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.
Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision Hello Reader Before I introduce this month's Audio.TST, I'd like to take a moment to mark the passing of John Eargle earlier this month. John was a well respected presence in the audio community that will be missed. He was a Grammy Award winning recording engineer, a fellow and past president of the AES, a prolific author and lecturer on a variety of audio topics, a past vice president at JBL and a mentor to many. Please visit johnmeargle.com for more information about John and to share a thought about him with those he left behind. This month's Audio.TST features a PDF that summarizes all the keyboard shortcuts for AP software and an introduction to dither. As always, we welcome your questions and comments at audio.tst@ap.com. Thank you for your continued support of AP and I look forward to seeing you out on the road (I will be in Japan and Denmark later in the summer, please see Events for details) or at the next trade show. -- Bruce Hofer Output: Tech tips and new applications from AP
AP2700 Keyboard Shortcut Quick Reference Guide Everyone knows the power and flexibility of AP's control software. A side effect of the flexibility is there are a lot of commands and panels, which can make navigating from setting to setting somewhat time consuming. Fortunately, the software contains numerous keyboard shortcuts that can save users a lot of effort... if they know the commands. For those of us who don't have the time to memorize 30+ keystroke commands, Tech Support has created a PDF "quick reference guide" with all the AP2700 & ATS software shortcuts in one easy-to-read location. Just print it, cut along the dotted line and pin it up somewhere useful. Download the AP Keyboard Shortcuts here Sound Advice: Audio Test Q&A
Q: What is dither? Why do I need it? When should I use it? A: Any time a signal is quantized, error is introduced. Although dither is most often considered when performing an A-to-D conversion, error occurs regardless of whether or not the conversion is cross domain. The error is highly correlated with the signal, so when the signal is small, the correlation renders it highly audible. Even worse, at very low signal levels the signal will drop out completely because its excursions are smaller than one quantization step. For high resolution ADCs, this is not actually a problem because the noise floor of the analog front ends is much greater than a quantization step. These systems are therefore self-dithering. However, for digital wordlength reduction (e.g. reducing a signal from 24-bit studio master to 16-bit CD), and for low resolution ADCs, quantization introduces unacceptable distortion. To counteract this error, dither is added before quantization. Dither is a noise signal, usually generated pseudo-randomly. The ideal dither adds the minimum amount of noise power while achieving the goals of dither. In audio, these goals are accepted to be (a) zero harmonic distortion from quantization and (b) no noise modulation. The lowest power dither which achieves this is triangular probability distribution function (TPDF) dither with an excursion of +/- 1 LSB. Several benefits result from the addition of TPDF dither before quantization:
Rectangular dither (RPDF) is non-ideal because although it eliminates distortion, it suffers from audible noise modulation. Shaped dither, or wordlength reducers embedded in psychoacoustic feedback networks, can achieve better performance than TPDF dither in that the noise floor power spectrum can be shaped for minimum audibility. The actual noise power is not reduced over TPDF dither, just its audibility. This is used in some commercial CDs to improve performance. Test Results: AP News & Events
APx Midwest Introduction Tour Visit the AP Events calendar for dates and contact information. IBC Amsterdam -- September 7-11 123rd AES Convention New York -- October 5-8 Tokyo AES Convention -- July 17-20 Technical University of Denmark -- August 22 |
