Audio Multi-tone testing using FFT analysisdScope Series III using synchronous multi-tones is fast and powerful. It allows many parameters of the equipment under test to be measured simultaneously, using a single stimulus waveform, and yields results much more quickly than sweeps or sequences of spot measurements.
Synchronous multi-tone analysis is based on calculating an FFT of a captured multi-tone stimulus after passing it through the equipment under test. The complex nature of the stimulus means that many different properties of the equipment under test can be measured from a single, short acquisition of a buffer of audio data. The results are calculated by means of a frequency-domain analysis of the captured buffer, using a Fast Fourier Transform (FFT) algorithm.
Each tone in the generated multi-tone waveform is arranged to repeat exactly over the number of samples in the FFT buffer. This means that a rectangular FFT Window function can be used, and each tone will occupy a single 'bin' in the resultant FFT, without leaking into adjacent bins.
To achieve this, the requested tone frequencies may need to be slightly modified in order to correspond to the bin centres. The larger the size of the FFT buffer, the smaller will be the maximum necessary frequency modification. Furthermore, by arranging that each tone occupies an 'even-numbered' bin in the FFT, it is guaranteed that all harmonic and intermodulation products which result from the tones also occupy other even-numbered bins, and can thus be measured independently from the general noise floor, which occurs in the odd-numbered bins.
Frequency response, ripple etc. can be measured by comparing the relative amplitudes of the recovered tones. Balance-related measurements can be made by comparing FFTs from the two channels. By generating certain tones at slightly different frequencies for each channel, it is possible to measure inter-channel cross-talk at these frequencies.
The power of the technique lies in the fact that all these measurements can be calculated simultaneously and for both channels, after acquiring a short buffer of the recovered multi-tone signal from the device under test. Using multi-tone analysis, it is possible to make many different measurements at the same time – for example you could measure: Distortion, Noise, Total Distortion + Noise, Frequency Response, Ripple, Gain, Channel Balance and Cross-talk on both channels at the same time, view the results in either graphical or numerical form (or both), and check them against your acceptable limits, all in a few seconds!
Until now, multi-tone analysis has been difficult: it has been possible only with expensive and specialised equipment, which has been difficult to program. Merely generating appropriate multi-tone stimuli with such equipment has been complicated enough, but tailoring the required analysis functionality has been next to impossible. dScope provides the solution, bringing multi-tone techniques within the grasp of any operator, without the need for ANY programming whatsoever. Using dScope’s 'Multi-tone Generation and Analysis' feature, you simply tell the dScope the range of tones you want to generate, and what numerical or graphical results you need – dScope does the rest.
All this works thanks to versatility of dScope's VBScripting capabilities within the Signal Generator and Analyzer, but when you use the 'Multi-tone Generation and Analysis' dialogue box, dScope writes and loads the scripts automatically, sets up all required parameters, and arranges the Readings and Traces on the screen.
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