On Sun, 16 Jan 2005 14:27:51 +0000, Steven Gibbs wrote: >I recall using Rhode & Schwarz test-sets for filter >alignment, also Tektronix tracking-generator and >spectrum analyzer combos. Further back I recall >similar devices called wobbulators used for IFs. > >- when was the technique of using wideband noise > followed by audio or video spectrum analysis first > described? > >- ditto, for amateur radio applications?
For roughly 35 years, pink noise and real time analyers have been used to evaluate the amplitude response of audio systems and and equipment, adjust system equaliztion for a desired response. In the late 60's, the late Dick Heyser developed and patented a powerful analysis technique the he called "Time Delay Spectrometry," whereby a generator and narrowband detector were swept at the same rate, but with time offset that could be set by the user. This allowed coherent measurement of two-port systems -- that is, magnitude and phase -- and it also allowed the direct sound of a loudspeaker to be separated from reverberation in a room. Heyser demonstrated the technique to the pro audio community using a collection of HP and GR gear that filled a van. It took about ten years before Bruel and Kjaer built a dedicated product to do it. Crown Int'l introduced a dedicated product in 1982. Although it wasn't widely known, that box included FFT capability, and many of us, including myself, used the FFT capability for various functions. About that time, a number of folks began using single channel and dual channel FFT analyzers to to analyze and tune sound systems. With the single ended technique, the source was usually pink noise and you got only an amplitude response. With dual channel analysis, one could compare any excitation signal (including program material) with the output. These two forms now dominate the landscape in pro audio. A program called Smaart, at least some of which was written by a programmer who had been associated with the Crown box, was introduced at least ten years ago. It runs on any Windoze box, and uses the sound card for I/O. It is QUITE widely used to tune live systems. Nearly 25 years ago, I used my portable real time analyzer to determine the response of systems where I had access to both input and output. The analyer could store and display two measurements, and I subtracted them point by point to get the system response. I did this with the telco links between the Chicago FM Club's 146.76 repeater and two remote receivers in the late-70's -- the input was my radio listening to a station on the input, the output was the same radio listening to the repeater output. We then adjusted an equalizer at the repeater site to optimize (or at least improve) system response. I also used the technique professionally to analyze the response of the duplication systems used for TV spots. There are now dozens (hundreds?) of FFT programs written to run on PC's, and for use in many disciplines. Spectrogram was written to facilitate the analysis of bird calls! A very powerful new one called EASERA, for which I have been a beta tester, was written by some good friends in Berlin. It is primarily a tool for acoustic testing, but from a single log sine sweep, it can quickly computer and display harmonic distortion vs. frequency, BY HARMONIC ORDER! Jim Brown K9YC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
