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Meeting held Saturday, January 9, 2016, noon-5pm at a small, private audio research facility in Redmond, WA.
The PNW Section kicked off 2016 with a Saturday afternoon session about sampling rate conversion (SRC). AES/IEEE Fellow James D. (JJ) Johnston gave a tutorial on sampling, how sampling creates and controls the whole process, what a SRC does, how it works, and what it should and should not do. Bob Smith, of the AES PNW committee and an acoustic scientist at Physio-Control, then gave a presentation about SRC testing, with results showing aliasing and step response of different filters. Snack breaks were held about every hour. The meeting was held at a new, as-yet-to-be-named audio research company in Redmond, WA. 41 people attended, about 18 being AES members. Full A/V documentation will be available as well as the Powerpoint decks of the entire meeting.
James D. (JJ) Johnston earned the BSEE and MSEE degrees from Carnegie-Mellon University, Pittsburgh, PA in 1975 and 1976 respectively. JJ temporarily retired in 2002 but worked 26 years for AT&T Bell Labs and its successor AT&T Labs Research. He was one of the first investigators in the field of perceptual audio coding, one of the inventors and standardizers of MPEG 1/2 audio Layer 3 and MPEG-2 AAC, as well as the AT&T Bell Labs or AT&T Labs-Research PXFM (perceptual transform coding) and PAC (perceptual audio coding) and the ASPEC algorithm that provided the best audio quality in the MPEG-1 audio tests.
Most recently he has been working in the area of auditory perception of soundfields, electronic soundfield correction, ways to capture soundfield cues and represent them, and ways to expand the limited sense of realism available in standard audio playback for both captured and synthetic performances. He was most recently employed by DTS Audio and his current status is retired.
Mr. Johnston is an IEEE Fellow, an AES Fellow, a NJ Inventor of the Year, an AT&T Technical Medalist and Standards Awardee, and a co-recipient of the IEEE Donald Fink Paper Award. Mr. Johnston has presented many times for the PNW Section, most recently on the issues surrounding "Dynamic Range." In 2006, he received the James L. Flanagan Signal Processing Award from the IEEE Signal Processing Society, and presented the 2012 Heyser Lecture at the AES 133rd Convention: Audio, Radio, Acoustics and Signal Processing: the Way Forward.
Bob Smith Bob Smith has a BSEE from the University of Washington and has worked in the Biomedical industry for over 40 years. The last 30 years he has spent developing acoustic research and audio engineering disciplines for Physio Control to improve speech intelligibility for medical device voice prompting and voice recording systems in noisy environments. He is responsible for voice prompting in 30+ languages. The department now handles acoustic measurements of components such as drivers, microphone capsules and system measurements including Thiele-Small parameters, polar plots, waterfalls, frequency response, impulse response, several speech intelligibility methods, etc. When he's not playing acoustic/audio monkey for his corporate master, he runs an acoustic lab, soundSmith Labs. From time to time, he can also be found recording local musical talents. Currently he is working on audibility and measurements of DAC jitter.
JJ began with The Shannon Sampling theorem (i.e. Nyquist criterion) which dictates all with PCM digital audio. An SRC must never allow aliasing, where unfiltered spectrum images overlap the desired baseband audio signal, otherwise "stuff" happens. JJ played an example that shall never be reproduced again, but that did prove the point of "don't do that".
So, now you need to interpolate data? Yes, and to satisfy what the ear is senstive to, including the frequency domain. He continued with problems with filter design and how that can interact with the original signal, which might have used half-band filters to bad effect. JJ has stats on thousands of CDs showing much questionable antialiasing (also much poor quantization, much clipping, and a whole host of other problems in various 'don't do that' catagories.)
Continuing with filter design, JJ covered FIR, IIR, an attempt at apodizing, and half band filters, as well as filter calculation costs, efficiency, and multi-step filtering. Rate conversion at fractional rates, i.e. 8/3 fractional rates were introduced. Some mention of how to do rate reduction in steps was included, perhaps including a bit of confusion on how to do that (n.b. jj added italicised comment). Finally, how to do conversion ratios with terrible least-common multiples was briefly discussed. FIR filter design was demonstrated using Matlab (expensive) or Octave (freeware!) showing ripple and pre-echo problems.
Throughout the discussion, the use of SRC for subsample delays was mentioned repeatedly, both for fixed and changing delays.
After a snack break, Bob Smith discussed SRC tests. He began with a photo analogy of incremental degradation - the photo gets blurrier with more aliasing. Aliasing (spectra not filtered reflects back into the passband) examples were played. JJ pointed out that this aliasing demo relates exactly to bad clipping scenarios during digital processing as well as for SRC.
Charts of step response (square waves) through different filter types (linear phase, minimum phase, hybrids) were shown, with different "ringing" at transitions. Such tests may help you determine what filtering is used in a DUT (Device Under Test). Lastly, SRC example test result charts were shown, using the Octave script for testing..
During a break, door prizes of booth swag obtained from the 2015 AES 139th Convention in NYC, courtesy of Gary Louie were awarded:
The message was that you should test your SRC yourself with your setup - Octave script provided.
He showed examples of charts of a good, and bad SRC, upsampling and downsampling.
Reported by Gary Louie, PNW Section Secretary