Metering is a critical element of any audio recording system, and the modular nature of DK Audio's extensive range of high‑quality units makes it possible to purchase a system that exactly meets your requirements. Hugh Robjohns is impressed...
A good meter provides visual detection or confirmation of things that might be extremely hard to hear, even in good monitoring environments. Not only average and peak levels, but also panning offsets and phase errors can be hard to hear but easy to 'see'!
The wholesale move towards digital recording formats is forcing the 'virtually useless' VU meter and the misnamed Peak Programme Meter (PPM) into retirement while true peak‑reading digital meters are becoming the standard (see 'Meter Types' box). Growing interest in surround sound is also placing much greater demands on metering technology, which few manufacturers have yet addressed.
DK Audio, a small Danish company, produce a comprehensive range of systems which address every aspect of metering for the modern age. Their range of Master Stereo Displays spans a wide price range and has an extensive list of options. Although they may appear expensive, it must be remembered that these meters provide far more information and functionality than other typical stand‑alone models.
The base model is the MSD100 which employs a monochrome LCD display panel. The midrange units are the MSD200, MSD500 and MSD800, with a yellow monochrome electroluminescent display, while the range‑topping MSD600 boasts a slightly larger, colour LCD. All share a common basic display screen with a phase‑correlation meter running vertically up the left‑hand edge, an audio vectorscope in the centre (see 'Up Vectorscope!' box), and selectable bar‑graph level displays (including conventional VU and PPM options) on the right‑hand edge. The amount of information presented by these three complementary displays is simply enormous, and once you are familiar with them, they're hard to work without!
Depending on the model, options include digital inputs (with interface status information), a real‑time spectrum analyser, multi‑channel monitoring (of 4, 6, or 8 inputs), test signal outputs (tone, white or pink noise and impulses), and surround‑sound 'jellyfish' display modes. The systems are configured through 'soft keys' below the screen — three buttons on the MSD100, six on the MSD200, MSD500 and MSD800, and eight on the MSD600. All models except the MSD100 have an RS232 connection, allowing the installation of operating software upgrades downloaded from the company's web site; this connection also allows the user to modify metering scales from a supplied PC program. None of the units work directly from mains power, but external power adaptors are sold separately if required. Mounting brackets are supplied.
The most 'inexpensive' base model of the MSD100 is equipped with analogue inputs on unbalanced phonos, whereas the other MSD100s have either transformer‑balanced XLR analogue or AES‑EBU digital inputs, and it is the only meter in the range without a real‑time spectrum analyser. For comparison, a pair of AB/MS PPMs, a true peak‑reading meter, and a phase‑correlation meter would cost close to the asking price of this base model, and these still wouldn't convey as much information.
Physically, the machine measures 179 x 129 x 39mm (whd) with a 320 x 240‑pixel monochrome LCD display. This has a relatively narrow viewing angle, although a contrast control allows it to be optimised for your particular installation. The display brightness can also be adjusted and even inverted giving the choice of white graphics on a black background, or black on white.
All the MSD models have seven metering scales to choose from. These are a Nordic scale, the BBC‑style PPM and its equivalent EBU version, a DIN scale, the classic VU, and two digital meters derived from the Sony 1630 standard — one covering a 60dB window, and the other just the top 6dB with correspondingly increased resolution. The reference level can be adjusted for each meter, and overload LEDs illuminate when the signal enters a designated area. A full gamut of peak‑hold options is provided and one of the soft keys introduces a 20dB boost to enhance the monitoring of low‑level signals.
The digital‑input version can decode the status data in the incoming bit‑stream to provide information on pre‑emphasis, sample frequency, word length and so on — all this is tabulated in an window overlaid on the vectorscope display. The current input sample rate is always shown at the top of the screen.
The spectrum analyser, where provided, uses a 1024‑band FFT algorithm to display the signal content between 44Hz and 20kHz, covering the dynamic range from ‑70 to +10dBu.
These are probably the most familiar models as they are often installed in high‑end mixing consoles — the bright yellow‑on‑black display is instantly recognisable. All share the same screen dimensions as the MSD100 and incorporate balanced analogue inputs and outputs. The MSD800 models provide either four or eight analogue inputs, with corresponding outputs, while the MSD200 and MSD500 models have stereo analogue I/O augmented by an AES‑EBU stereo digital input. All connections for the MSD200 and MSD800 units (as well as those for the flagship MSD600 range) are accessed via 25‑pin D‑Sub sockets: not only the audio interfacing, but also the RS232 data port and DC power connections. D‑Sub connectors are supplied with these units for you to wire them into your own system manually if you wish, but ready‑made break‑out leads are also sold separately by Canford Audio, providing XLRs for the analogue and digital I/O, a 9‑way D‑Sub for the RS232 port and a latching 5‑pin DIN plug for power. The MSD500 models, being designed for installation within high‑end mixing consoles, have a more compact casing and use different interfacing connection standards designed for this purpose (NTP‑compatible or EDAC/ELCO connectors).
Although the screens of these units have the same 340 x 240 resolution as the MSD100, the electroluminescent display has the advantage that it has superb contrast and a 160‑degree field of view. However, there are many other important enhancements too, not the least of which is the provision of three extra control buttons.
The main display page provides the correlation meter, vectorscope and bar‑graph level display — the last displays either simple Left and Right (with the option for simultaneous sum‑and‑difference metering) or multi‑channel level meters. The same seven metering scales are available as on the MSD100, but this time they can be accessed directly from the panel buttons rather than by having to step through a list. The supplied DK‑Scale PC program allows the user to rename, modify, reorder or even custom‑design these meter scales as desired, although careful thought and concentration are required to avoid mistakes, because the software interface is a little crude and there are few safeguards to prevent you entering nonsensical values accidentally.
Aside from the 20dB boost facility, other user‑selectable metering preferences include adding peak‑hold bars (permanent or self‑resetting) and true‑peak bars which flit around above the main metering columns. There is also an overload counter, and the correlation meter can be switched between fast or slow response times. The 'Statistics' menu shows the dynamic range of the signal over time. This peculiar display provides a 'nose' which grows horizontally out of a vertical bar — the fatter the nose, the wider the dynamic range, and the higher the nose is up the bar, the greater the peak level. Strange, but useful and always intriguing!
The spectrum analyser function is much the same as on the more expensive of the MSD100 models, but with more facilities included. For example, as well as covering the entire spectrum from 44Hz to 20kHz, the display can be 'zoomed' to show only the region from 4Hz to 2kHz. There is also a moveable cursor to display the amplitude at any required frequency, and the amplitude display can be normalised relative to a selected frequency band. Various FFT windowing algorithms can be implemented and a pre‑emphasis option introduces a 3dB/octave tilt to the display to provide the visual appearance of a typical 1/3 octave analyser.
Another facility not available on the MSD100 is the test‑signal generator, which is able to output sine‑wave tones of various frequencies (31Hz to 15.5kHz) and levels (‑2 to +18dBu), along with pseudo‑random white and pink noise. Analogue input signals or those decoded from the digital input can also be 'echoed' to the outputs if required. When the digital input is selected its sampling rate is displayed at the top of the screen, and the status data can be decoded and displayed for each channel independently. The meter can be customised to automatically display different meter scales with the analogue or digital inputs.
The most interesting development over the MSD100 is to be found in the MSD800 models, which include the unique 'jellyfish' mode for metering surround‑sound levels. This is a mode I referred to earlier in which the vectorscope display is replaced with a representation of the speakers in a choice of surround monitoring arrangements (see 'Jellyvision' box). This circular display writhes in response to multi‑channel input signals, with a peak on any one channel causing an 'arm' to grow outwards in the appropriate direction — a centre‑only signal looks like a bullet and an LR‑only signal resembles a cardioid pattern.
The MSD600C and MSD600M are the flagships of the range and are slightly larger than any of the other units, at 184 x 144 x 49mm (whd). The colour LCD screen is around 20 percent larger with a wide 90‑degree viewing angle and a sharp 640 x 480‑pixel image. Although they have this enhanced screen, a VGA output is still provided via a 15‑way high‑density socket on the back panel, and this is accompanied by a 9‑pin D‑Sub connector which provides not only RS232 and power connections, but also a single AES‑EBU digital 'sync' input for supplying the meter with a master digital clock.
The MSD600M has been designed to be a completely modular system — the main unit has no audio inputs at all until channel expansion cards are added, each providing extra stereo inputs or outputs up to a maximum of eight channels. Each expansion card provides both balanced analogue and AES‑EBU digital connections with sample‑rate conversion, and flexible routing options mean that the MSD600M can easily serve as an audio matrix or A‑D/D‑A converter. However, each expansion card has an individual D‑Sub connection, which takes the cost of break‑out cabling for a fully expanded MSD600M to just under £800.
The MSD600C is a pre‑configured version of the MSD600M offering four channels of analogue I/O along with four channels of digital input. While not expandable, it's a little more cost‑effective if it meets your needs, not least because all its audio I/O is accessible through a single D‑Sub connector.
Even a stereo‑in, stereo out MSD600M without break‑out cabling or power adaptor is going to set you back more than £2700 — substantial money by any standards — but you're getting a lot more than 'just a meter'. Adding the cost of separate AB/MS PPMs, a digital bar‑graph meter, an AES‑EBU data analyser, a correlation meter, a program‑level logger, and a test‑signal generator you would not be far off the cost of the most basic MSD600 — and that is before you add in the benefits of a spectrum analyser and a uniquely informative surround‑sound
The main display and general functionality of the MSD600 range are almost the same as on the less expensive units, although the provision of two extra soft keys further simplifies the menu structure. The correlation meter, vectorscope and metering are all identical in terms of both display and configuration except that the colour scheme can be altered to a house style (or for personal taste!). However, no bar‑graph can be coloured red, as DK have reserved this colour for overloads, so those of you wishing to use the familiar BBC metering colour code of red, green, white and yellow for L, R, M and S will have to use another colour instead.
Pairs of analogue and digital inputs can be displayed alongside one another and the bit stream of a digital input interrogated in the same manner as previously described. The spectrum analyser and test generator also hold no surprises, and the 'jellyfish' display is similar to that of the MSD800 models.
DK Audio's MSD units provide a wealth of precision metering tools to suit a wide range of budgets. Although possessing only the basic metering facilities, the MSD100 range are every bit as capable and precise as their more expensive siblings and their only real weakness is in the limited field of view of the LCD display. The MSD200, MSD500 and MSD800 probably represent the best value for money with their superb displays and wide range of useful metering options. However, the model to covet is the MSD600 which is truly excellent in every sense and should be a standard requirement in every mastering house in the land!
When stereo was introduced, existing mono meters were complemented with additional meters (denoted 'A' and 'B' in broadcasting) for the two stereo channels. The mono signal (denoted 'M') can be derived from the stereo busses by summation of the left and right channels (in other words M=A+B), usually with a ‑3 or ‑6dB level adjustment to make the three meters more comparable in level. Often a fourth meter (denoted 'S') will also be provided to show the difference between left and right (in other words S=A‑B), representing the width of the stereo signal.
AB and MS meters can provide a great deal of useful information to the trained engineer, but they are not the only solution. The audio vectorsope (or Lissajous display) was developed as an
alternative means of displaying information about a stereo signal and many engineers now rely on it. Early systems used an oscilloscope with the left signal controlling the position of the dot along the X‑axis and the right signal controlling its position along the Y‑axis. Inputting a mono signal would, therefore, result in a straight line being traced, the angle of which would represent the relative amplitude of the two channels.
The display of modern audio vectorscopes is rotated 45 degrees anticlockwise to generate a more intuitive image. Consequently, a central mono signal is represented by a vertical line, with fully left or right signals tracing lines at 45 degrees to the vertical. With normal stereo signals the dot's position varies continuously according to the instantaneous voltages of left and right inputs, drawing what is known as a Lissajous figure. A typical full‑width stereo signal will tend to create a 'blob' with a circular outline, whereas a narrow stereo image results in an elliptical outline and a lot of out‑of‑phase information results in a figure that is wider than it is tall. If the figure occasionally exhibits obvious holes, this means that there are time‑delays inherent in the signal (this is typical of a recording made with a spaced‑mic array, for example), which could compromise the signal's mono‑compatibility or blur the stereo imaging.
The audio vectorscope provides an astonishing amount of information about the nature of a stereo signal. However, it does not convey peak levels very well so it is usually employed in conjunction with dedicated peak‑level metering.
As the MSD range is modular, systems are to a large extent freely configurable. Here are some sample system prices including VAT, but contact Canford Audio for exhaustive pricing information.
- MSD100 two‑channel meter: £681.50 or £893 with either balanced analogue or AES‑EBU digital inputs.
- MSD200 two‑channel meter: balanced analogue I/O, AES‑EBU digital input, £1674.38.
- MSD800: four‑channel version £2455.75; eight‑channel version £2908.13.
- MSD500 two‑channel meter: £1932.88 or £2038.63 for NTP‑compatible version.
- MSD600C four‑channel meter: £2614.38.
- MSD600M/SA modular multi‑ channel meter with four slots for stereo input modules (£299.63) and four slots for stereo output modules (£176.25): £2273.63.
- MSD100, £29.38; MSD500 and MSD800, £14.50; MSD200, £41.13; MSD600, £58.75.
- From £88.13 to £152.75
The original Dolby surround format allowed four audio channels — Left, Centre, Right, and (mono) Surround — to be encoded (or 'matrixed') so that they could be recorded onto the stereo audio track of film and video. These encoded tracks are denoted LT and RT, and can be decoded by a special decoder to recover the four original channels. The domestic Pro Logic system is such a decoder and a simple version of this is incorporated into surround‑capable models of the MSD series meters.
Current surround formats employ either five (left, centre, right and stereo surround) or seven (the same plus half‑left and half‑right) entirely discrete channels. A separate low‑frequency effects channel (denoted as '.1') is also frequently employed. The DK 'jellyfish'‑display metering mode shows nominal loudspeaker locations according to the selected surround format (4, 5, or 7 channel). The signal amplitude corresponding to each channel is represented by a closed curve which grows from the centre of the screen, stretching towards each loudspeaker according to its respective level. The result resembles a dancing jellyfish — hence the bizarre name!
There are three generic meter types. The first and most widely used is the Volume Unit (VU) meter, originally developed for the American telephone industry in the 1920s. This simple meter displays an averaged signal level which gives an approximation of the perceived volume. As a result, its calibration is accurate only with steady test tones and it under‑reads the levels of transient peaks dramatically.
The Peak Programme Meter (PPM) was developed by the BBC in the 1930s to allow broadcast engineers to control sound levels more accurately. It has a slow fallback time and a simplified scale which was designed to be easier to view over extended periods. However, despite its name, the PPM does not indicate true peak levels, because it was designed to have a finite response time in recognition of the inaudibility of very brief transient overloads in analogue recording and transmission systems. The PPM under‑reads true signal peaks by a surprisingly consistent 4dB.
Digital equipment — in particular analogue‑to‑digital converters — cannot tolerate signals exceeding their maximum quantisation level, no matter how transient or brief they may be. Consequently, genuine peak‑reading meters have become essential to control signal levels entering the digital domain.
Different scales are used to conform with national standards and to optimise each of the different meter types for specific applications. For example, all digital meters are scaled to count down from a maximum value of 0dBFS, typically to around ‑60dBFS. The BBC PPM's simplified scale is numbered arbitrarily from 1 to 7 with PPM 4 at 0dBu, whereas the EBU‑standard PPM meter is scaled directly in dBu from ‑12 to +12. The DIN and Nordic‑standard PPMs present much greater dynamic ranges, from ‑50 to +5dBu and from ‑42 to +9dBu respectively.
- Informative and accurate metering.
- Superbly integrated display.
- Easy to use.
- Powerful options.
- Unique 'jellyfish' and statistics modes.
- Upgradeable and customisable.
- External power supply needed.
A range of trustworthy and informative metering systems which integrate a correlation meter, user‑selectable bar‑graph meters and an audio vectorscope in an easy‑to‑use package. Optional extras include a spectrum analyser, test signal generator, and 'jellyfish' surround display.