Measuring Non-flat Devices With Equalized Multitones

Created on 2010-04-20 23:13:00

When testing certain types of devices or systems that are known to have a non-flat frequency response (e.g. phonograph preamplifiers and other pre/post equalized signal chains), it is often desirable to inversely equalize the audio test stimulus signal, so that frequency response measurement of an ideal device appears as a flat curve.

The flat curve makes it easy to visualize and quantify defects in a device’s equalization circuits. In many cases, equalizing the stimulus signal is superior to the alternative of mathematically correcting the results, as it keeps the stimulus signal within the device’s normal operating range at each frequency, thereby preventing errors due to overload. In the case of a phonograph preamp, for example, a 20 Hz input stimulus may clip at only 30 mV whereas a 20 kHz stimulus might not clip until it exceeds 500 mV

Using the APx Multitone EQ Utility

The utility creates an equalized multitone stimulus signal, with the inverse response curve calculated either by measuring an existing device with an APx analyzer, or by importing a Microsoft Excel Worksheet file with frequency and level data.

Figure 1  APx Multitone EQ Utility using “Measure with APx” as the data source.

When “Measure with APx” is selected as the data source, the APx signal path and measurement boxes default to “Signal Path1” and “Multitone Analyzer”. Note that the active APx project must contain a multitone measurement within the specified signal path—if one is not present, then it should be added to the measurement navigator.

Figure 2  Measured frequency response of the reference device (bottom), with the derived inverse equalized stimulus (top).

The reference device is now connected to the analyzer, after correctly setting the signal path connections and generator level. Any subsequent device tested using the equalized multitone that we are about to create, will measure as flat if exactly matches the reference. Click the “Measure Frequency Response” button to perform a multitone measurement and collect the frequency response data.

Figure 3 Retesting the reference device or a perfect clone, using the equalized multitone, will result in a flat frequency response curve.

Now that the data is collected, it’s time to create the inversely equalized multitone. Select the desired sample rate and bit depth, assign a filename, and choose if you want the file automatically uploaded to the APx generator upon creation (recommended). With the equalized multitone waveform loaded into the generator of the Multitone Analyzer, retesting with the same device should result in a flat frequency response.

Figure 4 APx Multitone EQ Utility using “Read from APx exported file” as the data source.

If “Read from APx exported file” is selected as the data source, you can click the “Import…” button to specify the desired Microsoft Excel workbook file. The workbook data must be in a specific format—to make a template, run the APx Multitone Analyzer measurement and then choose “File | Export Graph Data” to create an Excel workbook. Then, open the workbook, go to the Relative Level worksheet, and replace the data in the Y column(s) with the desired numbers.

Figure 5 Microsoft Excel worksheet with exported graph data.

Typical reasons to edit the data are to correct for errors in the reference device’s response, or to use a curve that is a published standard. If the frequency you actually want to modify is not listed in the worksheet, then you will need to interpolate accordingly. The frequencies included in the APx 32-tone multitone are essentially the ISO preferred 1/3-octave frequencies from 20 Hz to 20 kHz. Note that if the Relative Level source data contains only one channel, then both the left and right channels of the stereo .wav file will be equalized using the Ch1 data.