Signal Chain Basics Audio metering enjoying the VU

Posted on Feb 5, 2014

Audio metering is present in almost any equipment that plays or records audio from cell phones displaying bar graphs of audio output levels, to home stereos with flashing LEDS, to live broadcasting. There are multiple standards for audio metering that deal with the levels being monitored and time constants used for calculating the average. The

Signal Chain Basics Audio metering enjoying the VU
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behavior of volume unit (VU) meters is defined in ANSI C16. 5-1942, British Standard BS 6840, and IEC 60268-17. These standards define 0 VU = +4d Bm, with an integration time of 300 ms. Traditionally, with a moving needle meter, this means that from zero signal to 0 VU, the needle should reach 0 VU within 300 ms (same for decay). This makes a traditional VU meter excellent for getting an idea of loudness, but a poor method for monitoring transients. This was fine in traditional broadcast and recording systems, mainly based on tubes and tape, both of which tend to saturate elegantly. In today`s digital systems, clipping inputs or outputs sounds awful, so peak program meters (PPM) are used. PPMs are similar to VUs, but, integration time is much faster (typically around 10 ms). Note that there are multiple standards bodies with definitions for the signal levels and exact integration times. Display technology is a big part of these standards. Getting the ballistics right to move a needle through 90 degrees (or more) with one percent of accuracy is much harder and slower than lighting a few LEDs! Using LEDs, the quality of audio monitoring varies drastically. For example, a simple voice recorder product requires a different metering system from a mini-micro system using LEDs as a visual effect. In a live sound or broadcast environment accuracy is important, as well as seeing the difference between average loudness and the...

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