Audio.TST Archive

By Bruce Hofer, Chairman & Co-Founder, Audio Precision

Hello Reader

Now available- Technote 104: Introduction to the Six Basic Audio Measurements

Technote 104 discusses methods of making a set of basic measurements using an Audio Precision 2700 Series or ATS-2 audio analyzer. Whether keeping to "the big six" or developing more esoteric measurements, a strong understanding of the principles behind these basic tests will ensure that users make the measurements correctly and have a firm foundation when employing techniques such as FFTs, MLS, multitone, and log chirp.

The six measurements covered in the technote are:

• Level
• Frequency Response
• THD+N
• Phase
• Crosstalk
• Signal-to-Noise Ratio

More advanced tests can be found on the Audio Precision Resources Disc, available on the Software section of the AP.com website. Users looking for more technical audio test content can also download the industry-standard Audio Measurement Handbook.

APx Owners:
This Technote is directed towards users of a 2700 Series or ATS-2 only. With its more advanced interface, APx users can simply select the measurements they need from the Measurement Navigator and run a sequence of the tests they need to make.

Coming soon:
Next month we'll release the new High Speed Test v2.0 application, designed to make production test faster and easier. We're also working on an update to Technote 4: Loudspeaker Testing.

If you have an idea for a Technote or some audio test problem that needs a solution, please email AP Tech Support.

Downloads & Links

• Technote 104 from the AP Audio Resource Library
• AP Audio Resource Library

Tutorial: Using LabVIEW with a SYS-2722

AP Senior Applications Engineer James Williams runs through a practical example of taking a THD+N measurement with a 2722 analyzer controlled through LabVIEW. This tutorial assumes the user has a basic knowledge of LabVIEW and its measurement architecture (visit the National Instruments website for LabVIEW help). For initial setup of your 2722, please download the Getting Started Guide for the AP2700 LabVIEW driver from the Software section of the AP website.

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Initialize and Analog Generator Setup
When developing a custom VI (LabVIEW virtual instrument) that includes the AP2700 driver, the first step is to define the VISA resource for the instrument and connect the resource to the AP2700 Initialize VI. Find the GPIB address to which the 2722 is connected (in this example we're using Address #2) and enter the string as shown into the VISA Resource control.

Note: For each VI setting, you can either look at the Front Panel view or its equivalent icon in the Block Diagram view.

When this VI is run you will hear a series of relay clicks indicating that the instrument is set to its default state and is ready to accept measurement commands.

The first step in taking a measurement is to set up a stimulus, in this case we will need a 1 kHz 1Vrms sine wave, which we will obtain via the AGEN SINE VI. AGEN SINE is a VI created by Audio Precision. Like all the VIs in this tutorial, it's available for download at http://ap.com/ap/download/AP2700_LabVIEW.zip.

The AGEN SINE VI requires parameters to configure the output voltage and frequency values. Use the AGEN SINE VI Control Panel to input the values and configure the hardware connectors (in this example we're using balanced BNCs).

This VI completes the signal generator portion of the measurement.

AGEN SINE VI: Front Panel View

AGEN SINE VI: Block Diagram View

Analog Analyzer Setup
Now we need to set up the analyzer to take a THD+N reading.

The first step here is to define the physical connectors for analyzer input. Here we use the AP2700 ANLR INPUT VI, the source can be Balanced XLR, Unbalanced BNC or generator monitor. Again, use the VI's Control Panel to define the parameters.

A more complex analyzer input VI would look like the one below, with selections for weighting filters and other input test conditions:

Taking the reading
To take a reading of the measurement that has been set up, click the "Run" command on the LabVIEW tool bar. The front panel of the AP2700 ANLR FUNC M VI returns the THD+N value as a numeric double. In this example, the value of .0205% was returned.

Here is the block diagram view of the entire THD+N VI

Advanced example VIs
To see a more advanced VI design, open the THD+N Vs. FREQUENCY VI created by Audio Precision. This VI is one of several advanced sample VIs saved in the Examples folder of the AP LabVIEW Driver zip file.

Downloads

• THD+N sample project (.VI file)
• AP2700 LabVIEW driver (ZIP file)
• LabVIEW driver for AP2700 Getting Started Guide (PDF)

Events

• NAB Show 2008 | Las Vegas
  April 14-17
  Visit the show website

AP in the news

• Bruce interviewed in Electronic Design
  http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=16804
  
• AP at the AES show
  http://www.eetimes.com/news/semi/showArticle.jhtml?articleID=202401094

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