TAPE GUIDE (Jun. 1984)

Playback Equalization

Q. My cassette deck has a four-position equalization switch: 70 µS for Type II and IV tapes, 120 µS for Types I and Ill. What is the difference between these settings? What frequencies do 70 and 120 µS use equally? And what is the equation for converting microseconds to Hz?

-Ray Segura; Jefferson, La.

A. With 120-µS equalization (used only for Type I ferric oxide tapes), playback bass boost starts (up 3 dB) at 1,326 Hz; with 70-µS equalization (used for all other tape types), it's up 3 dB at 2,274 Hz.

To convert turnover frequency in microseconds to Hz, divide 159,155 by the number of microseconds. Thus, for 70 µS, we have: 159,155 divided by 70, or 2,274 Hz. To convert turnover frequency in Hz to microseconds, divide 159,155 by the frequency. For 2,274 Hz, we thus have: 159,155 divided by 2,274, or 70 µS.

Tape Saturation

Q. I have a problem when recording with Dolby C but not with Dolby B. When I record live material from radio or phono records, the music always saturates the tape; the result is scratchy and blurred sound. I keep the recording level at the point suggested by the instruction book and have also tried lowering the level. Should live material be recorded with Dolby B instead of Dolby C? Any help would be appreciated.

-Douglas Brenner; Douglaston, N.Y.

A. First, let me point out that "live" material, as you call it (truly live material would be the music source itself, not a radio station or phono record), tends to contain sharp transients which challenge the capability of the tape system with respect to tape saturation.

I suspect that something is wrong with the calibration of your Dolby C circuitry in recording, so you are not getting correct tracking (match between recording and playback levels) for the tape you are using. If anything, Dolby C has been devised to provide even greater headroom (protection against tape saturation) than Dolby B. This is achieved by reducing the treble emphasis at the upper end of the audio range in recording. As you may recall, the Dolby system variably emphasizes treble in recording, with low sound levels getting more emphasis than high sound levels, and in complementary fashion it variably de-emphasizes treble in playback to restore flat response. For all this to work properly, there has to be a match between recording and playback levels.

If Dolby B works better than Dolby C. this provides further evidence that something is wrong with the Dolby C circuitry in your deck. You should have your deck checked by a competent technician; take along one or more of the cassettes you plan to use so that adjustments can be made on the basis of these tapes.

Dubbing, Dolby, and dbx

Q. I have to dub about 20 cassettes encoded with Dolby B NR which were made on a deck with incorrect azimuth adjustment. I couldn't play a friend's tapes, nor could he play mine because of my deck. I have a new deck which provides Dolby B, Dolby C, and dbx noise reduction. I want all my newly dubbed tapes encoded with dbx. I'll be dubbing from the deck that was originally used to record the Dolby B tapes. Will I have any trouble dubbing the old tapes and encoding them with dbx? Do I decode the Dolby B tapes as if I were listening to them? Do I have to be overly concerned with level adjustments on the new deck? And how do I set the bias switch on the old deck? I use chrome tapes.

-John Flanagan; Fall River, Mass.

A. When playing the original tapes on the source (old) deck, do so in the Dolby B mode, which will give you a correctly equalized and quiet source signal. When you make your dbx-encoded dubbings on the new deck, you do not have to be concerned with level (sensitivity) adjustment, as is the case with Dolby. When playing your original tapes on the source deck, the equalization switch should be in the 70-µS mode-the mode for all but ferric tapes. Bias selection is not involved when playing tapes; bias is used only in recording.

(Source: Audio magazine, Jun. 1984, HERMAN BURSTEIN)

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