digital VU meter Part 1
This VU meter design considerations 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. The circuit presented in this two-part article is a variant of this meter that can for direct measurementsof digital audio signalsThe introduction of digital audio (CD, DCC, DAT, MiniDisc) in the 1980s has drasticallybe inserted directly inseries with the a.f. sig-changed the world of audio and hi-fi. Manynal line. Its specifica-analogue circuits have been replaced by black boxes like
tions are reminiscent ofprofessional equip-digital filters and sig-nal processors. The a.f. data has been ment. We are notchanged from a seriesentirely certain, but of waveforms to a The present vuthink that this is the firsttrain of binary digits meter is geared to the new tech-(bits). nology. Where in earlier times a net-Brief technical data Display double alphanumeric 31/2-digitdouble 30-segment led bar with peak indication Brightness individually presettable138.5 dB (with 24-bit input) ResolutionInput 32 kHz, 44.1 kHz, 48 kHzMeasuring range 0.1 dB (±0.005 dB) S/PDIF 16–24 bit; i2s 16–24 bit Sampling frequenciesMeasurement peak, PPM, and RMS Status indication by 10 LEDS on front panel DIY VU meter ever pub-lished in a magazine.* Sony/Philips Digital Interface Format – the consumer version of the AES/EBU standard. This stan-dard was devised by the American Audio Engineering Society (AES) and the European Broadcasting Union (EBU) to define the signal format, electrical characteristics and connectors to be used for digi-tal interfaces between professional audio products.
(text) Contentswork consisting of a capacitor, a resis-The basic design of the meter is shown in the block diagram in Figure 1. A large part of the meter is taken up by the displays. Apart from the two 30-LED bars, there are also two 31/2-digit-wide alphanumeric displays. Both dis-play groups are controlled by a dedi-cated controller from Maxim, the Type MAX7219. The LED bars give a a good visual indication of the signal level: they sim-ulate the moving pointer of VU meters of yesteryear. The alphanumeric display shows the peak level measured during a recording session. There are also several LEDS that in-dicate which functions of the meter have been selected. The input of the meter is formed by an S/PDIF* receiver connected to a mul-tiplexer. Several inputs of the multi-plexer are (as yet) unused, but are in-tended for connecting an analogue-to-digital (A/D) converter which we hope to publish in a future issue. Elektor Electronics 4/96 tor, a diode and a mini moving-coil meter was used for level indication, in modern equipment this network is re-placed by a digital signal processor— DSP. This results in a rather more com-pact instrument that gives excellent performance. The VU meter is based on a Type 2105 DSP from Analog Devices. This 16-bit device is designed and pro-grammed 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 ob-viates the use of special components (to keep any errors down). This arrangement also keeps the cost down and results in a compact, flexible meter.