G'day,

Last time we looked at ' VU.' - Volume Unit meters. Now we'll take a look at Peak Program Meters or 'PPMs'.

PPMs are a 'Bargraph' type of meter using LED's as indicators. They are more expensive than to produce than VU meters because of the much more elaborate circuitry, and partly because of the precisely defined characteristics of the physical meter itself. [Technically, the specifications state that 'Type I' meters have an integration time of 5mS and take between 1.4 and 2.0 seconds to fallback 20dB from displayed peaks. 'Type II' meters have double the integration time figure and should fall 24dB within 2.5 to 3.1 seconds…. Phew!!]

As they have no mechanical damping affecting the meter's operation, the instantaneous peaks of the programme material can be more accurately displayed and monitored on PPMs than moving-coil meters. For example, cymbals will show little movement on moving-coil VU Meters, yet their high transient level signals can easily overload electronics. By using a PPM meter we are able to more closely monitor the signal's peak values.

There are limits to PPM ability to catch the very fastest of transient peaks though. As a result, PPMs are often referred to as 'quasi-peak' meters. They can only show transients which are sustained for a defined time. The result of this is that the levels of transients will usually exceed the PPM reading by between 4dB to 6dB. This design encourages overall programme levels to be driven slightly higher - giving better signal-to-noise performance. As we saw last week, overloading the briefest transients will not cause overloading, and distortion will be inaudible.

Many Desks with bargraph LED's for meters have the ability to switch between VU. and PPM representation. A good compromise is regularly incorporated in metering which is a combination of both types called a 'Peak and Hold Meter'. This constantly displays the average or VU value of the signal - as a solid bar, while momentarily 'holding' the peak or PPM value at the top of the meter - as a floating 'dot.' This dot often has a temporary or permanent 'hold' function to give us time to notice the peaks! [Technically PPMs are Type II meters which incorporate a delay of between 75mS and 150mS before the fallback occurs, helping to reduce headaches and sore eyes!]

Metering And Loudness
Although the VU meter was designed to give us an indication of volume, level meters in general display information about signal voltages rather than what our ears hear. (The 'perceived loudness'.)

Our perception of loudness depends not only on the level of a signal, but also on its frequency and bandwidth. The wider the bandwidth, the louder a signal seems to be, even if its peak levels don't change. Our ears are most sensitive around the 2-4kHz region, so signals in this frequency band will sound much louder than low or high-frequency signals of a similar peak*. For example, signals centred around 100Hz or 10kHz can be almost 15dB higher in level than those around 4kHz, yet all will be perceived as sounding equally loud!

Digital Metering?
All analogue meters have a nominal alignment point - the zero reference - with a notional headroom above. The idea is that signal peaks are routinely allowed into the first 8dB or so of this headroom, though peaks of +12dBu will usually start to cause distortion which becomes more and more noticeable with increasing level until clipping occurs, usually at between 18dBu and 22dBu.

Unfortunately, the nature of digital systems is such that even the briefest of signals above '0' - technically 1Volt - will cause REALLY UGLY DISTORTION!! So neither VU nor PPM metering is accurate enough. The majority of digital recorders, mixers and converters therefore use true peak-reading meters whose displays are derived from the digital data stream. As these don't rely on analogue level-sensing electronics they can be extremely accurate.

Well that's about it for this time.

Until next............have a good one !!

Willy T. This is extracted from the book: "LIVE-AUDIO WORKSHOP: A Hands-on Guide to Rigging and Operating PA Systems." ISBN 0 646 36408 1. © Giraffe Communications 1998. Order via e-mail or at all good Music & Book shops.