Notes from the Test Bench
By Bruce Hofer, Chairman & Cofounder, Audio Precision
Members of the AP team have just returned from a trip to visit with manufacturers of Bluetooth products, and many were eager to test and troubleshoot their new products with the APx525 analyzer the AP team brought with them. Many of these developers are still relying upon "can you hear me now?" testing with Bluetooth, which clearly leaves something to be desired.
The APx525 analyzer offers complete Bluetooth analysis with a built-in radio, and when combined with our new PESQ software option becomes an ideal platform for evaluation of Bluetooth audio. This point was driven home as our team was able to identify key problems in products in very short order; these are issues could have taken weeks to solve otherwise.
That's what we bring to audio test: the performance and assurance that allows engineers and manufacturers to move ahead with confidence. We look forward to hearing how you use AP analyzers to bring your projects and products to life.
Output: Isolating Harmonics in THD Measurements
The APx Acoustic Response and Continuous Sweep measurements can display a graph of THD vs frequency, as well as any single harmonic or group of harmonics vs frequency. AP’s Tech Support department has developed a new “THD Sweep with Individual Harmonics” project that leverages APx500’s data import/export feature to display many single harmonics on the same graph.
The ability to identify the specific harmonic content in distortion products has several useful applications. A good example of this is the characterization of specific types of loudspeaker distortion, as the magnitude of distortion in loudspeakers is much higher than in most electronics. While two different loudspeakers may demonstrate nearly identical levels of THD, analysis of individual harmonics can reveal important and potentially audible differences that depend upon the specific harmonic products and where they occur in the frequency spectrum; for example, significant distortion occurring at harmonics within the range of 1 kHz to 4 kHz will likely be far easier for a human ear to detect that an identical amount near 15 kHz.
Amplifier design and diagnosis can also benefit from this type of analysis. One would typically arrange to see a summation of all odd harmonics in one test, then compare the results to a summation of all even harmonics. If odd harmonics dominate, then the nature of the non-linearity is “symmetric” meaning it is a function of the absolute value of the signal voltage (i.e. the same for both negative and positive peaks). If even harmonics dominate, the non-linearity is “asymmetric” meaning that it is different for positive and negative peaks. This is extremely useful information for the designer in order to optimize distortion performance.
Figure 1: Sine wave with no clipping
Figure 2: Sine wave with asymmetric clipping (even harmonics dominate)
Figure 3: Sine wave with symmetric clipping (odd harmonics dominate)
Another application may occur in devices that seek to deliberately generate harmonic distortion of a specific nature, such as electric guitar effects. In this case, the harmonic profile of distortion can be used to characterize the audible qualities of the effect.
Other uses for individual harmonic analysis include checking passive component non-linearity.
AP Support has developed a APx project that employs our Acoustic Response measurements in combination with automated export and re-import of data to perform the operations needed to separate the harmonic content of distortion data from a DUT.
Figure 4: Individual measurements within the THD Sweep with Individual Harmonics Project
The project takes advantage of the automated data export and import features in APx500. Individual harmonics up to H10 are graphed, and you can modify the project yourself to extend the range to H20
The results shown below were created using a small powered loudspeaker as a DUT and a measurement microphone. We expect this small loudspeaker to generate sizable amounts of distortion, and it does not disappoint.
Figure 5: THD plus individual harmonics up to H10 plotted against input frequency for a loudspeaker
The graph above displays all of the first 10 harmonics, each on an individual colored trace. Total harmonic distortion (THD) is also shown. By simply turning selected traces on and off, it is easy to see the individual harmonics as a function of stimulus frequency, and to compare any one to another.
Figure 6: THD (olive green) and 2nd harmonic only (dark green)
In the image above, we can see both THD and the contribution from H2 (second harmonic) on the same graph. It is clear that at frequencies below 600 Hz, THD is dominated by second harmonic.
Figure 7: THD (olive green) and 3rd harmonic only (blue)
This next image shows THD and the contribution from H3 (third harmonic). It is clear that most of the distortion in the decade spanning 1.5 kHz to 15 kHz is contributed by H3, which indicates that the loudspeaker is behaving quite differently at these higher frequencies.
The ability to easily measure the contributions of individual harmonics to THD extends the possibilities for connecting measurements to audible effects in a significant manner. It can be especially useful in amplifier design, and in examining and characterizing electro-mechanical acoustic devices, such as loudspeakers. We look forward to hearing from you about additional uses and suggestions.
Sound Advice: Expanding APx with Automation
At the heart of the APx family of audio analyzers is our powerful APx500 software. APx500 allows engineers and designers to easily create sequences of measurements tailored to any type of audio product, using any of the audio interfaces that are built-in or available as options for the APx515, APx525/526 and APx585/586. In this article, we show how APx500 can be further extended through the use of external programs in sequences. This powerful ability is especially useful as compact computing devices continue to evolve into mainstream audio products that require comprehensive testing.
The example for this article is automating a test station for a tablet computer. A tablet computing device, like its smartphone sibling, is a multi-function media player and recorder that supports several types of audio I/O:
In order to conduct proper tests over each of these ports, the tablet must be configured via internal software to direct test signals to the appropriate audio interface. Ordinarily this would require human interaction with the tablet, but because this DUT (device under test) is itself a computer, we can leverage its capabilities to automate our process, illustrated in the diagram below:
Figure 1: Test setup diagram
In this example, two small programs were developed. One resides on the PC that hosts the APx500 software, while the companion piece is installed on the DUT; each program facilitates a network connection between the two devices. The program on the PC running APx500 contains command line arguments that allow it to specify audio files for open-loop testing and the desired audio port on the DUT, while the program running on the DUT carries out these instructions as they are received. Audio test files may be sent over the network or may reside locally on the DUT.
These programs are leveraged using an automated sequence in the APx500 project. With no additional code required, an APx500 test sequence can easily contain calls to external programs and use messages from these programs to trigger measurements or other actions, such as pass/fail notifications, etc.
Adding an external program to an APx Sequence
In the example here, a simple sequence has been created to measure Level and Gain on one of the DUT‘s audio interfaces. It is of course necessary that any physical connections required by these interfaces be made in advance so that the APx instrument can receive the test data.
As part of the sequence shown below, two calls are made to an external program residing on the PC. They are given easily-read names in order to make the sequence clear to anyone who must use it. The first call instructs the DUT to stop any ongoing audio, while the second call loads and starts a new audio signal from a specified file.
Figure 2: APx Sequence Step Properties
Editing the "Start WAV Audio" step in the sequence reveals the available options, shown below:
Figure 3: Editing an individual sequence step
The program "RemoteAudioControl.exe" is called, and arguments passed to the program as shown. The companion program running on the DUT may then send messages back to RemoteAudioControl.exe, which in turn can be used to trigger appropriate responses from APx500. In this example, it is expected that the DUT will normally send a "PASSED" message back to the PC, which will be used to validate the program output in the sequence. A failure will cause APx500 to display a message to the user, indicating that the DUT has failed or perhaps requires attention. Once this step in the sequence is validated, the APx analyzer will begin measuring Level and Gain on the desired DUT interface.
Note: while the situation described here is real, we are not posting the RemoteAudioControl.exe code as it is specific to the DUT in this proof of concept.
The easy integration of external programs with APx automated sequences is powerful and useful in many ways, especially so with the rapidly growing class of networked consumer audio devices, as they are capable of responding to commands issued from such sequences using simple applications developed on standard platforms.
Test Results: AP News & Events
AP2700 3.3 SP2 Released
The latest version of AP2700 Control Software is available now. In previous releases, Cross–domain delay measurements were off by one sample. A fix was applied to remove the one sample error when making measurements that use analog out / digital in or digital out / analog in. This change affects the digital generator and the triggerable measurements (FFT, FASTTEST and MLS).
APx500 v3.0 SP2 Released
The latest version of APx500 3.0 is now available for download. This is a minor update that contains an enhancement to the APx DSIO software, allowing the user to shift the position of the Frame Sync rising edge relative to the first bit of sample data. This enhancement allows the APx DSIO to be configured to be compatible with Cirrus Logic’s TDM format.
Venus Transits the Sun
On June 5, 2012, the planet Venus passed between Earth and the Sun, an event that will not repeat until 2117. Despite have nothing to do with audio measurement, the crew at Audio Precision were not about to allow this to go unobserved. We are Oregonians who are fascinated by stories of the bright light in the sky, since we don't see it all that often.
We congregated in the parking lot while the estimable Dr. Tom Kite, VP of Engineering, set up a telescope to project an image of the Sun onto a white card.
Behold, Venus appears. We are suitably impressed by Dr. Kite, again.
© 2012 Audio Precision, Inc.