array comprises green, yellow, orange, and red LEDs configured much like a conventional bar-graph meter. Working up either side of the array, the first seven LEDs are green, followed by yellow, amber, and red. These colours extend into the corresponding rows across the rest of the array, with the exception that the centre vertical column is made up entirely of red LEDs. In this way, the colour of the display provides a useful indication of peak signal levels, and the central axis is indicated by the red LEDs.

Watching The Box

I used the meter by hooking it up to the headphone outputs of various recorders and mixers, as recommended. The headphone level is critical to the correct operation of the meter, but this is established simply by replaying a constant-level signal on both channels at a nominal 'zero level' and adjusting the headphone level so that all of the centre red LEDs are illuminated.If the read-out is more like this, then it means that the stereo image is quite narrow.If the read-out is more like this, then it means that the stereo image is quite narrow.

The display then indicates signal levels in the usual way, with yellow, orange, and red LED's warning of headroom incursions. Adjacent LEDs illuminate at roughly 3dB increments, as on a normal bar-graph meter, but low-level 'dynamic compression' ensures that the stereo image display remains fairly consistent when the signal level falls.

The provision of a mono button on the rear panel is handy to check that the meter is accurately aligned internally, and provides confidence that when it says the signal is in the middle of the stereo image, it really is! An accurate dual mono signal is shown as a thin vertical red line, and if any of the adjacent greens are illuminated there is something wrong somewhere — either the balance is off, or there is some sort of phase or timing error. I detected a one-degree phase error on a 1kHz stereo calibration tone recorded on a prototype test disc using an earlier version of The Box, so it is a remarkably accurate tool.

This read-out is characteristic of an over-extended stereo image or excessive out-of-phase components.This read-out is characteristic of an over-extended stereo image or excessive out-of-phase components.Panning the signal off towards the left results in the vertical line also moving to the left, with the bottom of it running up the left-hand side of the diamond pattern. A fully-left signal runs entirely up the left-hand side of the diamond pattern. Full-width stereo material is shown as a fairly full pattern of LEDs, where the outside edges are the same length and brightness as (or slightly smaller and dimmer than) the middle. If the pattern is fairly high but narrow — looking a bit like a Christmas tree — then the stereo image is quite narrow.

Should one channel be reversed in polarity the display shows a clear 'V' on the outside LEDs, with nothing illuminated in the middle at all. If the stereo image is excessively wide, or if there are phase anomalies, the edges of the display will be longer or brighter than the middle. It may sound complicated, but the display is actually very intuitive, and after a little experience you will wonder how you ever managed without it. This form of metering often highlights potential problems before they become obvious through listening. The Box 2 offers a compact, convenient, and very usable stereo sound-stage meter which is recommended for anyone involved in live stereo mixing or post-production.

Published January 2005