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Q. Why do the meters of my preamp and those of my DAW give me different readings?

Most analogue 'peak meters', including those on the Focusrite ISA Two, shown here, are actually 'quasi-peak' meters, which display an average value, but with a much faster integration time than you'd find on RMS meters. So their indication of signal levels with real sources will not be the same as on sample-peak meters in a DAW.Most analogue 'peak meters', including those on the Focusrite ISA Two, shown here, are actually 'quasi-peak' meters, which display an average value, but with a much faster integration time than you'd find on RMS meters. So their indication of signal levels with real sources will not be the same as on sample-peak meters in a DAW.

I use a Focusrite ISA Two preamp with a Focusrite Clarett 4 Thunderbolt interface. The ISA Two has a control to adjust the sensitivity of the meter so it can be aligned with that of the DAW, but I'm having trouble getting the two meters to match. I can get them to align when I use a test tone but when I switch to using a microphone, the mic level appears way too hot on the DAW meters compared with what the meter on the ISA Two is showing me. I tried aligning the meters using white noise and that was even worse! Have you any ideas what the problem could be?

SOS Forum post

SOS Technical Editor Hugh Robjohns replies: I suspect this is the common problem of comparing 'programme material' on meters with different characteristics and ballistics. There are a great many different types of audio meter, even though many share the same apparent decibel scaling. Some are true-peak reading, some are quasi-peak reading, some display the true-RMS level, some a quasi-RMS level, and some now indicate 'perceived loudness'... and they can all have different attack (integration) and recovery times.

As a result, different meters can give very different indications — often 6-12 dB different — when displaying music or speech (or noise), simply because some are responding to the actual waveform peak amplitudes and some only to an approximation of the waveform's peaks. Others, like the classic VU meter, try to give an indication of the signal's 'energy' (which relates to the perceived loudness) in one form or another, so they ignore the amplitude peaks and show higher readings for sustained sounds. If you're to interpret their indications, then, it's important to understand and appreciate the types of meter on different devices and recognise how they respond to typical programme signals.

Your description implies that the DAW is indicating actual peak levels, while the ISA Two's 'peak meter' is responding slightly slower and showing quasi-peaks, as is typical of analogue peak meters — so it will inevitably miss the very fastest transients, and tend to under-read on music and speech. Provided you're working with sensible headroom, it isn't a problem: you just need to learn to recognise a typically healthy level on each type of meter.

As different kinds of audio signal have different relationships between their peak and RMS (effectively the average) amplitudes, we can only use a steady sine wave tone to check and calibrate the alignment of different meters. A sine wave has a reliable and known 'crest-factor' (the difference between the waveform's true peak amplitude and its RMS value) of 3dB. Because it's a steady, unvarying sound, the meter's attack and recovery ballistics are irrelevant too, so manufacturers can calibrate any type of meter to read at the reference mark when monitoring a signal of the correct alignment level for the equipment. That means we can easily adjust equipment input and output levels to ensure that signal is at the optimum operating level (safely below the clipping point, but well above the noise floor) for each device in a signal chain. The usual test tone frequency is 1kHz, but 440Hz is often used instead because it's easier on the ears and doubles as a handy tuning reference! And the standard alignment level is normally either +4dBu or, for European broadcast environments (and their metering systems), 0dBu.

I assume you're trying to achieve a reasonably consistent meter display between the preamp and DAW, so that you can set levels on the preamp without fear of overloading the interface. Given that the two metering systems clearly respond differently to programme material, the best way to achieve your goal is to ignore the recommended tone alignment and adjust the ISA Two meter by eye. Try recording some typical solo material (vocals, guitar or whatever) and adjust the ISA Two's meter trim control to provide a similar indication to that on the DAW when you're recording optimum signal levels. It will never match perfectly but you should be able to find a setting that's close enough for reliable operation. And remember, if you ever need to recalibrate the meter to a standard reference, always use a static sine tone!

Published March 2020