TAPE GUIDE (Nov. 1989)

Test-Tone Anomalies

Q. My cassette deck has the Dolby symbol marked on its meters midway between the 0 and +4 dB points. A few months ago, I acquired a cassette containing some test tones, including a Dolby 400-Hz calibration tone and left and right-channel 400-Hz tones recorded 3 dB below Dolby level. When this cassette is played on my deck, the meter reading for the Dolby calibration tone is at the 0-dB point on both channels; meter readings for the left- and right-channel tones are at -4 dB, and readings on both channels sometimes fluctuate-for example, from -4 dB to midway between-4 and 0 dB. What is the correct interpretation of the meter readings I have described? What, if any, are the undesirable audible consequences of these readings? I find it hard to believe that such readings are normal. It seems that a problem exists, or will exist, but I am uncertain if I have encountered it. Is my deck in need of alignment or other servicing?

-Victor S. Zupancic; Kirkland Lake, Ont., Canada

A. Your deck may be slightly--about 2 dB--misadjusted with respect to Dolby level. The meter markings, too, seem slightly off--about 1 dB. Alternatively, the test-tone levels may be off. A high-quality, accurate test tape is expensive, costing up to $100; inexpensive test tapes can easily be somewhat inaccurate. The variations in the recorded tones' levels also seem to be an artifact of the test tape, particularly since they vary upward. If they were downward, this would suggest momentary loss of good contact between the tape and the tape head.

If the meter readings, instead of the tape, are at fault, they are too minor to be of real consequence. If you follow these readings, you may fail to achieve as high an S/N ratio as is theoretically feasible, but not by much. On the other hand, you'll get added protection against recording at excessively high levels, which can cause noticeable distortion and treble loss. In the case of the meter fluctuations-if these are slight, brief, and not particularly audible--again, you have nothing important to worry about. Even if they are audible on single tones, keep in mind that such imperfections tend to disappear on normal program material. Also, as I said before, these fluctuations may be due to the test tape rather than your deck.

Your ears are an excellent measuring device. If your deck can produce a good aural facsimile of the original source, it is working properly. Remember the maxim: If it ain't broke, don't fix it.

More on Hi-Fi VCR Recording Reader John H. Markell has more illuminating information on high-fidelity recording with a VCR: I agree with Brent Jessee ("Tape Guide," January) that operation at the highest speed on a Hi-Fi VCR is desirable in order to minimize dropouts.

This is the only reason for operating at the highest speed, because frequency response and SIN are not affected by tape speed. Also, for Beta Hi-Fi, L-500 tape is recommended over L-750 because the former is thicker and therefore less subject to dropouts.

However, it appears that the theory of dropouts should be examined. It seems to me that a tape dropout's duration is too short to be heard directly through the audio decoding system.

Dropouts longer than the sync pulses could cause loss of synchronization, turning on the muting circuit. Thus, what would actually be heard would be the dropping in and out of the muting circuit, which has time constants in the audio range. If this is indeed the case, the type of modulation (PCM or FM) would make no difference in the number of dropouts heard, inasmuch as the problem is in the sync circuits. I suspect that differences in dropout susceptibility between various VCR brands are due to differences in their muting circuits.

I have tried Hi-Fi recording without a sync pulse, using the simulcast setting of the VCR with no TV station tuned in. Needless to say, this resulted in many dropouts as well as inconsistent tape speed.

The character of the internally generated sync pulses, when recording audio, may also have an effect on frequency of dropouts. I seem to have fewer dropouts when recording a TV signal, which uses the TV sync, than when recording an audio-only signal, which uses the internal sync. It could be that the TV sync is better than the VCR's internally generated one. Differences in the internal sync generators might be why audio-only tapes made on one brand of VCR sometimes will not play properly on another brand, even though videotapes will. There may also be incompatibility between VCRs if the relative azimuth positions of the audio and video heads aren't maintained.

Dolby for Dubbing

Q. Should I use Dolby noise reduction to record CDs onto tape? If so, should l use Dolby B or C NR?

-Alex Ly, Pasadena, Cal.

A. Definitely use Dolby C NR rather than Dolby B NR to copy a CD. Dolby C NR will provide greater noise reduction than Dolby B NR, better enabling you to cope with the wide dynamic range of many CDs. Also, it will better guard against tape saturation in the treble spectrum.

Skew, Torque, and Speed-Up

From his vantage point as service manager and then owner of an audio service shop over a period of eight years, reader Daniel E. Mull of San Luis Obispo, Cal. illuminates some interesting tape problems: I have run across skew and related speed aberrations on a variety of cassette machines, ranging from minor skew problems (primarily on dual-capstan decks) to major skew and speed problems on cheap units. My experience has been very frustrating at times. Typically, when a deck has excessively high take-up torque along with worn or glazed pinch rollers or inadequate pinch-roller pressure, the fact that most Type II tapes have very highly polished coatings and backings will cause the tape to skew. If the takeup torque is great enough in this situation, the tape will be pulled past the capstan and pinch roller, causing tape speed-up. If a Type I tape of medium to poor quality is used, these symptoms occur far less often.

Recently we serviced two high-end dual-capstan decks that exhibited major skew problems, with resultant tape damage, but only when using Type IV tape; cassette behavior was perfect with Type II and Type I tapes. In both decks, the problem was eliminated by increasing pinch-roller pressure on the supply side and decreasing the takeup torque back to the unit's original specifications.

Conversation Piece

Q. I have some questions about tape recording and equipment for the purpose of transcribing conversations.

Presently, I am using an old Realistic tape deck, a Numark equalizer, and an old self-amplified Ampex speaker. My objective is to hear as much conversation detail as possible. What do you recommend?

I also transcribe microcassettes.

Would it be best to copy these onto a C-60 or C-90 cassette, or would you recommend just hooking the microcassette deck directly into the equalizer?

-Herbert Joe; Houston, Tex.

A. It seems that your best option is to take advantage of the facilities provided by your equalizer. Treble emphasis, starting in the range of about 2 or 3 kHz, would enhance the clarity of taped conversations. Trial and error will help you optimize the equalizer settings. If you use excessive treble boost, you may get objectionable ringing. Also try bass reduction below 200 or 300 Hz to improve clarity.

I see no advantage in copying microcassettes onto compact cassettes before transcription; that is, simply go from the microcassette deck into the equalizer. A better speaker system might also help, particularly if the tweeter level is adjustable.

Variations in Output Level

Q. I have a JVC deck and an Akai deck. The output level of the JVC deck is very low compared with that of the Akai, even though, according to the record level indicators, they are recording at the same level. Why?

-Robert G. Nicolella, Scotia, N.Y.

A. The output of home decks tends to vary considerably, roughly from 0.33V to 1V or slightly more on signal peaks.

Even though the meters of two decks read the same in playback, their output levels can differ appreciably, owing to different gain in the stages following the point at which the playback signal is fed to the meter. A 2:1 voltage ratio--say, 1V versus 0.5V--is a 6-dB difference in level, which is quite distinct. A 3:1 ratio is a 9.5-dB difference, which is even more profound.

Just Plain Dolby

Q. If a prerecorded tape just has the words "Dolby System," does this mean Dolby B or C NR or HX Pro?

-Nelson Chin, Wollaston, Mass.

A. If a prerecorded tape just says "Dolby," this signifies that it was recorded in Dolby B NR. The reason is that many low-price decks still provide only Dolby B NR and not Dolby C NR. HX Pro probably was not used, but this has no relationship to the Dolby decoding required in playback.

High-Frequency Outage

Q. I have a new deck with auto reverse. Every time I make a recording of 15 minutes or more, I get a very annoying dropout problem. The dropout is cyclical--about every 3 or 4S--and the high frequencies suffer the most.

The longer the recording, the worse the problem becomes, to the point where the dropouts convert to a continuous absence of high frequencies, except for an occasional moment of normal response. I take meticulous care of the deck, with regular cleaning and demagnetization. Please help me with this problem.

-Douglas J. Bayne, Huntington Station, N.Y.

A. When high frequencies suffer the most, I am inclined to suspect an azimuth problem or poor contact between the tape and the heads. Inasmuch as your difficulty is intermittent, at least early on, poor azimuth alignment would also have to be intermittent. This could be caused by skewing of the tape within the cassette shell, due to a defect in either the cassette or the deck. Poor contact between the tape and the heads could be due to failure of the cassette's pressure pad to operate properly; perhaps something in the deck's mechanism prevents the pad from fully exerting pressure. Other possibilities are improper adjustment of the torque of the take-up and supply reels, or a spot of dirt which cleaning hasn't adequately removed from the tape head.

(Source: Audio magazine, Nov. 1989, HERMAN BURSTEIN)

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