Metering: Digital and Analogue

I noticed that the meters on two pieces of equipment I have used on this module are quite different to each other, and so I decided to research into metering to find out what the difference is, and what the need for the differences might be.

An article on the website Sound On Sound gives a good introduction to the two types of audio meters and what their differences are.

The simpler meter I have encountered is called a VU (Volume Unit) meter on the Wendt X3 mixer. The article explains that this was an early piece of metering technology, and the following quote has helped me understand how a VU meter works and why it might not be the most reliable way to check your levels when recording in the field:

Because the VU meter measures 'average' levels, a sustained sound reads much higher than a brief percussive one, even when both sounds have the same maximum voltage level: the reading is dependent on both the amplitude and the duration of peaks in the signal. In addition, the standard VU response and fallback times (around 300 milliseconds each) exaggerate this effect, so transients and percussive sounds barely register at all and can cause unexpected overloads.

 The other type of audio meter is that found on the Marantz PMD661 recorder. I've looked at specs online, but can't seem to find what type of audio meter this recorder has. However, the Sound On Sound article tells me that:

"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."

In a sound & camera workshop during this module, Ron explained that the 0dB reference tone the Wendt mixer can send to a recorder should meter at around -18dB on the Marantz. According to this information, this is because 0dB would measure very differently on the Wendt's VU meter to how it could on the Marantz's "true peak-reading" meter. This is important to know because, as the article explains, with digital recorders such as the Marantz, there is no headroom to rely on - if a sound peaks at or above 0dB for any time at all, digital clipping immediately distorts the signal for as long as the sound peaks. Unlike the various types of analogue distortion people often claim add a nice character to music, such as valve overdrive or tape compression, digital clipping is a harsh-sounding type of distortion that is never desirable on a recording, as this distortion cannot be removed as it is, in a sense, the sound of missing digital data.