Notes from the Test Bench
By Bruce Hofer, Co-Founder and Chairman, Audio Precision
Hello and welcome to the latest edition of Audio.TST from Audio Precision.
As you can see, our newsletter is evolving as we work to bring you the most useful news and tips on audio test every month. Of the new columns, I'm anticipating the most popular will be "Sound Advice". If you have an audio test problem that's stumping you, I encourage you to ask us via this webpage. We promise to answer each query individually and every month we'll select one or two of the best for the newsletter. For every question we publish, we'll send the author(s) a free copy of our 5 DVD set Fundamentals of Audio Test and Measurement.
I look forward to your comments, and I hope you find the new format useful.
Bruce Hofer
Output: Tech tips and new applications from AP
Dolby Digital 5.1 DVD Certification made easier
Last month the AP Tech Support department introduced QuickTest to help with testing playback-only devices like MP3 players. Not content to rest on their laurels, this month Tech Support has released a new Technote containing a series of macros for the AP 2700 Series audio analyzer to help with your Dolby Digital 5.1 DVD Certification. About 25% of you are involved with Dolby in one way or another so we hope this is useful to you.
Dolby certification requires you to take about 30 different measurements on your Device Under Test. Each macro included in the new Technote corresponds to one of those tests. Just connect your device to the analyzer, play the stimulus signal, and start testing.
You can download the Technote for free from our website. The Technote presumes that you have already obtained the Certification materials from Dolby and that you own or have access to an Audio Precision SYS-2722 analyzer running under AP2700 version 3.10 and an appropriate AP input switcher.
Please note that this program has no official endorsement from Dolby and we make no claim that devices evaluated by the testing procedures offered here will be certified by Dolby Laboratories.
To download the Dolby Digital Technote, click here (login required).
Sound Advice: Audio Test Q&A
Question: When should I use an IMD measurement instead of THD+N?
Answer:
IMD (Inter-Modulation Distortion) measurements provide useful information about the high frequency non-linearity in a relatively sharply bandwidth limited system that would not otherwise be revealed with THD+N (Total Harmonic Distortion + Noise) testing. Such systems include CD players, portable media players and cassette tape players. Measurements of THD+N using a fundamental that is higher than about 1/3 of the system bandwidth are subject to serious error because one or more potentially significant harmonics will simply fall outside of the audio band. Indeed, THD+N measurements with a fundamental frequency above ½ of the system bandwidth will include NO harmonics at all!
Inter-Modulation Distortion measurements are made by stimulating a device with a complex signal made up of two component sinewaves. A non-linear device will reproduce the two original sinewaves plus a number of IMD products expressed by the equation mF1 + nF2, where m and n are integers and F1 and F2 are the frequencies of the two component sinewaves.
The advantages of IMD measurements are that the two frequencies can be arranged so that the distortion products appear inside the audio band. Traditional THD+N measurements can not practically be made above 7 kHz because the majority of the harmonic products would be above the bandwidth of the device being measured, typically 22 kHz for consumer digital audio devices. In addition typical IMD techniques such as DFD (aka CCIF) are more sensitive because they use high frequency tones, usually above 10 kHz, where many devices exhibit greater sensitivity because distortion usually rises with frequency as the feedback factor falls.
The disadvantage of IMD measurements is that they do not measure noise, which excludes buzz, hum, aliasing products, and all other non-harmonic components present in the output signal of an audio device.
Test Results: AP News & Events
NAB - Las Vegas 2006 April 24 - 27, 2006
We're looking forward to seeing you in Vegas in April. Make sure you stop by our booth N1317. We can't promise fancy chochkies, but we will have engineers on site ready and eager to talk anything audio.
We've also made a special arrangement with NAB for our customers to get free access to the hall. Just go online to www.nabshow.com before April 19, select "Register Now", then select "VIP Exhibits Pass" and enter Source code: RC1065.
Audio Engineering Society (AES) - Paris May 20 - 23, 2006
We'll be in Paris in May for the AES Pro Audio Expo & Convention.
Look for more details in next month's Audio.TST newsletter.
Record growth second year in a row
by Alan Miksch, President of Audio Precision
When we celebrated Audio Precision's twenty-year anniversary in 2004, we set the objective of making our third decade our best. In that context, we're pleased to announce that in 2005, Audio Precision enjoyed record growth for the second year in a row, giving us a good start on achieving our goal.
So a big thank you to all our customers for putting your trust in Audio Precision. And thank you to the staff and distributors of AP for making it all possible. With your support, we promise that we will continue to set the standard in audio test and measurement for many years to come.
back to top
