TAPE GUIDE (May 1985)

Dolby C and dbx

Q. Does dbx offer an improvement over Dolby C? Why would both circuits be present in one deck?

-Freeman Matthews, Columbus, Ohio

A. Dolby C, like Dolby B, works on the principle of variably boosting the treble range in recording and cutting it by a complementary amount in play back, which simultaneously cuts back upper-frequency noise. The amount of this treble boost or cut in recording and playback varies with signal level; the lower the level, the greater the change. This avoids applying excessive treble to the tape, which would cause distortion.

The dbx NR system compresses the entire audio range in recording and expands it back to normal in playback, with the downward expansion in play back reducing noise, just as it does in Dolby NR. By compressing the entire band, dbx NR effectively permits more signal to be recorded than would otherwise be the case.

Dolby C NR achieves about 20 dB of noise reduction, Dolby B about 10 dB, and dbx NR about 30 dB. Thus, Dolby C can attain a signal-to-noise ratio of more than 70 dB, Dolby B over 60 dB, and dbx more than 80 dB, all on a weighted basis referred to the 3% harmonic-distortion level at 315 Hz.

Dolby C requires matching the re cording and playback levels for each tape formulation. This requires adjusting the deck's internal or external controls so that treble cut in playback will match (or "track") treble boost in re cording to achieve flat response. (Re member that, in both Dolby systems, treble change varies with signal level.) The dbx NR system requires no such matching. However, noise-reduction systems tend to have unwanted side effects, namely "pumping" or "breathing" (audible changes of the background noise level as the NR action begins and ends). The greater the degree of noise reduction, the greater these effects tend to be. Most people hear little or none of these effects; some claim that they do hear them and are bothered by them. Dolby C tends to have lower distortion than dbx at low and moderate signal levels, while dbx tends to have less distortion at high signal levels.

Having both Dolby and dbx in one deck enables the user to play tapes recorded on other decks with one NR system or the other. And it gives the user the choice of whichever system sounds better to his own ears with respect to tapes both recorded and played on his deck.

Switch-Setting Confusion

Q. I own an old cassette deck which has a "Normal/CrO2-equalization switch. The service manual indicates that the CrO2 setting rolls off the high frequencies during recording and does nothing during playback, which agrees with the audible effect of the switch. I note, however, that most cur rent decks, including my car cassette deck and my Walkman-type player, include a normal/CrO2 switch that seems to roll off the highs during playback.

Assuming that it would be inappropriate to equalize during both recording and playback, which would be more advantageous-to record with the CrO2 setting and play back with the normal setting, or to record with normal and play back with CrO2?

-Gerald Zuckier, New Haven, Conn.

A. Normal (Type I) and CrO2 (Type II) tapes require different bias settings in recording and are normally used with different equalization settings in both recording and playback. Some decks have separate switches for bias and EQ, while others have a single switch handling both.

The "Normal/CrO2" switch (or the Type I/II switch on some decks) ordinarily does the following: In recording, it supplies more treble boost (and, if there's no separate bias switch, more bias) for Type II than Type I tapes. In playback, it supplies more treble cut for Type II than Type I tapes, a difference reaching about 1 dB at 1 kHz, about 3 dB at 2.7 kHz, and an eventual maximum of 4.4 dB at 20 kHz (using 400 Hz as the 0-dB reference). I realize that this contradicts your service manual, according to your letter. But unless your deck differs uniquely from most others, that is the case.

If your Type I/II switch controls both bias and EQ, then you must record CrO2 tapes in the "CrO2" (or Type II) position to avoid excessive distortion and exaggerated treble, and you must record normal ferric tapes in the "Nor mal" (Type I) position to avoid excessive treble loss. That being the case, response should be flattest when you set the EQ switch to match the tape in playback, too. If you mismatch the tapes and EQ switch in playback, you will get a bit more highs when playing CrO2 tapes in the " Normal" position and a bit softer highs when playing ferric tapes in the "CrO2" position; with some recordings, you might even prefer these deviations from flat response.

If your deck has separate bias and EQ switches, you still must match the bias to the tape in recording, but you are free to try using the "wrong" equalization in recording and playback; you will still get reasonably flat response, as long as you use the same EQ set ting for both recording and playback.

Using the " Normal" EQ setting will give you the best high-frequency response your tape is capable of, but riot quite the lowest noise. Using the "CrO2" set ting wilt give you the lowest noise, but not quite the most extended high-frequency response. The latter equalization was deemed better for CrO2 tapes because those tapes have more high-frequency response to start with.

Limiter Switch

Q. My tape deck is equipped with a limiter switch. Judging from the user's manual, this limits excessive peaks while recording, but the manual is far from clear. I have several questions:

What exactly does the limiter do? Should it be used only in recording, or in playback as well? At what levels should I record with the limiter on and with the limiter off (the meters are peak-reading)?

-Stuart Munro; Brighton, Mass.

A. A limiter is a compressor which only goes into action when the signal being recorded approaches the tape saturation level. It therefore reduces the likelihood of distortion due to over recording, though at the expense of compressing the dynamic range of loud sounds. This is better than com pressing the entire signal (as an ordinary compressor would), but still not as good in terms of fidelity as recording without a limiter at levels just low enough to prevent over-recording. Limiters are most often used when recording live events, where signal levels are unpredictable. They work most satisfactorily for nonmusical material, where accurate dynamic range is usually less critical.

To record with the limiter off, without distortion, you should set the recording level so the meters do not exceed approximately 0 VU. However, you should experiment. Depending on the tape you use, and quite likely on the program material too, you may find that you can go a few dB above 0 VU without noticeable deterioration in sound quality, thus improving the S/N ratio. With the limiter on, you may be able to aim a bit higher still.

Companding

Q. I am a retired bass-guitar player. I tape my own arranged backgrounds and add a bass line plus other effects for guitar-chord jazz. I am interested in a compressor for recording and an expander for playback, and need information concerning this technique. There is no one near me to help, and San Diego is 40 miles away.

-Jerome Geller, Lake San Marcos, Cal.

A. I suggest that you review the listing of noise-reduction units (including companders, expanders, etc.) on page 240 of the October 1984 issue of Audio. Select those manufacturers who offer units of possible interest to you, and write to them for information.

Essentially, the combination of a compressor and expander will enable you to get more information on the tape and, with expansion in playback, re duce the noise of the tape recording system. (Plans for building such a unit were in our February '85 issue.) It seems well worth your while to go into San Diego to hear for yourself what such units can do. If your tape system already has a high signal-to-noise ratio--say, up in the high 60-dB range, or into the 70s--they might do little or nothing for you.

Tape Hiss

Q. I have noticed that a new (unrecorded) tape produces less hiss than an erased tape. Is this normal?

-J. Carl Shrader, New York, N.Y.

A. If you are referring to a tape erased by the tape deck, yes, it is normal for such a tape to have some what greater hiss than a virgin (new) tape. The reason is that any distortion in the waveform from the deck's bias oscillator, which also drives the erase head, causes noise. There is almost always some distortion in the wave form, although usually less in decks of high quality than in others.

To minimize hiss, some people disable their deck's erase heads and use a bulk eraser prior to recording. A tape that has been carefully bulk-erased has about the same amount of hiss as one that is new.

(Source: Audio magazine, May 1985, HERMAN BURSTEIN)

= = = =