Created on 2008-07-01 20:53:00

Using Multitones in Audio Test

AP’s manager of Technical Support, Joe Begin, looks at the theory behind multitone audio test.

Multitone testing is the foundation on which HST, Audio Precision’s High Speed Test Application for the 2700 series and ATS-2 audio analyzers is built. HST was featured earlier this year in the January issue of Audio.tst.

As the name implies, a multitone stimulus signal consists of multiple sine waves (or tones) at different frequencies combined together. Any number of tones can be used, but 3 to 30 tones spaced logarithmically across the audio band is typical. This feature alone offers several advantages over traditional single tone testing:

1. It allows the performance of a DUT to be evaluated over a range of frequencies with a single measurement.
2. The characteristics of a multitone waveform (e.g., spectral content, histogram, crest factor, etc.) give it a much closer resemblance to typical audio program material like music or speech, than a single sine wave.
3. Multitone distortion is generally better at detecting real-world problems involving clock jitter and sample rate conversion than traditional THD+N measurements.

But the real power of multitone testing comes from the speed advantage that it provides. Multitone testing can result in test speeds that are 10 to 100 times faster than conventional swept sine techniques.

Multitone testing uses FFT analysis and some special digital signal processing techniques to derive a number of measurements from a single acquisition. First, the multitone signal is made “synchronous” with the FFT transform buffer. A synchronous signal results when the frequency of each sine wave present is chosen such that an integer number of cycles of the waveform fits exactly into the FFT transform buffer (Figure 1). With synchronous acquisition, a window-less FFT can be performed resulting in a leakage-free measurement: i.e., for each sine wave present, all the signal energy is contained in one FFT bin, with no leakage to adjacent bins (Figure 2).

Figure 1. A synchronous sine wave.

Figure 2. Example of a multitone spectrum with logarithmically spaced tones.Once a leakage-free FFT has been performed, a number of measurement results can easily be extracted. If the individual tones in the multitone are spread across the spectrum, the amplitude and phase of the FFT bins corresponding to these tones provide the frequency response and phase response of the DUT. The FFT bins between the tones will contain only distortion products and noise. Power-summing the amplitude of these “non-tone” FFT bins yields the total distortion (both harmonic and intermodulation distortion) plus noise.

Multitone testing can also be used to derive crosstalk measurements in a stereo DUT. For this application, a stereo multitone is constructed with tones in one channel at different frequencies from the tones in the second channel. Then, the power at frequencies unique to Channel 1 is measured in Channel 2 and vice versa. This yields a measure of the crosstalk at these frequencies.

Multitone analysis can take advantage of another property of FFT processing: If the FFT transform buffer is made twice the length of the generator buffer, the analyzer will have twice the frequency resolution of the generator. Using this scheme, even numbered FFT bins may contain fundamental tones, distortion products of those tones, and noise. Odd numbered FFT bins, however, can not contain generator-related signals; i.e., the odd numbered bins can only contain noise. This provides a very powerful means of measuring a DUT’s noise in the presence of signal.

Figure 3. Example of multitone measurements available for a signal with a 1.5 kHz fundamental frequency.

While multitone testing often provides significant advantages over traditional test methods, it’s not always the best choice. For example, in an R&D environment, you may need to measure overall distortion performance, or you might want to use the simplest stimulus possible. In this case, traditional single tone sine testing is a must.

So what audio measurements are good candidates for multitone testing? The most obvious ones are those where test time is at a premium – for example, production line environments where a large number of devices need to be tested in a minimum amount of time. Another often time-sensitive application that is well suited to multitone methods is power amplifier testing. Sometimes, an amplifier can only be driven at high power levels for very short periods of time – for example, if one wanted to evaluate a power amplifier’s performance before heat sinks have been installed. In cases like this, multitone testing can be a powerful tool.

Broadcast applications are also well-suited to multitone testing. To minimize program interruption, a multitone stimulus signal can be made very short (less than one second long). And, as mentioned above, multitone stimulus signals are inherently more “program like” than a single tone. A multitone can be made even more program like, by varying the amplitude of the individual tones to more closely match the spectral content of music or speech.

Other applications where multitone methods should be considered are testing of devices whose noise level varies with signal level, such as noise gates, compressors, and some DACs which shut down with zero digital input. For these devices, measuring noise in the presence of signal is a fundamental requirement, and multitone testing provides this important capability.

Audio Precision’s AP2700 series and ATS-2 analyzers provide a rich multitone feature set, with many multitone measurements built-in. AP’s multitone (or Fasttest) analyzers also include a number of features that provide the flexibility to tackle virtually any multitone measurement, including sophisticated multitone triggering and synchronous, windowed or frequency-error-corrected processing. In addition, a powerful multitone creation utility is included to help users build an optimum multitone waveform for their test.

If you haven’t yet used multitone testing for your audio application, we encourage you to consider it. And audio test engineers with production line test needs should be sure to check out HST. It provides a special multitone stimulus bundled with a convenient User Interface and some additional measurement results.