Ben Duncan fathoms the ins and outs of phase checking.

The TP-1151 comprises of a pair of black boxes, namely a Pulse Generator and a Pulse Detector. Together they can be used to diagnose and verify both acoustic and electrical polarity (phasing), wherever lots of mics, speakers and balanced lines are gathered together. They can also be used to check cables and verify signal flow, and are therefore invaluable in any recording set-up that's gone beyond the 'Portastudio and two mics' stage.

Let's start from basics. How does it work? Figure 1 illustrates the Pulse Generator and Detector schematically.

Operation

The generator produces a Uni-Polar pulse, which is heard as a Click, over the speakers. Inside the PD-11 detector, the signal is received either via the line level input socket, or the built-in mic (for acoustic measurements). The mic is unswitched and sensitive at all times, but it won't respond to ambient sounds below some 85dBSPL. Thus PD-11 won't in general pick up enough extraneous noise to upset the line level measurements (unless you actually shout into it). The absence of switching is justified on the basis of simplicity; there's one less part to break or go wrong!

A Phase Detector then decides whether the received is positive or negative going, (in other words, whether the signal's polarity is positive or negative), and lights a red or green LED accordingly. Polarity detection is straightforward enough when the received pulse he has undergone a clearly defined polarity reversal, either normal (0°) or inverted (180°), but if we send the test signal through an EQ section, or any loudspeaker, we can expect varying amounts of phase shift. This could prove confusing: if the received pulse is shifted 90° (which is exactly midway between 0° and 180°), is the received polarity positive or negative? An earlier (and well-known) phase checker design used ½ sine wave pulse, and had problems making up its mind if there were incremental phase shifts in the system, but thanks to the carefully chosen pulse shape, the TPC-1151 can easily cope with the borderline phase shifts around 90° and 270°: Quite simply, if it's >90°, the polarity is negative, if <90°, it's positive. The TPC-1151's resolution is also improved by the gate, which locks out ringing and extraneous noises after it's received the first pulse. This is analogous to gating a percussive instrument, with the same sort of advantages.

Construction and Facilities

The PG-11 Pulse Generator has a proud XLR input, without needing a patch lead. The On/Off power toggle is sensibly recessed at the opposite end of the enclosure, so it won't get knocked to the On position accidentally. Nonetheless, battery consumption is minute. Even with the unit left switched on, it would last the battery's shelf life, somewhere in excess of twelve months.

Up at the output end, the PD-51 Pulse Detector receives its input from two sources. First, there's a complimentary female XLR, and once again, it pokes out far enough to plug into any chassis-mounted output socket. Secondly the tiny capacitor mic capsule, hidden behind a foam wind-shield (which is itself recessed). At the opposite end of the panel, beside the power toggle, lie the two polarity readout LEDs, marked positive (green) and negative (red). Because the TPC-1151 is made for Turbosound in the USA, these relate to the 'pin 3 hot' convention, but we've assumed it'll be used with 'pin 2 hot.' This transposes the colours, as shown in the shortform spec below, but it only affects our assessment of absolute polarity, and since most checks are about relative polarity, it's not as important or as confusing as it sounds. Again, power consumption is small; the battery lasts about 100 hours with regular use, but less if it's left connected without the LEDs illuminated, as these consume about 90% of the power.

"...the unit was designed by Chris Strahm to withstand being dropped from a height of 40 feet (from a flying PA onto a concrete floor), and still survive."

Application

I'll assume that you've already digested the 'Polarity Unveiled' article in this very issue, or you're already in the know about polarity. If neither of these is the case, you might be well advised to read the aforementioned article so that what follows does not appear utterly incomprehensible. Let's do some testing.

Beginning with microphones, we plug the Pulse Generator into a power amp, feeding into a full range speaker, then put the mic in front on it. In turn, the mic goes into one of the console's channels in the normal fashion, and the Pulse Detector plugs into the nearest output, either the channel insert, or the channel's output routed solo to a sub-group output.

If we now switch the Pulse Detector on, we can wind up the channel gain so that one of the LEDs lights up solely in response to the room noise, picked up by the mics. Of course, we don't want this, but now we know 'where it is', it's easy enough to back off the channel gain by 15 or 20dB — just enough to be really sure that room noise won't intrude. The LEDs should now go out, after which we can switch on the Pulse Generator, and adjust the amplifier's gain until one of the LEDs comes on, and stays put. If this demands a horrifically loud click, either the channel gain is wrong, or the mic needs to be closer to the speaker, and ideally on-axis. Sometimes, the LEDs will flash in sequence, say a burst of green, then settling down to a steady red. This is normal for any phase/polarity tester, since some 'thinking time' is necessary.

Unless you're bothered about absolute polarity, it doesn't matter much whether the red or green LED lights up. It is much more important that, having set up the test rig, we should try every mic on the same premises: every one should indicate the same polarity. Working to this formula, Matthew Kemp (at The Chapel 24-track) borrowed the TPC-1151 for a weeks maintenance, and promptly found that four mics had opposite polarity, out of the studio's collection of 45. This seems a typical figure, meaning that roughly 9% of mics are 'out-of-order.' (Unless you remember to use the console's phase invert switch;) It certainly explained the odd sound they'd been getting on their SM57.

Testing Lines and Speakers

Balanced XLR-to-XLR cables can be checked directly, by simply plugging them in between the TPC-1151's twin boxes. Equally, for balanced cords featuring B gauge jack plugs, all we need is a standard conversion lead. At this stage, Matthew's maintenance work with the TPC-1151 uncovered seven 'out-of-phase' leads, and two faulty ones. A good leads checker might have identified these, but it won't then go on to reveal speakers or mics which have been wired back to front.

For unbalanced connections (including anything with mono jacks or mono plugs), it's important to note that the Pulse Generator's balanced output is of the floating variety, meaning it's across pins two and three, with no reference or connection to pin one. In fact, pin one just connects to the box, for shielding. In any event, for unbalanced connections, we simply need to check that pin three is tied to pin one on the conversion lead, as is the normal practice on an unbalanced system, with pin two as hot. (If pin three is hot in your system, pin one should be linked to pin two.)

For monitoring and PA systems; and speakers in general, the procedure begins as with microphone checks, namely feeding the Pulse Generators up the speakers via the power amplifier's input. But this time, we don't connect anything to the Pulse Detector, but just turn up the click level on the PA and waggle the unit's integral mic in front of the speaker, until the LEDs respond.

"The absence of switching is justified on the basis of simplicity; there's one less part to break or go wrong!"

To confirm correct polarity for every drive unit in a system, it's generally adequate to just poke the mic close up to the cone's apex, or the mouth of any horns. Of course, to discount any possibility of the mic picking up any sound reflected from another unit, you may wish to mute or disconnect the other units.

Unless you know the design parameters of a speaker, it's unwise to jump to any conclusions about the polarity differences between drive units working in different frequency bands. But we can say for certain that all the drivers working on the same frequency band should give the same polarity readout, whether it's the mid-range section on a mono PA, or the bass drivers on a pair of stereo studio monitors.

Although the TPC-1151 isn't confused by substantial increment phase shifts, designer Chris Strahm warns that it could give misleading results on some (possibly badly designed) active crossovers, which may exhibit wild phase shifts, well in excess of 360°. It follows that the two are best treated as a separate issue. Which means confirming the crossover's polarity independently, and then firing the Pulse Generator directly up the power amplifier. In other words with an active system, we check the bass, mid and HF speakers separately, via their respective amplifiers.

Presentation

The best word to describe the TPC-1151 is 'utilitarian'. Because Turbosound are principally innovators of high-tech PA speakers and related products, and having spent 15 years running their own hire company, just to find out what happens to gear on the road, the TPC-1151 comes already packaged to withstand the real world.

The XLRs are held in place with chunky self-tappers (meaning they're easily replaced), the LEDs and switch are recessed, and the exposure is composed of 3mm extruded aluminium, finished in extra heavy duty, black anodising. In fact the unit was designed by Chris Strahm to withstand being dropped from a height of 40 feet (from a flying PA onto a concrete floor), and still survive! This is achieved by totally encapsulating the circuit components in epoxy, to protect them from shock.

Since its first introduction in the States four years ago, designer Chris Strahm reports shipping hundreds of units with a failure rate of just one: a broken toggle switch. It follows that, unlike the prettier but more dilettante studio products from Japan, the TPC-1151 can be expected to work for many years, even with daily use and much trampling of boots.

The Turbosound TPC-1151 costs £227.70 including VAT and further information can be obtained from: Thatched Cottage Audio, (Contact Details).