Operating Levels

EBU or SMPTE? VU or PPM? Dbu or DBFS? An array of standards and specifications with which the new entrant into broadcast audio confronts. Broadcasters have standardised on one or the other as a matter of history, and of good engineering practice as defined by specifying bodies.

But how many professionals understand the difference and the consequences of operating level? How does it relate to digital consoles, DBFS scales, and to the programme content?

EBU R68 as used throughout most of Europe specifies a test tone level at -18db DBFS which corresponds to a voltage output of 0.775V RMS. Most recognise this as the 0dbu reference, that is, the resulting voltage across a 600 ohm resistance in order for 1mW of power to be dissipated. A throw-back of the old days in the UK to the General Post Office (GPO) and BBC specification relating to balanced line feeds terminated into 600 ohms. What goes in, was what was supposed to come out - but it's okay leaving me!

SMPTE RP155 used throughout most of North America specifies a test tone level of -20db DBFS which corresponds to a voltage output of 1.23V RMS (+4db greater) recognised by users in North America as the 0VU reference. Therefore, it can be seen that things start to hot up very quickly at the business end of the scale if levels and standards are ignored. Allowing content to reach 0DBFS using EBU operating level corresponds to an output of 6.16V RMS (+18dbu), but using the SMPTE operating level 0DBFS corresponds to a colossal 12.28V RMS (+24dbu) - a +6db difference, in practical terms a voltage difference twice that of the EBU operating level. Consequences too for the audio console as output headroom becomes all the more important. Therefore console specifications of more the +24dbu output headroom should be available, typically +26dbu, or even better, +28dbu.

With colossal voltage levels appearing at the output possible, specifications relating to maximum (operating) output levels would generally be held at +12dbu (3.09V RMS). This corresponds to -6db DBFS EBU. This maximum output level applies to many of the European broadcasters. In North America, maximum (operating) output levels would be from +3dbu to +7dbu (1.10V RMS to 1.74V RMS). Therefore, employing good practice and the correct engineering standards whatever the operating levels will guarantee programme content is held within the dynamic constraints of the transmission path.

One further element to be considered however is the ballistics, dynamics, and integration time of the signal measuring instrument itself. The simultaneous use of the Volume Unit instrument (VU) and the Peak Programme Meter (PPM) with typical programme content will show marked differences in results. One may seem to underdrive, while the other might seem to overdrive. Therefore an appreciation of exactly what each instrument measures and how it responds mechanically should be realised.

The VU meter has a typical integration time of 300ms, simply, its ability to 'see' the signal and mechanically register its value takes about one-third of a second. For this reason the VU meter is considered to be relatively slow. However, the instrument detects averages not peaks - the Volume Unit so to speak. For signal paths terminating at an audio recorder using ferric oxide tape, the VU meter was considered by some a bonus, enabling maximum signal to tape in order to minimise the effects of tape noise and to maximise signal to noise ratios - tape saturation and print-through allowing (0VU at 320nw/m, DIN +4dbu). However, this does not help a transmission path where the output may be a radio transmitter. Often, a limiter device is employed to brick-wall the peak signals to the transmitter location.

The PPM on the other hand has a very fast integration time, typically a rise time of about 5ms, and a decay time of 1.5 to 2 seconds. As the name Peak Programme Meter implies, it detects peaks of the programme content. For transmission paths and transmitters sensitive to peak clipping, and at worst transient shutdown, it provided accurate monitoring and safety.

Measuring Average or Peak values has advantages and drawbacks, but providing good practice and recognised standards are employed consistently, and more important, understood, then all should be well. This being the case, the audio engineer is able to concentrate on the content and not whether their output is melting someone's transmitter, video tape or audio tape. This being true, then they can confidently say: "It's okay leaving me!"

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