Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
First, I’d like to give a special thank you to conference chair Nathan Bentall and the rest of the AES UK chapter for their excellent conference “The Ins and Outs of Audio.” I really enjoyed giving the keynote speech, and the theme lent itself well to a balanced and informative couple of days.
Meanwhile back in Portland, APx500 v2.8, the APx Bluetooth option, and the upgraded HDMI option with Audio Return Channel (ARC) all started shipping at the beginning of this month.
As for me, it’s all eyes forward to the next release of hardware.
Output: Taking Analog Filters into the Digital Domain
By Matt Bell, special to Audio.TST
Dr Thomas Kite, Vice-President of Engineering at Audio Precision, worked on the implementation of audio filters in the high performance hybrid analog/digital 2700 Series analyzers, and more recently on the newer almost wholly digital APx Series. In particular, he has been closely involved in replicating many of the high-specification analog designs of the 2700 as digital processes in the APx. He explains the reason for the move to digital, beginning by looking at why filters are so important in test and measurement:
"Any decent piece of audio test and measurement equipment will have at least three main uses for filters. First, to define the bandwidth of your input signal; that’s very important when carrying out basic measurements, like THD+N, for example. Second, for notch filtering; if you’re measuring THD in the signal from a device under test, a very common filter requirement is to use a notch filter to remove the fundamental before analysis (more on this later). And third, signals are often weighted by filtering. Many manufacturers state their noise or THD+N results with A-weighting applied as a matter of course, for example.”
2722 audio analyzer (analog filter implementation).
"So AP’s older analog analyzer designs, such as the System One, the System Two, and the current 2700 Series, feature many simple tunable notch filters engineered to a very high standard. They also have some extremely intricate analog filter designs available as optional plug-in cards: high-order filters with lots of poles, op-amps, and switchable topologies. These filters are all very precise, but they are also very costly."
Plug-in analog option filter board.
Fortunately, as Dr Kite explains, digital implementations of these filters offer a number of advantages.
"Digital designs are very stable, and you can add complexity—more poles—more easily and affordably than in analog, which requires more complex, and therefore more expensive, circuitry. Of course, you may need extra DSP power, but the cost has come down so much in recent years, it’s not the concern it once was. From a cost point of view, it’s easier to achieve higher performance in digital than in analog.”
APx525 audio analyzer (digital filter implementation).
"In some cases, the digital implementations are more effective. We can notch out a fundamental very easily now, for example. AP’s analog notch filters are very deep, but they still have finite roll-off, and attenuate some frequencies either side of a fundamental. In digital, we can measure the frequency, amplitude and phase of a fundamental and cancel it with the inverse signal, leaving everything else intact. We do that in the APx Series, for example, when measuring THD+N."
Translating many of these analog circuits into the digital domain is straightforward, using standard design tools like MATLAB.
A-weighting filter, analog implementation on 2700 Series.
A-weighting filter, digital implementation on APx Series.
"The mathematical behavior of standard low-pass filters is very well understood. You can choose from a number of models to implement these digitally, and their behavior digitally is very close to that of the analog designs, with the same response as the as analog version, and the same sharpness of cutoff."
Even where extremely complex analog circuits are involved, Dr Kite has found it possible to create very faithful digital implementations of the 2700 Series’ analog filters by working methodically through the schematics of the original circuits.
"AP’s co-founder Bruce Hofer is a very gifted analog designer, and some of the best results I have achieved digitally have come from replicating his circuits exactly in the digital domain. For example, the complex filters either side of the RMS detection circuit in one of the early AP analog analyzers were painstakingly designed by Bruce to give a flat response in the frequency domain, and also a good transient response in the time domain, so I reproduced the analog schematic in every detail, and the response of the resulting digital version is exactly the same. You can measure a test signal using the analyzer’s analog circuits, and then do the same using the digital implementation. When you’ve got it right, the results are exactly the same: the plots fall on top of one another."
Time vs. Frequency
As the cost of DSPs has fallen, it has become easier to design analyzers with the processing power needed to convert input signals from the time to the frequency domain before undertaking any processing. This has been the breakthrough in AP’s APx Series analyzers—the first to undertake frequency domain processing. But what’s the advantage of this approach?
"When you’re working digitally in the time domain, digital filters are usually referenced to the incoming sample rate. Imagine, for example, you’re designing a filter that gives a certain gain at 1 kHz. In digital systems at a constant sample rate, that’s easy: if the incoming rate is 48 kHz, you filter at 1/48th of the sample rate.
"However, if the sample rate of the incoming audio can be variable, and audio starts coming in at 96 kHz, then your filter will no longer work correctly. You need to find a way to map the filter shape to the sample rate, so that the shape is maintained irrespective of the sample rate—and this can be very difficult in the time domain. Even simple low-pass filters often have complex Bessel functions at their heart, so mapping them is not trivial.
"However, if you frequency-transform your incoming signals, and do all of your filtering in the frequency domain, you can scale your frequency transformations at different sample rates, and all of the time-domain problems go away. Low-pass filtering, for example, becomes very easy. You just throw parts of the spectrum away, as with a graphic EQ. Even complex weighting functions are much easier—you just draw the shape of the filter you want in the frequency domain, and that’s what you implement. Having said that, frequency-domain filtering isn’t applicable to everything. Even in the APx Series, some algorithms, like quasi-peak detection, still have to be implemented in the time domain."
Still Analog after All These Years
Despite the move to digital processing, there are still uses for analog filtering.
"I mentioned the use of notch filters to remove fundamentals in test signals earlier, and even in digital analyzers, there’s a lot to be said for using these in the analog domain, before a signal even goes into the A-D converter. Imagine you’re trying to measure the low-level distortion that results from putting a test sine wave at a certain frequency through a device. You just want to measure the harmonics, not the fundamental. But the fundamental will be much higher in level than the residuals you’re interested in. So that’s a massive dynamic range for a simple A-D converter to deal with. If you can remove the fundamental before it hits the converter, the input signal will be much more manageable in terms of dynamic range, and the converter on the analyzer’s input will itself be less prone to distortion, which could skew your measurements.”
APx Series input signal path (simplified).
"It’s very expensive to do pre-converter filtering, so the APx Series doesn’t offer that. But the 2700 Series does take that hybrid approach: it has an analog tunable notch filter which seeks the fundamental in the input signal and removes it prior to the residuals being passed to the converter. When that extra few dB of performance matters, for example if you’re an A-D converter manufacturer, that’s the standard you need."
2700 Series input signal path (simplified).
Not all of the analog designs AP still uses are precision notch filters.
"We still use very simple analog AC coupling filters—just a capacitor and a resistor. If the signal under test is sitting on top of some DC, it’s sensible to remove the DC before the signal hits the converter—again, it’s a dynamic range issue. And the brickwall filters in our oversampling converters are analog. Nothing complicated or expensive is required, just a resistor-capacitor circuit really."
Past, Present, and Future
If AP’s analog filter designs are the company’s past, and the digital ones are the future, Dr Kite feels it appropriate that the company uses both approaches at present.
"We offer the relative affordability of digital with the APx Series, but the quality benefits of pre-converter analog filtering, for example, are still there in the 2700 Series for those customers that need it. That seems a sensible balance to me."
Sound Advice: Click and Pop Measurement with APx
Clicks and pops in the output of audio equipment can be annoying, as well as be potentially dangerous if a user is listening on headphones. These unwanted noises will typically occur when a unit is turned on, turned off, switched, or muted. While many audio IC solutions have integrated click and pop suppression, some circuits still require suppression measures to be taken.
We’ve designed two APx projects that utilizes our APx Sound Level Meter Utility to do click and pop detection, taking advantage of the file recording capability of the APx500 software. This solution is superior to making measurements at discrete intervals. Even at a relatively fast measurement rate of 128/sec, a transient click or pop can get missed between acquisitions. By making a digital recording at a 48 kHz sample rate, we can be sure that any audible transients will get captured.
Measuring Clicks and Pops
If you do not already have it, download and install the APx Sound Level Meter Utility. Start APx500, and then start the utility. When the utility asks if you want to open the default project, click “no.” Then, from APx500, open the ClickAndPop_NoGen.approjx project included with this article.
The APx Sound Level Meter Utility.
Connect the outputs of the device under test to the Analog Inputs of the APx instrument. The device should be loaded as it would be under normal operating conditions. For a headphone or speaker output, that typically would mean 32 Ω or 8 Ω respectively.
Now, click the Acquire button on the utility to start recording the file. The recording runs for 16 seconds and then stops. During this time, you can turn the device under test on and off, or otherwise manipulate it to try to induce clicks and pops.
Measurement Recorder Level vs. Time view.
Once the recording has stopped, click the Analyze button. The utility will now display three views in a new window—the original data, the data after it has been passed through an optional A‑weighting filter, and the data after it has been averaged. If you change the filter, time constant, and averaging settings in the window, the graphs will automatically redraw.
Analysis window, muting the DUT.
Controlling the Device from APx500
DUT/analyzer connections with APx instrument controlling the DUT.
The second project file, ClickAndPop_SqGen.approjx, employs a clever technique to control the device automatically. Analog Output 1 on the APx analyzer is used to generate a square wave that can control logic pins, such as sleep and mute, on the device. Because the square wave generator on the APx instrument has a minimum frequency of 10 Hz, we’ve created a waveform file with a 0.25 Hz square wave at 0 dBFS that’s already loaded into the project. The Signal Generator Level in APx500 is preset to 5 volts, but should be adjusted to the required logic voltage.
APx generator square wave output.
You can also use the square wave to control a relay that interrupts power to the device. To protect the analyzer output, we suggest that you use a solid state relay or relay buffer and not drive the relay directly.
Analysis window, turning the DUT on and off.
Test Results: AP News & Events
Audio Precision has two open positions (more details online):
Senior Software Engineer: Senior software development of extremely high performance digital audio test and measurement instrumentation and applications. Individual contributor as well as providing technical leadership to the software team.
Engineering Manager: Manage software and hardware engineering department for development of extremely high performance digital audio test and measurement instrumentation and applications. Leads cross-functional project teams to deliver quality products and features to the market on time. In conjunction with the Vice President of Engineering, develop long-range positions on programs or technologies for consideration of the Product Planning Team.
Both jobs are at the factory in Beaverton, Oregon. Competitive salary and benefits. Audio Precision is an Equal Opportunity Employer. See our Careers page for more details.
©2011 Audio Precision, Inc.