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Audio.TST February 2009: APx500 Automation with LabVIEW, APx585/586 600 Ohm Output Impedance

Created 11 Mar 2009

Audio.TST February 2009

Notes from the Test Bench
Output
Sound Advice

Notes from the Test Bench

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

Hello addressee_placeholder

Two things I’d like to share.

First, I have some excellent news about a new lease program AP is introducing.

I think it’s fair to say that in difficult times, R&D budgets can suffer. No one doubts that times are tough right now, but I think everyone would also agree that we can’t just stop working and put our heads in the sand. It’s innovation and perspiration from people like you and me that are needed to get the economy moving again.

Under this new lease program, monthly payments allow engineers to get around frozen capital budgets and put the gear they need on their benches. We’ve made the program as flexible as possible (no down payment, no early buyout penalty, etc.) and at the end of the lease, you can own the instrument outright. I’m confident it can be used by just about anyone who’s currently having to choose between getting their job done and maintaining cash flow.

Leases are only available in the US for now, but we hope to expand this program to other countries in the future. Visit ap.com/sales/leasing, download the pdf, or ask your local AP partner for more information. AP's rental program is still available for US customers with short-term projects.

Second, speaking of AP Partners, I hope you’ll share in my congratulations to AP’s 2008 Partners of the Year.

US/Americas: Chris Gill, Northern California
International: Equipements Scientifiques, France

We made these selections based not solely on revenue, but on overall contribution to audio test. The winners distinguished themselves by offering seminars, coming up with creative solutions for tough applications, and generally going above the call of duty for their customers. It’s never an easy decision, as all of AP’s partners are very good at what they do. AP (and our users) are extremely fortunate to have such a network of audio experts at their disposal nearly anywhere in the world.

On behalf of the Board, the company and as an audio engineer myself, thank you.

Bruce

Output: Tech tips and new applications from AP

This month our Director of Technical Support and LabVIEW expert Joe Begin writes about the new APx LabVIEW drivers and the additional automation and output processing options that they bring to the APx Series.

APx500 Automation with LabVIEW

Audio Precision has developed a full-featured LabVIEW driver for its family of APx500 Audio Analyzers. The APx LabVIEW driver is a collection of LabVIEW subprograms that enable users to access the functionality available in the APx500 Application Programming Interface (API) with a minimum number of programming steps, using high level LabVIEW-typical functions. Using the driver, LabVIEW users can quickly and efficiently develop APx applications with clean, concise LabVIEW code.

LabVIEW is a graphical programming language and development environment from National Instruments. It uses icons instead of lines of text to create applications. LabVIEW programs are called virtual instruments, or VIs, because their appearance and operation imitate physical instruments, like oscilloscopes, volt meters, or FFT analyzers. Since it was first introduced in 1986, LabVIEW has grown steadily in popularity, especially for data acquisition, instrument control, and industrial automation. Today, thousands of engineers and scientists around the world rely on LabVIEW for applications in test and measurement, process control, automation, monitoring, and simulation.

Figure 1 Production test block diagram in LabVIEW using APx VIs.

A key reason for LabVIEW’s popularity is the easy connectivity it offers to a vast range of test and measurement instruments. In addition to supporting all the common interface buses (GPIB, RS-232, USB, FireWire, Ethernet, etc.), LabVIEW instrument drivers are available for thousands of test instruments from hundreds of vendors.

LabVIEW and APx in Production Test

The APx500 software has many test automation features built in, including the measurement sequencer, customizable user prompts, lockable projects, and the ability to call external applications. Users that want to go beyond the built-in automation features of APx500 can take advantage of its comprehensive API. The API is built on the Microsoft .NET platform, allowing custom APx programs to be developed in any .NET capable language, including Visual Basic, C#, and LabVIEW. With the driver, LabVIEW users can easily create automated audio test procedures by making simple block diagrams with very little programming (Figure 1). And experienced LabVIEW users will be able to take advantage of LabVIEW’s feature-rich library of front panel controls and indicators to create intuitive and highly customized user interfaces (Figure 2).

Production test ui
Figure 2 Production test user interface (custom LabVIEW VI).

So, if in addition to controlling a high quality audio analyzer, you need to control instruments like programmable power supplies, function generators, video devices, RF generators, mechanical actuators, switches, etc., LabVIEW and APx make a great combination.

LabVIEW and APx in Research and Development

LabVIEW has thousands of mathematical and scientific functions available, including VIs for waveform generation, waveform conditioning (filtering, DSP, etc.) probability, statistics, and much more. It also has extensive graphing capabilities. Combine this with the APx LabVIEW Driver which greatly simplifies acquiring and accessing APx audio test data from LabVIEW, and you have a powerful R&D platform. For example, if you want to make polar plots of a loudspeaker’s directivity response, do custom calculations on the spectral peaks from a multitone FFT, or plot the spectrogram of a chirp signal (Figure 3), LabVIEW and APx are a powerful combination.

Figure 3 Chirp Spectrogram

There’s even a driver VI to get back all FFT points from all channels of the APx signal analyzer measurement. But be careful! The APx supports FFTs up to 1 million points in size, and with channel counts as high as 16 in the APx586, that can end up being a lot of data.

Other LabVIEW Options and Resources

LabVIEW connectivity to Audio Precision instruments is summarized in Table 1 below. The most convenient way to access an instrument with LabVIEW is through a LabVIEW driver. In addition to the APx500 LabVIEW Driver, there are LabVIEW drivers for the 2700-G Series instruments and the Portable One/ATS-1. If, for some reason, you prefer not to use the driver, APx500 also allows interfacing to LabVIEW through .NET connectivity (using the APx API Developer Tools). The 2700 Series and ATS-2 also support LabVIEW over the APIB interface, using Active-X Automation (see TN-101). Finally, on units equipped with a GPIB interface (standard on the P1/ATS-1, and optional on the 2700 Series and ATS-2), LabVIEW users can control the instruments using low-level GPIB command strings.

	APx500	AP2700	ATS-2	P1/ATS-1
Driver	x	x		x
.Net	x
Active-X Automation		x	x
Low level GPIB commands (-G units)		x	x	x

Table 1 LabVIEW connectivity

Related Downloads

APx LabVIEW Driver
The APx LabVIEW Driver is a collection of Virtual Instruments (VIs) that provide LabVIEW users the ability to access the functionality available in the APx500 Application Programming Interface (API) using high level, LabVIEW-typical subVIs, with a minimum number of programming steps.

Sound Advice: AP Knowledge Base

APx585/586 600 Ohm Output Impedance

Question:

We want to use the APx585 to test some older professional audio equipment that is specified using a signal generator with a 600 Ω output impedance. The APx585 only offers a fixed 100 Ω output impedance. What can we do?

Answer:

Unlike its 2 and 4-channel cousins, the APx520/525 and 521/526, which offer 40, 100, 150, 200, and 600 Ω output terminations, the balanced outputs of the APx585 and 586 are fixed at 100 Ω. This is due to the high channel count in these units—there just wasn’t enough room to add the resistors and relays to make all the output termination choices available. It’s easy, however, to achieve 600 Ω output impedance by using external resistors. The most convenient way to do this is to mount the resistors inside an XLR barrel adapter, such as the Switchcraft S3FM (Figure 1).

Image of switchcraft adapter.
Figure 1 Switchcraft S3FM barrel adapter.

Not only is this convenient, but putting the resistors in individual barrels prevents crosstalk problems. You will need one adapter for each channel to be converted. The wiring diagram is shown in Figure 2. For balanced outputs, each of the two terminals of the balanced connection should have one half of the impedance. Since we are adding 500 Ω to the 100 Ω that already exists, two 250 Ω resistors are required. We recommend 250 Ω 1 Watt resistors with a tight 0.1% tolerance. You should use heat shrink or plastic tubing on the resistor leads to prevent shorting, and connect pin 1 (ground) to the shell terminal of the adapter at the female end.

Figure 2 Barrel wiring diagram. R=250 Ohms.