Audio.TST Archive
Audio Precision sends out the Audio.TST newsletter once a month to approximately 13,000 audio engineers and other audio professionals. Each issue looks at current events in audio test, answers readers´ technical questions and announces any AP news.
As you can imagine, we've been busy following up on May 15th's launch of the APx585 multichannel audio analyzer. Three AP teams traveling across Asia, Europe and the US have seen many hundreds of engineers in 13 countries, including one meeting in Taiwan with 246 attendees. Upcoming US road show dates are listed in the Events section below and other venues in other countries will be added soon. In other news, the ATS-2 was recently adopted by both CNET and PC World as the baseline audio analyzer for their reviews of audio portable devices. You can see the stories (as well as APx highlights) on our news page at ap.com/press. You may also be interested in a talk I gave to the AES in Los Angeles covering the history of faster test techniques. I enjoyed the opportunity and it was good to see so many familiar faces. Bruce Output: Tech tips and new applications from AP Creating Audio Precision graphs for the web or other reports Engineers often have to create reports of test results for others. Graphs convey a lot of information very quickly and are a standard part of any test summary. "Export Graphics" and "Copy Panel to Clipboard" are the two ways AP2700 allows graphs to be exported. This article will detail how to use them. Graph setup
Export Graphics command
Copy panel to clipboard command: The Configuration menu controls how Copy panel to clipboard works. You should set your preference before you start using the clipboard. Go to Utilities, then Configuration, then Graphs and select which format you want to send to the Clipboard.
Once your preference is set, just click on the graph you want to copy, then go to Edit and select "Copy panel to clipboard." Then go to the other application to paste the image directly into an existing document. Remember, in all cases, the size and shape of the graph panel as its displayed is what gets exported, so make sure this is right before you start your export process. If you do a lot of graphing and reports, you may want to check out the APx585's automated graphs: every time a measurement is made, an HTML report with PNG graphs is generated automatically. It can be a significant time saver and worth a demo.
Faster Test It wasn't until the mid-1970s that audio test first started to experience significant improvements in speed: "auto set level" reduced distortion measurement times from 50 seconds per data point (if you were good) to less than 15 seconds. Then in 1980 Tektronix's AA501 became the world's first automatic audio analyzer with auto-ranging of all measurement functions, auto set-level, and digital numeric readout. The addition of GPIB ("HPIB") soon followed, enabling automated measurement sequences. With new advances in the HP8903 in 1980 and the Tektronix AA5001 in 1982, a 16-point 20-20kHz graph of THD+N that used to take 15 minutes could be created in less than one minute. While automation decreases setup time and reduces the chance of human error, it does not impact actual measurement time. At the same time, point-by-point sine or stepped-sine analysis often misses unexpected problems and behaviors in very narrow bands that require greater resolution to be identified properly. The need for faster and more thorough testing gave rise to several non-sinusoidal techniques over the years such as maximum length sequence (MLS) and multi-tone testing. MLS is extremely fast, using a pseudo-random noise signal where every possible bit pattern occurs once during its repetition interval. A Hadamard transform of the output signal yields the impulse response of the device under test. MLS techniques are particularly attractive for acoustic measurements but their usefulness is compromised by non-linearity in the device under test, as non-linearity causes response errors that cannot be readily identified. Multi-tone analysis offers attractive speed benefits and is useful when testing playback only devices such as MP3 players (the single tone can be stored on the device and played back just once versus having to repeat the acquisition for each step in a traditional point-by-point sine measurement). The test signal is a superposition of between 5-31 sine-waves from which measurements are calculated in a windowless FFT. The disadvantage of multi-tone is that it suffers an inherent lack of correlation with traditional techniques. The measurements aren't wrong, they're just different. This can be very confusing when trying to make comparisons using the different test techniques. The newest approach to increasing the speed of test is swept-sine or chirp testing. While the concept of linear chirp (where frequency varies linearly with time) has been around for over 30 years, it has limited application especially in calculating THD+N measurements. The value of chirp changed dramatically when Dr. Angelo Farina of the University of Parma introduced the first logarithmic chirp technique at the 108th AES Convention in 2000. Dramatic improvements in speed were made possible because frequency increases exponentially over time: with a log chirp, the time to sweep from 20 Hz to 40 Hz equals the time to sweep from 10 kHz to 20 kHz Farina showed that a deconvolution of the input and output signals yields time separated impulses of the linear and harmonic responses of the system being tested. By carefully time-windowing and transforming the various impulses, the individual response functions can be recovered from which many different measurements can then be derived mathematically. In other words, almost every common audio measurement can be obtained from a single acquisition. Farina's technique was recently extended by Audio Precision to simultaneously test multiple channels through use of very small time offsets between channels (as seen in the new APx585 multichannel audio analyzer). This enables one to measure crosstalk in addition to response and distortion in multiple channels simultaneously. Incredible amounts of data can be extracted from a single, multiple-channel test. New techniques will come along, but for now, chirp offers the most promising avenue for test speed. With the increase in computing power in today's PCs, we should see significant performance improvement year on year. It's an exciting time for audio test.
Events North American APx Introduction Tour | USA | June - August, 2006
International Broadcasting Convention (IBC) | Amsterdam | Sept 8 - 12, 2006 Audio Engineering Society (AES) | San Francisco | Oct 6 - 8, 2006
Tom Kite is one of AP's principal engineers. We caught up with him during half-time of the England vs. Trinidad &Tobago World Cup match. You've been at AP for over seven years now, what exactly is your role and what are you concentrating on right now? The chirp? What do you like most about working at AP? Other than APx, what else have you been working on? What got you started in audio? You work primarily in the digital realm. How important is analog today? When you get home from work, what do you like to do? And is it true you've been seen bringing down the house at Karaoke bars late into the night?
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