Audio Measurement with HP-41

by Christoph Klug

The HP-IL Interface Specification 82166-90017 describes a scenario for realizing audio measurement with HP-IL : The application use the HP-85 Personal Computer, controlling the HP 3468A IL-Digital Multimeter for measurement of AC voltage levels. Additional HP 82166A IL-Converter is interfaced to a Multiplexer-Device for connecting Digital-Multimeter to different points of measurement object, controlled by software.

Today the modular, battery powered I/O-Board make possible similar measurement application with minimal hardware expense. Chapter XVI of I/O-board manual contains detailed informations : By using modern analog IC´s we expand the existing I/O-Board plug in module family by two special modules. The Micro Linear ML2038CP is a one chip sine-wave-generator (internal wave-table-rom and 8 Bit DAC), controlled by parallel 16 Bit data word. And for measuring AC voltage levels we take the SSM2110 RMS-Log-Converter chip (from Analog Devices) as frontend for the existing 12 Bit ADC-Module.

The rest is done by software : For HP-41 we program control routines for setting frequency of sine-wave-generator, and for reading voltage level to X-Register. Working with high accuracy reference measurement equipment during calibration (Rhode & Schwarz Audio Analyzer UPA 3) and using curve fitting routines (HP-41 AEC-Rom) our low cost system reach amazing performance : Generator frequency runs from 5Hz to 60KHz with 1Hz resolution. Measurement of AC voltage levels runs from -80dB (=0,1mVeff) to +20dB (=10Veff), typically accuracy is +0,3dB !

For determining the complete frequency response (for example of a Hifi-Device) we need additional software. By entering start- stop- and increment-frequency we execute an automatic measurement sequence. Voltage level readings may be stored in X-Memory data-files. But much better is working with CCD-Module matrix-files. A measurement sequence creates two matrix-files, one for parameters (name, date, time, start- stop- increment-values ...), belonging second matrix-file containing measurement data (AC voltage levels).

By using CCD-Module math-matrix commands we program a helpfully math-function set for calculating frequency response data : Invers frequency response, adding frequency response data, shifting, finding max peak, smoothing (for acoustical measurement). For completing this audio-measurement system we add some HP-IL Devices : IL-Cass-Drive for extensive data storage, IL-Printer and IL-Plotter for data output. Using HP-IL*PC Interface Card, we transfer measurement data to PC, making possible data post processing with EXCEL.

Part III of IL2000 manual continues chapter XVI of I/O-Board manual and describes additional HP-41 software for carrying out third octave frequency response measurement. Additional software make possible Multi-Plots using IL-Plotter. Furthermore some optional hardware extensions given like adapting a microphone for acoustical measurement or an expanded input multiplexer section with high impedance line level input, 10KW input, 16W input and a balanced input.

For more informations about using HP-41 for Audio Measurement write to :

Christoph Klug Körnerstraße 47 B 31141 Hildesheim Germany

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Page last modified : 10th March 2000