Audio.TST April 2009 APx DSIO Configuration Files, External Sweeps with Noisy Signals on the 2700 Series and ATS-2

Notes from the Test Bench

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

I’ve just returned from NAB. Not a great turnout, but that’s to be expected. The good news is that everyone who came by the booth was working on new designs. There was a lot of interest in APx, especially around HDMI and multichannel I2S.

Meanwhile, Tech Support has released the first batch of APx digital serial config files. You can read more about that below.

Next up is AES in Munich May 8-10. We’re looking forward to giving a preview of the latest APx software release, including several digital audio protocol analysis features that will greatly assist with debugging transmitters and receivers.

I look forward to hearing what you think.

Bruce

Output: Tech tips and new applications from AP

APx DSIO Configuration Files Released

We recently introduced the new APx Digital Serial I/O module, which lets you do chip- and board- level serial audio testing with our APx Series analyzers. Now we've released the first set of DSIO Input and Output configuration files which help to make serial testing even faster and easier.

DSIO transmitter settings.

The DSIO configuration files include pre-configured and tested settings for a range of popular A/D, D/A, and sample rate converters. Loading the appropriate .stx (serial transmitter) or .srx (serial receiver) file takes care of all the serial settings on the APx instrument. We’ve supplied the files for each chip in a range of protocols (LJ, RJ, I2S, TDM) and sample rates. If you need a variation that isn’t supplied—for example you want a 96 kHz sample rate and we've supplied 192 kHz—simply start with the 192 kHz configuration file and make any necessary changes needed for the new sample rate. Then, save it under a new name for re-use later. Besides existing as stand-alone files, configuration files are automatically embedded within APx projects.

DSIO evaluation board settings chart.

A chart with all the jumper and DIP switch settings for each chip’s evaluation module is also included. This saves a lot of time spent searching through datasheets and manuals to set the multitude of board options.

The configuration file package will be regularly updated – whenever you download it, you’ll get all the latest chips we’ve added, plus the previous ones. If the converter you’re testing isn’t included, you can still set up the DSIO panel and the evaluation board – just not as rapidly.

Related Downloads

• APx DSIO Configuration Files
  This file collection includes DSIO presets for a number of A/D and D/A converters, using various protocols and sample rates. Also includes a chart with evaluation module jumper and DIP switch positions. Updated on an ongoing basis.

Sound Advice: AP Knowledge Base

This month's Sound Advice column is contributed by Applications Support Engineer William Rich.

Making External Sweeps with Noisy Signals on the 2700 Series and ATS-2

Introduction

Many real world signals, like FM radio, are noisy. When performing an external sweep, a noisy signal can prevent the frequency meter from settling. Since external sweeps depend on accurate frequency readings to identify tones and plot data points on a graph, the sweep may fail to start or may miss points.

Figure 1 AP2700 Sweep and External Sweep panel configuration.

To better understand this, we’ll review how an external sweep measurement is conducted. The sweep panel configurations we are using for the SYS-2722 and ATS-2 are shown in Figures 1 and 2 respectively.

Figure 2 ATS-2 Sweep panel configuration.

The sweep starts by looking for the first measurement point, as defined by the sweep start parameter. For this example, this frequency can be anywhere between 19 and 21 Hz, according to the 5% tolerance setting on the AP2700 external sweep panel, and the spacing setting on the ATS-2 sweep panel. The external sweep algorithm monitors the Source 1 instrument parameter (Frequency A meter) for the required value. When it gets a settled reading for the frequency, it then looks for settled level readings from the meters defined by the Data 1-6 instrument parameters (Level A). When it gets those, it checks the Source 1 meter again to see that the value (Frequency) has not changed. If the 20 Hz signal is stable, and the settling parameters for the meters are satisfied, then the 20 Hz measurement is recorded and the sweep continues by waiting for the next frequency as allowed by the panel settings.

Now, if excessive noisy signals from the DUT (device under test) cause us to have unstable readings that interfere with proper triggering of the sweep, what can we do?

Harmonic Distortion Analyzer Solution

Both the AP2700 and ATS-2 control software have a DSP program that can make real-time frequency measurements which are virtually immune to noise. The Digital Analyzer “Harmonic Distortion Analyzer” shown in Figure 3 uses FFT technology to make its measurements. The frequency meters display the frequency of the highest-amplitude component of the applied signal. Because noise is usually at a much lower level than the level of the highest-amplitude component of the stimulus signal, the readings are stable and the settling algorithms produce a settled reading quickly.

Figure 3 ATS-2 Harmonic Distortion Analyzer.

To set this up, on the sweep panel change Source 1 to the Distortion meter’s Channel 1/A Fundamental Frequency (Distortion.Ch.1 Fund Freq in AP2700, or Harmonic.ChA Fund Freq in ATS-2), as shown in Figure 4. The AP2700 control software can continue to use the analog analyzer level and function meters for measurements. The ATS-2 control software only allows one analyzer to be selected at a time, so use the Amplitude and Harmonic Sum meters to make Level and Distortion (THD) measurements, as shown for Data 1 in Figure 4.

Figure 4 ATS-2 Sweep panel, using harmonic distortion analyzer.

External Sweep Triggering Improvement

There are a number of ways to improve external sweep triggering, so let’s briefly review them to make sure we are choosing them correctly:

• Use the distortion meter, as described above, when the instrument is having trouble measuring the frequency of a tone with a high level of noise or distortion. The FFT based frequency meter picks out the strongest tone present and ignores everything else.

• Adjust the threshold (AP2700) or Min. Lvl (ATS-2) sweep parameter to gate out any noise, tones, or program material that might falsely trigger a reading to take place. Note that once the threshold is exceeded, no suppression of noise occurs.

• Loosen the settling parameters in the settling panel when the frequency or level of the tone is unsteady and won’t trigger. This may compromise the accuracy of the measurements, so don’t alter the settings more than necessary.

Other Recent Posts

• I2S jitter measurement with the SYS-2722
• Interrogating installed hardware in the 2700 Series and on the APIB bus

Test Results: AP News & Events

AP at AES in Germany

In May we're off to AES, exhibiting May 8–10 in Munich, Germany. So, if you are in Europe, come visit us at booth #1503. And as our guest, we are again offering you free passes to the exhibits. Advance registration deadline is April 30, 2009.

©2009 Audio Precision Inc.