kz08+@andrew.cmu.edu (Ken Zuroski) (03/05/90)
Hello: I'm not sure if this is the appropriate newsgroup to post this; but here goes anyway. If there's a better place to post it, please let me know. Here is a problem I hope you can help me with. I'm involved in a research project--very complicated affair, so on and so forth. One of the tasks I and a fellow graduate student are expected to perform is interviewing a dozen or so "principle investigators" involved in this project--medical doctors and college professors, mainly. These people have proven to be by and large difficult to find, very busy and therefore are reluctant to spend time talking with us, and generally difficult to interview. Nevertheless, we managed to do so, talking with each of them for about an hour each, recording our questions and their answers on tape. A couple of days ago I received a frantic call from my fellow worker. It seems that the tape recorder she was using to interview these people somehow malfunctioned during three of the interviews she did (we had split the job up, and each of us had six people to interview). To make a long story short, it seems that the condensor microphone in the unit wasn't properly grounded or whatever, and that there is a very persistant and annoying sixty-cycle AC hum masking the voices on the tape. Uh oh. Now, the frustrating thing is that it is almost possible to decipher the voices behind the buzz. Almost. If you turn the volume up, naturally the noise increases as well, and so it's a futile attempt. And so I had the idea of using a graphic equalizer to filter out some of the noise. Here is what I am going to try to do, and what I would like you to comment on as to the possibility of success. I will use two tape recorders, the graphic equalizer (when I find one) in line between them. I will use headphones to mask out as much of the buzz as is possible using the graphic equalizer, and duplicate the equalized signal on recorder number two; I'll boost the overall level of signal in the process, thus hopefully amplifying the voices. Then, if I still can't make out the tapes, I'll do the same thing again, placing the recording into tape player number one, masking and boosting the signal, then recording again on a third tape. What I'm trying to do is recursively filter out the bad frequencies while boosting enough of the good frequencies to make out the dialog. What do you think of my chances of making this work? Anyone out there ever try this? Any other techniques I should try? Obviously I don't have a whole lot of financial resources, but I'm all ears to any technique that might be of help. Many thanks for taking the time to read and reply. --ken zuroski ----------------
peg@psuecl.bitnet (PAUL E. GANTER) (03/05/90)
In article <YZwNWoG00VQc8GOWpD@andrew.cmu.edu>, kz08+@andrew.cmu.edu (Ken Zuroski) writes: > To make a long story short, it seems that the condensor microphone in > the unit wasn't properly grounded or whatever, and that there is a > very persistant and annoying sixty-cycle AC hum masking the voices on > the tape. Uh oh. > > Now, the frustrating thing is that it is almost possible to decipher > the voices behind the buzz. Almost. If you turn the volume up, > naturally the noise increases as well, and so it's a futile attempt. > And so I had the idea of using a graphic equalizer to filter out some > of the noise. Here is what I am going to try to do, and what I would > like you to comment on as to the possibility of success. I will use > two tape recorders, the graphic equalizer (when I find one) in line > between them. I will use headphones to mask out as much of the buzz as > is possible using the graphic equalizer, and duplicate the equalized > signal on recorder number two; I'll boost the overall level of signal > in the process, thus hopefully amplifying the voices. Then, if I still Ken-- I think you are on the right track, but instead of using a graphic equalizer, see if you can get a notch filter. CMU has plenty of EE's--see if you can find a laboratory filter that you can use, or have an EE throw together a filter with a couple of op amps. The notch filter will have much better attenuation than a graphic EQ would, and you could tune it to 60 Hz (an equalizer may not have a slider near that--a parametric equalizer is another possibility--they allow you to tune the center frequency of the slider). The cost of a simple op amp notch filter should be < $10. If you need schematics for one, let me know, but any op amp circuit cookbook should have one. Good luck!! Paul