Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
Audio Precision has recently attended two international AES events, the AES Expo Brazil 2012 in Sao Paulo, Brazil and the 132nd AES Convention, held this year in Budapest, Hungary. I traveled to Budapest, and while I found the number of exhibitors to be lower than expected, the quality of technical papers that were presented was excellent. Budapest is a beautiful city with a very rich history, and I hope that AES will work to increase attendance at these European events.
The AES Expo in Brazil was a first for us, and was a great opportunity for members of our team to meet key players in the audio industry and share the latest advances in audio measurement techniques. You can read about some of the show highlights later on in this month's issue of Audio.TST.
Be sure to check out our new feature additions for our APx family of analyzers, including support for PDM microphone testing and PESQ measurements. We are excited to continue to release products that serve the changing needs of the audio industry.
As always, I welcome your feedback.
Output: AES Brazil 2012
Audio Precision exhibited at the AES Brazil conference in Sao Paulo, May 8 through the 10th.
A highlight of our visit was the impromptu evaluation of a large power amplifier on display with one of the other exhibitors. After engineering an appropriate test load, it was run through a battery of tests. The amp’s designer was impressed to see that the tests executed by the APx’s automated sequencer took only two minutes to accomplish what requires well over an hour of manual testing at their factory. Given that each amp is tested as it comes off the production line, this was quickly seen as an opportunity to eliminate a production bottleneck.
Another visitor observed that they could use APx’s custom reports to create a personalized datasheet for each device that comes off their production line, with the exact performance measurements for that particular unit embedded in the company’s existing document. What a great idea!
AES Brazil was productive and exciting for us, and AP looks forward to returning to Sao Paulo next year.
Sound Advice: The ups and downs of Square waves
Our Support engineers are frequently asked about square waves and the differences they might see between different types of wave generation and measurement. Square waves may not occur often in nature, but they are frequently employed in test and measurement for audio and other technologies, and present special issues in audio analysis, especially in the digital realm.
An ideal square wave is a periodic, symmetrical waveform that alternates instantaneously between two levels, creating a familiar image when viewed in the time domain:
The power spectrum of an ideal square wave consists of the fundamental and an infinite series of its odd harmonics. The level of each harmonic is proportional to the harmonic number; thus the level of the third harmonic is 1/3 that of the fundamental, the level of the fifth harmonic is 1/5 that of the fundamental, and so on.
In practice, square waves show non-idealities in the time domain, such as finite risetime and imperfect symmetry. In the frequency domain, these manifest themselves as harmonic roll-off and even harmonic content, respectively.
Square wave stimuli have been used in audio test for many years. For many audio engineers, the square wave is an old friend:
Traditionally, square waves have been used to quickly diagnose issues such as poor frequency response and instability. Typically, the results are interpreted graphically by inspection of an oscilloscope trace rather than by quantitative measurement. However, it is possible to measure characteristics such as risetime and slew rate using square waves.
Square waves are largely a child of analog technology. Although the fundamental frequency of square waves used for audio test is usually confined to the audio band, the harmonic content extends to frequencies far beyond the limits of typical digital systems. In addition, the discontinuities of the ideal square wave present a unique problem for sampled-data systems, as we shall see.
Analog square waves
In the analog realm, timers, multivibrators and op-amps are used to create circuits that rapidly switch between low and high states, creating a waveform that looks like an excellent square wave when viewed on a scope. But since none of these devices has an infinitely high slew rate, the rise time of the square wave is finite. It appears less and less ideal as the fundamental frequency increases, with the rising and falling edges appearing to be less and less vertical as the high frequency limit of the system is reached.
While still subject to the effects of bandwidth limiting, these analog techniques can generate very high quality square waves when used within specifications.
A sine wave can be used for a quick distortion test by examining the DUT’s response to it in the frequency domain. (Peaks in the spectrum at harmonics of the fundamental indicate the presence of harmonic distortion.) In a similar way, a square wave can be used for a quick response and stability test by examining the DUT’s response to it in the time domain. For example:
Square waves can provide some rapid visual insight into the frequency response of a DUT. They can easily show the slew-rate limiting inherent in a low-pass filter, or the inability to pass DC inherent in a high-pass filter. Examples of each are shown below.
Analog square waves are also used in the DIM (dynamic intermodulation distortion) test.
Digital square waves
Square waves can be generated digitally in two ways:
The first method generates square waves that are perfect when viewed digitally in the time domain. However, their fundamental frequencies are restricted to those whose periods are 2N samples, where N is an integer, and their spectral content includes low-level aliases throughout the passband.
The second method generates square waves with perfect spectral content, but with overshoot and ringing in the time domain, even when viewed digitally. This is a consequence of the fact that the Fourier series does not exhibit uniform convergence; in particular, it does not converge to the value of the underlying function at jump discontinuities. (This is known as the Gibbs phenomenon.)
When converted to the analog domain using a conventional DAC with a brick-wall anti-imaging filter, both methods result in square waves with overshoot and ringing, making them unsuitable for the traditional analog uses discussed above. However, digital square waves are useful for exposing certain flaws peculiar to digital systems. For example, systems engineered with inadequate headroom may overload when stimulated with full-scale digital square waves. This is a common problem in interpolation filters.
Square waves in APx analyzers
APx instruments generate signals digitally. When the selected output connector is digital, the signal is sent directly to the output. For analog output connectors, the digital signal is converted to analog through a DAC.
All APx models can generate square waves on the digital outputs. However, because digitally generated analog square waves are inadequate for traditional testing, basic APx models do not offer analog square waves. To cover this need, we created the AG52 generator option for APx525 models.
AG52 converts digitally generated sine waves into ultra-high quality square waves using dedicated circuitry. These square waves offer the fast risetime and low overshoot of analog, with the frequency stability and resolution of digital. They come very close to the ideal in both time and frequency domains. AG52 also adds DIM (dynamic intermodulation distortion) capabilities.
AG52/BW52/AO52 High Bandwidth Options for APx525
The APx BW52 Ultra-high bandwidth analyzer option for APx525 instruments offers unprecedented FFT performance: One million FFT points, DC to 1 MHz bandwidth and full 24-bit resolution. The companion AG52 high bandwidth analog generator option allows for the creation of near perfect square waves.
The AO52 option bundles both the AG52 and BW52 options for less cost than when each is purchased separately.
Square waves have limited application in audio test, but still can provide useful information about a DUT at a glance to a trained eye. In analog systems, square waves can disclose slewing and stability issues. In digital systems, square waves can highlight overload and headroom problems.
The APx525 Family offers precise, wide bandwidth analog square waves with the AG52 generator option and BW52 Ultra-high bandwidth input option.
Test Results: AP News & Events
© 2012 Audio Precision, Inc.