Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
With APx500 v2.5 now shipping, AP’s engineering department held its normal debriefing process to review how the release cycle went. Overall, I am happy to report that we are fit and firing on all cylinders (to use an expression from another industry). And so, the work of our engineers now pivots towards the next planned set of goals.
The product planning team has also been holding on-going meetings to review and refine our long range strategies, in light of the constantly changing mosaic of customer needs and new technologies.
Our job as the industry leader is to monitor the needs of our customers and foresee future trends. So, thanks to all who have contributed ideas and feature suggestions over the years. Keep them coming, and I will assure you that AP will always be listening.
PS. Special congratulations go to the two recipients of AP’s “Sales Partner of the Year” awards for 2009: Steve Peterson of ATECS in the US, and AP Technology in China. Of course, Sales Partner is a little bit of a misnomer. In reality, our partners are much more than sales reps. They’re on the front line of customer engineering challenges, and ultimately their job is to help you get your job done as quickly and easily as possible.
Output: File Playback Analysis
One of the major new features in APx500 v2.5 is wave file playback analysis. Wave (.wav) file analysis is an ideal tool for analyzing a wide variety of devices that record digital audio, from sound cards to portable recorders. Additionally, APx itself can save digital audio recordings of any device, such as a music player or power amplifier, using the analog or digital signals present at the analyzer inputs. This allows analysis to be performed on archived signal sources (with the ability to vary measurement parameters after-the-fact, and to compare multiple recordings), using the original analyzer or even remotely on another APx instrument.
The APx measurement recorder.
To capture an input signal to a file in APx500, the Measurement Recorder is used with “Save to File” checked. Digital signals are recorded at their incoming sample rate and bit depth, to make an exact copy. Analog signals are recorded at 24 bits, with the sample rate ranging from 48 kHz to 2496 kHz depending on the Input Bandwidth setting of the analyzer. In addition, APx stores analog voltage scaling data in the file, so that later analysis shows voltages just as if the analog signal were being measured in real time.
Input File List selection box.
Wave files can be selected as the input source for all APx measurements that allow external source (open-loop) testing—that is, measurements where the generator isn’t required to be turned on during analysis. When the files have been created by the APx Measurement Recorder, you simply browse to them on the host PC and add them to the Input File List. When the files have been created by a recording device, they are usually transferred to the host PC on a memory card, by using a proprietary software transfer utility, or by attaching a USB cable to the recorder (where it appears to Windows as an external disc drive). Either way, up to 8 stereo files, 16 mono files, or any combination of up to 16 tracks can be added.
Once files have been chosen, the instrument is used in exactly the same manner as when using real-time analog or digital sources, with no relearning necessary. The only difference is that bar graphs do not measure continuously, but make a single measurement on settled data when file playback is initiated, just as they always do when a sequence is run.
16-channel file playback analysis can be done on any APx 2, 4, 8, or 16 channel analyzer.
There are a number of distinct advantages and reasons to analyze wave files, both those made by recording devices, and those made by the APx analyzer itself:
Speed: Files analysis utilizes the power of the computer running APx to produce results as fast as possible, whereas digital audio streams have to be analyzed as they arrive in real time. This can result in some very large time savings when analyzing a recording made by a device—a 31 point frequency response sweep that normally takes 44 seconds, or an FFT acquisition with 20 averages that takes 57 seconds, can each be completed in under two seconds. Not only does this improve throughput, but it allows you to take advantage of techniques like averaging (which helps reveal low level detail in an FFT) without noticeably increasing analysis time. Of course, making the wave recording still has to take place in real time!
Multiple channels: Up to 16 channels of audio, with up to 192k/24 bit resolution, can be analyzed at the same time with any APx analyzer (including two-channel models). It isn’t necessary to have matching multi-channel hardware connections on both the DUT and the analyzer. Additionally, the multiple channels can be used to simultaneously analyze files from different devices, or files from the same device made at different times.
Troubleshooting: File analysis removes any contribution caused by DSP sections or digital transmitters in the playback signal chain of a device, helping to isolate problems.
Performance verification: When the recorded file is intended to be exchanged with other devices and computers, the file is the true end-product. By analyzing the file directly, instead of indirectly via a digital audio output, there can never be any concern over whether the two are identical.
Remote testing and servicing: The performance of recording devices anywhere in the world can be checked, with the files being sent electronically to the lab for remote analysis. Scientific field recorders in the jungle, in the Arctic, or aboard a research ship can be analyzed. It’s not necessary to have the audio analyzer there at the same time, in the same location.
There’re still of course many situations where it’s necessary or best to analyze digital audio streams in real time. One is when testing recording devices that store audio as encoded files (mp3, Dolby, DTS, etc), or that don’t provide file-based access. Additionally, it’s often desirable to analyze the entire playback chain of a recording device, to test for playback and transmitter errors. For devices that don’t store audio, it’s faster to make direct measurements, when features like delayed analysis or signal archiving aren’t needed.
Sound Advice: AP Knowledge Base
Cross-domain (Analog/Digital) Group Delay and Phase Measurement
How can I use an APx analyzer to measure group delay and phase shift through an A/D or D/A converter?
This may be done using the Continuous Sweep measurement. The group delay and phase are measured at the analyzer, using the generator as a reference. Because you may independently configure the generator outputs and analyzer inputs as digital or analog, cross-domain measurements can easily be performed. Group delay may be observed using the Impulse Response or Group Delay views, and phase may be seen in the Phase view (with the reference set to Absolute Phase).
When you want to measure phase between adjacent channels, use the Interchannel Phase measurement or the Continuous Sweep measurement's Phase view (with the reference set to Channel One).
Impulse Response view from the Continuous Sweep measurement in APx. The cursor marks the group delay through an A/D converter.
Group Delay view from the Continuous Sweep measurement in APx. The cursor marks the group delay through an A/D converter. Note the increase at low frequencies due to AC coupling in the ADC.
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Testing the Pro's Audio
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