Tape Guide (Q and A) (Mar. 1973)

Recording Stereo

Q. I recently bought a stereo tape deck and am very happy with it, except for the fact that I haven't figured out how to make stereo sound on sound recordings. The manual tells how to make mono sound on sound recordings, but not stereo ones. Can you make any suggestions for stereo sound on sound?

-Michael R. Martin, APO, New York, N.Y.

A. To achieve stereo sound on sound, you would require an extra playback head mounted before the erase head. And you would need the capability for monitoring the signal produced by this head (Signal 1) and at the same time feeding Signal 1 along with Signal 2 to your record head (through your present mixing facilities). Alternatively, you could use a four-track (quadraphonic) set of heads, along with suitable switching, so that you simultaneously go from Track 1 to Track 2, and from Track 3 to Track 4; and then simultaneously from Track 2 to Track 1, and from Track 4 to Track 3; etc.

Mr. Martin further writes to present a solution to a problem encountered with plastic dust covers for turntables:

"I have some information that may be of help to other owners of Dual and similar turntables with a plastic dust cover and a light-tracking cartridge.

My Dual is equipped with a Shure M-91E that I operate at one-gram tracking pressure. I have found that on cold days when the humidity is low the plastic dust cover develops a static charge strong enough to lift the tone arm from the rest in off position or from the record during play. The problem is made worse if the dust cover is wiped with a dry cloth. The situation is easily remedied by taping a four or five inch strip of aluminum foil one-half inch wide to the dust cover so that as long as the cover is in place the foil contacts the metal of the turntable base. This prevents the tone arm from rising."

Transformerless TV Hook-Up

Q. I want to take the TV sound from the set's volume control. This, of course, means connecting the TV chassis to the receiver chassis and to the tape recorder chassis. The TV is transformerless. It receives its power through a polarized AC plug, thus grounding the TV chassis.

If the polarity of the TV plug is not defeated, and if the AC wall receptacle wiring and polarity are compatible with that of the TV plug, would it be safe to make the sound connection described above?

-J.H. Ruder, Oklahoma City, Oklahoma

A. If all your premises are met, it seems safe to connect your TV sound to your audio system in the manner your propose. But please do not accept my guarantee for safety. On general principles, I just don't like the idea of connecting a transformerless component to an audio system.

2- or 4-Channel Recorder

Q. I am considering the purchase of a Tandberg 14 mono tape recorder. My problem is in deciding whether to get this model in two- or four-track form. I have had experience with four-track mono recording, and the garbled rumble produced by bass notes on an adjacent track during silent passages is a plague.

Tandberg specifies -60 db or more crosstalk. Would this be audible? Would I have this problem with a two-track machine? I understand that a two-track machine would put more signal on the tape, improving the signal to noise ratio about 2 db. Would this affect distortion?

-Stephen L. Kasioras, Allston, Mass.

A. Crosstalk of -60 db would be inaudible in as much as other noise would be at most about -55 db below the maximum signal level. With good tape machines I have not run into the problem of "garbled rumble" that you describe.

Inasmuch as there is wider spacing between tracks in the case of a halftrack machine than in the case of a quarter-track one, the former would be more free of crosstalk. Also, as you recognize, S/N would be higher for the half-track machine (about 3 or 4 db higher) inasmuch as this permits putting more signal on the tape. S/N is based on a given distortion level, so there would be no change in distortion.

Another advantage of a half-track machine is that the wider track permits better averaging out of tape defects, such as those causing dropouts. On the other hand, the half-track recorder has the disadvantage that azimuth alignment is more critical. For a given degree of azimuth misalignment, treble loss is greater for a half-track unit than for a quarter-track one.

Equalization Standard

Q. I own a Uher 4400 Report stereo portable tape recorder. This machine was purchased overseas, and I have a feeling that the equalization curves may conform to the European (CCIR) rather than the U.S. NAB standard. I note that, particularly at the 3 3/4 ips speed, the machine tends to attenuate the highs and slightly boost the bass.

Although less evident, the same problem occurs at 7 1/2 ips. I have discovered that tapes recorded on this machine do not display the same imbalance when played on other machines. Can you tell me whether it is possible to adjust either the equalization or bias frequency in order to correct this problem.

-David R. Kidd Jr., Lenox. Mass.

A. Regardless what equalization is used, a properly designed and adjusted tape machine should give you flat record-playback response. Inasmuch as tapes recorded on your machine sound O.K. when played on other machines, the problem appears to lie in the playback equalization of your machine. Changing the tape you use would be of no appreciable help now would change of bias level (nor frequency). I suggest that you take your machine to an authorized service dealer and have him check and adjust the playback equalization. Of course, all this is based on the assumption (per your letter) that tapes recorded on your machine do play back correctly on other machines.

Applying Equalization

Q. I have recorded about 60 LP's at 7 1/2 ips directly from the magnetic cartridge into the microphone inputs of my tape recorder, without any phono equalization between the cartridge and recorder. Can I equalize this material by playing my recorder at a low level into the phono input of a preamp?

-Robert Colvin, Hayward, Calif.

A. Yes, you can apply phono-equalization to the output of your tape recorder's playback preamp and come out with proper frequency response.

However, the ultimate playback signal may contain excessive distortion. Substantial treble cut should have been applied to the output of the phono cartridge. In the absence of such cut, the excessive signal level at treble frequencies may have overloaded the record amplifier of the tape machine and/or the tape. Keep in mind that in recording, a substantial amount of treble boost is applied to the audio signal.

Faster Copying

Q. Everyone knows that recording at 7 1/2 ips results in less hiss on playback than slower speeds. It is also known that copying a 3¾ ips tape at 7 1/2 ips and then playing it back at 3 3/4 ips results in a copy in one-half the time. Now does it follow that a 3 3/4 ips tape copied at 7 1/2 ips has less hiss when played back (at 3 3/4 ips) than if it were copied at 3 3/4 ips? Even if hiss is not reduced, one can still copy tapes in less time than it normally takes; or would this result in some loss of fidelity?

-Geo. B. Moore Jr., APO Seattle.

A. I don't believe you have anything to gain but time in copying at the faster speed. And, unless your tape machines are well equipped to handle high frequencies, you may lose in terms of treble response. When a 3 3/4 ips tape is played at 7 1/2 ips, all frequencies are doubled. Thus a 12,000 Hz tone becomes 24,000 Hz. If your playback and/ or record amplifier exhibits drop-off at this frequency, there will be treble loss. You won't gain in terms of S/N because as each tone is played at half the speed at which it was recorded, its level drops correspondingly. Furthermore, copying at elevated speeds raises the problem of obtaining a sufficiently high bias frequency (to avoid interaction with high signal frequencies) and of driving bias current at such high frequency through the record head (bias loss increases with bias frequency as the result of head inductance and winding capacitance).

Double Speed Copying

Q. Is there any disadvantage in copying 3 3/4 ips tapes by operating the playback and recording machines at 7 1/2 ips?

-George S. Hatch, Jr., APO.

A. When you play a tape at twice its normal speed, all frequencies are doubled. This raises the problem of exceeding the treble response capability of your tape amplifiers. Furthermore, in recording you should use twice as high a bias frequency to minimize the chance of beat frequencies between the bias signal and the audio signal.

(Audio magazine, Mar. 1973; Herman Burstein)

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