Simple VU Meter

Most of us have over the years become familiar with the nervously moving pointers or LED bars of the VU (visual unit) meter on the front panel of a cassette tape recorder or mixing panel that indicate the level of the a.f. signal. A simple VU meter circuit can be purchased at various places - such meter can be inserted directly in series with the a.f. signal line. the specifications of such device are reminiscent of professional equipment. The introduction of digital audio (PC audio, CD, DCC, DAT, MiniDisc) in the 1980s has drastically changed the world of audio and hi-fi. Many analogue circuits have been replaced by black boxes like digital filters and signal processors. The a.f. data has been changed from a series of waveforms to a train of binary digits (bits). The mentioned VU meter is geared to the new technology. Where in earlier times a network consisting of a capacitor, a resistor, a diode and a mini moving-coil meter was used for level indication, in modern equipment this network is replaced by a digital signal processor DSP. This results in a rather more compact instrument that gives excellent performance. Most VU meters are based on a Type 2105 DSP from Analog Devices. This 16-bit device is designed and programmed to enable data to be pro-cessed with a 64-bit resolution. This means that 24-bit wide data are processed with an arithmetical error that, in the end result, is smaller than 0.025 per cent. The arithmetic is carried out fast and accurately. The speed of it is provided by an integral multiply accumulator (MAC). For example, the multiplication of two 16-bit numbers which must be retrieved from the memory and the adding of the result to an existing number or storage into a memory location takes rather less than 100 ns. Since the VU meter is intended for measuring digital a.f. signals, it itself is designed on digital lines. Also, the processing is controlled by software wherever possible, which obviates the use of special components (to keep any errors down). This arrangement also keeps the cost down and results in a compact, flexible meter.

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