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Contour Effect Q. Please explain how the physical shape of some record heads produces a "contour effect," and what is different about those heads that do not produce this undesirable effect on low frequency response. Since this effect shows up on the record-playback response only, I have assumed that the recording head is solely responsible. What are the problems associated with maintaining uniform low frequency response down to 20 Hz in a tape recorder? -James Tankersley, Menomonee Falls, Wisconsin A. The contour effect occurs in play back and is responsible for irregular response at very low frequencies (i.e. long wavelengths recorded on the tape). What happens is that at such frequencies not only the gap of the head responds to the magnetic field on the tape, but also the entire head responds. There fore the size of the playback head, the shape of the head, and the wrap of the tape about the head affect bass frequencies. Minimizing irregularity of response at low frequencies requires taking the above factors into account and finding an optimum design. Such a design takes good research and engineering. On the whole, the problem decreases as the playback head is made larger. The problem of maintaining bass response down to 20 Hz involves appropriate bass boost in playback. But such boost also emphasizes hum. There fore many machines do not try to maintain full response below 50 Hz. In fact, the NAB standard permits play back response to be down 2 dB at 50 Hz and 3 dB at 30 Hz. Tape Copying Q. We do a lot of tape copying. Some friends copy at the speed at which the original tape was recorded. Others use a faster speed, such as 7-1/2 ips to copy 3-3/4 ips tape, in order to save time. I was wondering if we lose frequency response or anything else when copying at a faster speed than the original tape. Also, is there any correlation between tape recorder speeds and turn table speeds? Another problem we have is pickup of radar buzz on our tape recordings. It's a short buzz, in creasing in intensity, then ending abruptly. It cycles about every 30 to 60 seconds. Is there any way we can get rid of it? -Dennis G. Mueller, APO San Francisco A. When you duplicate a 3-34 ips tape at 7-1/2 ips, all frequencies are doubled in playback and recording. For example, a 15,000 Hz note becomes 30,000 Hz. This may exceed the frequency response capabilities of your playback and record amplifiers, of your playback head, and perhaps of your record head. Furthermore, there is increased chance of beat frequencies be tween the oscillator signal and the audio signal. On the other hand, if there isn't much above 10,000 Hz on the original tape, you may be able to copy at increased speed without noticeably adverse results. There is no correlation between tape and disc speeds. Perhaps the buzz problem may be alleviated by using capacitors of a few pfd between the input stage of your amplifier and ground. SOS and SWS Q. Could you let me know the difference between sound on sound and sound with sound? -Ghislain Gauthier, Chicoutimi, Quebec, Canada A. Sound on sound signifies that two or more audio signals are recorded in synchronization on the same track. Sound with sound means that two signals are recorded in synchronization on two different tracks. (Audio magazine, Oct. 1974; Herman Burstein) = = = = |
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