An investigation of this revolutionary 'new' audio metering device.
Mike Skeet takes a practical look at this 'revolutionary' new audio metering device that should prove as popular as the VU or Peak Programme Meter.
The history of audio signal level monitoring has a number of milestones and byways. Many will think of the Peak Programme Meter (PPM) as a milestone and look on the Volume Unit meter (VU) as being just as much a byway - although the latter does have its devotees! But peak reading accuracy is all the more necessary now that we are entering the Digital era. Since moving coil meters were the norm we have, in the last decade, seen LED columns, with peak indicating facilities, come to the fore. However it hasn't escaped my notice that some such systems have deliberately incorporated VU characteristics in their circuitry!
THE BOX, which we look at here, is a metering system which is most unusual and which incorporates a concept that might be seen as such a milestone. This, only time will tell, so suffice for us to introduce THE BOX as it stands, and discuss its attributes with illustrations.
It comprises 100 LEDs in a diamond matrix pattern, some analogue and digital electronics, inputs for Left and Right stereo signals and a calibration and line up switch and potentiometer system. All is housed in an attractive wooden and perspex cube which is about the size of a small Auratone speaker and is very sturdy indeed. Persistence of vision is relied upon to give what proves to be a panoramic display - yet only one LED is ever lit at any given moment!
As a stereo visual monitoring system, easily installed in a home studio set-up, a domestic system or, for that matter, in professional studios, there are two aspects to its use. Firstly, as a stereo metering system with music, and secondly as a test instrument with test tones or tapes.
As a stereo metering system, with music it shows:
1. Peak levels on stereo programme.
2. Stereo balance.
3. Stereo sound stage width.
4. Stereo soundstage positioning.
5. The presence of 'mains' borne interference in the programme.
As a test instrument, with tones, it shows:
6. Channel levels.
7. Channel balance.
8. Analogue tape recorder playback Azimuth errors.
9. Analogue tape transport defects.
10. The presence of 'mains' borne interference in systems.
Before we look at the ten points outlined, we must first look at the ergonomics of the display, then how THE BOX is installed in a system and calibrated to suit.
As shown in Figure 1, there are 58 green LEDs and 42 red LEDs in the front panel layout. The interval of triggering in any vertical column is 3dB. If an identical, in-phase, signal is fed to both of THE BOX inputs, at identical levels, with the internal calibration system appropriately set, then the centre vertical column will light. The number of LEDs triggered in the column depending on the applied signal level.
If the G7 LED at the top of the vertical red column is used as a Calibration and Nominal Level point (ie. the '0dB' point), then in effect one has +9dB and -18dB range above and below this point. More on calibration later.
If the signal mentioned above is varied in level between channels, the vertical trace displayed on the LEDs moves to the left or right as appropriate. It will however move 'off' the matrix when the difference is too great. How much difference it takes to do this will depend on the initial input level. With the peak level at the calibration point of G7 (at the top of the red column) a difference of 18dB between signals will move the column to the appropriate left or right edge so that only G1 or A7 LEDs are lit. A difference of 9dB in favour of the left hand channel will give the displacement illustrated in Figure 2.
If the input signal was 'at peak' (lighting the central vertical column all the way to L10) then it will leave the matrix when the difference is 27dB. The possible practical importance of this we shall discuss later.
To take the understanding of the matrix, and the triggering of the LEDs within it, a stage further, imagine it as being like an oscilloscope used to display Lissajous figures using its X and Y co-ordinates. Figure 3 shows Lissajous displays of equal amplitude signals with phase shifts to 90°. Then think of the whole display rotated 45° anticlockwise and only one quadrant displayed (see Figure 4A). Hence the diamond presentation of the matrix.
With two signal tones connected to THE BOX at calibration level, but with the phase of one of the tones shifted 90° relative to the other, the display shown in Figure 4B is generated.
THE BOX and its rear-mounted sensitivity controls allow a range of calibration point signal levels to be accommodated between 230mV and 1.7V. Thus it is plainly suited to both domestic and professional 'line' level signals.
What is meant by calibration or nominal levels? It is customary to have a nominal working level in a system which is a little below the 'absolute' or desired peak. For instance, take a compact cassette system. A flux level on cassette tape of 200nWb can be exceeded by 4 to 8dB, depending on the tape type, and still have an acceptable level of high frequency saturation or distortion. Similarly, with reel-to-reel working, but here it is possible to be even more than 12dBs 'hot' above what we know as Dolby calibration level.
Of course, with analogue systems the 'absolute' limit isn't fixed - it's determined by the degree of signal corruption one wishes to tolerate or is not really aware of! Judging by the levels some people work at in analogue recording, it seems to me that quite a lot is gaily tolerated! With digital systems however, there is an 'absolute' crunch point and the nominal working level needs to be dropped further down to prevent signal 'break-up'. For instance, with the Sony PCM F1, it is customary to work at -20dB on its scaling; this, in practice, is not as far down as it might seem for the reasons discussed in the various HSR articles on that system. With FM broadcasting it is usual to find an 8dB limit above the 'nominal' level - there are BBC test zones transmitted on Radio 3 FM stations after programmes, which can be used as a guide when calibrating your operating levels.
So, with THE BOX, my recommendation is that the G7 LED at the top of the central red column be used as the calibration point (0db) with the various systems outlined. For example, with a cassette deck play a Dolby level tape (or a tape recorded with a 400Hz tone on both tracks, to the Dolby level marks on its meter scaling. I assume that it plays back to the same level!) and calibrate THE BOX at the G7 LED. Similarly with the other sources, although there is a case with reel-to-reel to set it to the LED below, as these systems work to higher flux levels and can therefore accept higher level signals before tape saturation occurs ie. total distortion.
To assist understanding of what is involved with the Calibration switch and the Sensitivity potentiometers, at the back of THE BOX, refer to Figure 5. Notice in passing that the metering circuitry is buffered from the input. The Calibrate and Line Up switch has three positions:
L - feeds the LH input to both meter channels.
S — normal stereo run position.
R — feeds the RH input to both meter channels.
Before we can set the balance of the feed external to THE BOX, we must get THE BOX itself in balance as well as at the chosen calibrate indication. With the source to be lined up, feeding tone:
A) Switch to L. The LH input feeds both meter channels. Adjust both meter sensitivity controls to get the chosen calibration point and a central red display with equal fringing to the side green LEDs. This in fact, sets the meter 'in balance' and the sensitivity controls should not normally be moved again.
B) Switch to R. The RH input now feeds both meter channels. The same level should be available as on the LH input. If not take steps, external to THE BOX to achieve this. Knowing that THE BOX is 'in balance', switching to the S position will show the degree of imbalance between the signals being fed into the device.
C) Switch to S. This is the normal stereo position for THE BOX to show all it can.
The short answer is 'yes'. The longer answer is 'like all metering, it needs interpreting'. It certainly does not behave like an LED column as its display is very 'panoramic'. For me, LED systems have an interesting interpretation aspect - the 'time on' and 'brightness' perceived, give an indication of how loud a signal is. Odd, short flickering is not the same as continuous illumination.
(1 & 6) PEAK & CHANNEL LEVELS
It is implicit with the triggering of LEDs that a peak reading ability is available. Specifically, though, with THE BOX, the electronics employed show any signal level with a duration of 800 nanoseconds or more, as the device operates in real time. This means, as mentioned before, that only one LED (out of the 100) is triggered at any one time. Persistence of vision accounts for the panoramic display given on wide stereo programme. One can clearly evaluate the peaks even though there is no delay time as such or peak hold facility. As discussed in the paragraph on calibration, channel levels are easily set with test tones and equally on programme the levels can be clearly judged and suitably adjusted.
One of the most remarkable aspects of THE BOX is the extremely clear indication of channel balance with tones or stereo balance on programme. With tones 0.3dB resolution is available! In fact, it is closer than half the pointer width of my dual PPM! On programme the panoramic display shows any tendency for Left or Right bias either side of the centre red column. See Figure 6.
Again, this is a unique aspect of this product. One can clearly see where, in the related sound stage, the highest and intermediate signal levels are. There could be vocals in the middle, drums and bass around the middle, with instruments off to the left or right or partly left or right. Natural or manufactured ambience also fills out the stereo panorama — reaching the parts of monitoring that other meters fail to reach! See Figure 8.
(5 & 10) MAINS-BORNE INTERFERENCE
Installed in my system, and interestingly at another studio I visited, THE BOX showed occasional mains-borne interference from fridges, lights etc. as a very thin, dull trace up through the central LEDs. The unit is set to respond to even the briefest of transients, normally inaudible on my installation, which is a comforting thought.
(8 & 9) TAPE AZIMUTH & TRANSPORT DEFECTS
This was another revelation. Playing a known standard frequency tape, recorded to correct azimuth, it was obvious that my reel-to-reel mastering machines were not set exactly as I had thought, by conventional methods. Interestingly both mastering machines exhibited the same degree of small error. Azimuth check is done by playing, in stereo, a known standard tape of a frequency between 400Hz and 1kHz, and not the usual 10kHz or above. If the replay levels are identical and perfectly in-phase, it will show up as the straight vertical line of a mono feed. If the tape replay head on the tape machine is at an angle to the tape motion, the phase differences immediately 'speak out' with a 'void' in the display as shown in Figure 9. Just placing a finger lightly on the side of the playback head easily moved the display, in and out of vertical line! Such accuracy of indication! Of course, turning to compact cassettes one could see some ludicrous displays yet some cassette machines were remarkably accurate and consistent.
The amount of cyclic variation shown in the traces indicates all sorts of short term azimuth, tape path, tracking etc. problems. Interchanging tapes between machines is not good for the soul! Obviously a longer experience with the device would consolidate judgements in this area but I would certainly go as far as to state that THE BOX should be in every studio workshop for this reason alone, if not for its other areas of indication.
This is the first published survey of THE BOX and its potential. We cannot predict overall just how well the concept will be received but the writer (and the Editor — Ed) are firmly of the opinion that it deserves attention - not merely the casual 'oh that's pretty, what does it do?' — but a thorough evaluation in the longer term, properly lined-up (it only takes 30 secs!) to ones existing metering. Go as far as blanking off ones own metering and start relying on the thing - you might get hooked! We in the audio business are a pretty conservative lot, cynical (often rightly so?) of the latest gizmo, but THE BOX by Tapetalk is different and it should prove not to be a 'here today, gone tomorrow' idea.
Manufacturer: Tapetalk, (Contact Details).
THE BOX by Tapetalk: Model Mark IIIE £150 + VAT (£22.50) Total £172.50.
Mains to 12V DC power adaptor £5 + VAT (75p) Total £5.75
Carriage & Packing £2.50
Feature by Mike Skeet
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