TAPE GUIDE (Feb. 1989)

How's DAT?

Q. A recent advertisement for a DAT deck made some interesting claims.

Can a DAT recorder really "eliminate background noise and make perfect copies of old phonograph records," as the ad claims? What about noise from worn surfaces of 78-rpm discs?

-Allan Hibsch, Oroville, Cal.

A. My own experience with DAT decks, together with what I have learned from reliable quarters, confirms that a DAT deck provides extremely flat frequency response to about 22 kHz, very high S/N ratio (90 dB or better), extremely low (inaudible) distortion, and extremely little (undetectable) wow and flutter.

However, a DAT deck can only reproduce the signal which it is fed, preserving both its good and bad aspects. Therefore, it cannot eliminate or reduce noise already present in the signal. To cope with such noise, you would need a dynamic noise-reduction device-see your local audio dealer.

NR Mismatch

Q. What kind of results can I expect from tapes recorded with Dolby A NR and played back with Dolby B or C NR, and vice versa?

-Tom Posa III, Lebanon, Conn.

Q. If I record with Dolby C NR, can I play back with dbx NR, and vice versa?

-Boris Acosta, Los Angeles, Cal.

A. Dolby A, B, and C and dbx noise reduction systems have in common the fact that they are all compression/expansion systems, with the downward expansion in playback serving to reduce the noise of the tape system.

However, they each cover substantially different frequency ranges and differ in the amount of compansion they use.

Furthermore, Dolby systems are level dependent, meaning that the amount of compression in recording and the corresponding expansion in playback varies with the signal level. The dbx NR system is not level-dependent and therefore does not require matching the record and playback levels, as do the Dolby systems.

Dolby A NR divides the entire audio range into four bands and operates separately in each band for maximum effect. Dolby B NR operates essentially from about 500 Hz upward, and Dolby C NR from about 150 Hz up. Dolby C NR provides about 8 to 10 dB more noise reduction than Dolby B NR. The dbx system achieves even greater noise reduction, about another 10 dB, and operates essentially over the entire audio range.

Because of the above differences, encoding with one system and playing back with another tends to produce substantial anomalies in frequency response. This doesn't necessarily mean the results are unlistenable-that depends on the program material, the listener's tolerance for deviations from flat response, and the ability of the listener's tone controls and/or equalizer to compensate for the anomalies.

In such instances, I always advise listeners to experiment with the various combinations of noise-reduction systems available to them and to learn for themselves. No harm can come to the equipment, and one experiment is worth many words.

In closing, it should be noted that the incompatibility between Dolby B and C NR is sufficiently moderate so that correction via the treble control can often satisfy the ear. If the recording is in Dolby B NR and playback in Dolby C NR, treble boost is called for. If record/ playback is from Dolby C to Dolby B NR, treble cut is required.

Head Life

Q. I am having trouble telling whether or not my cassette deck's heads are worn out. The sound is fine, but there are two lines running through the middle of the pinch roller. How will I know when to replace the heads?

-Dave Marner Jr., Cedar Rapids, Iowa

A. Usually, with a three-head deck, the first head that needs replacement is the playback head; in the case of a two-head deck, it's the record/playback head. When this happens, the most obvious result is a noticeable drop in treble response.

The lines running through your pinch roller have probably been made by the tape as it sheds its coating. The roller (as well as the capstan and the heads) should be cleaned periodically. The usual rule is to clean them after every eight to 16 hours of play, using a fluid recommended by the deck manufacturer; 91% isopropyl alcohol, generally available in drug stores, is usually considered a satisfactory fluid. Often, a fluid containing trichlorotrifluoroethane is recommended.

Cons of HX Pro

Q. I would like to know why some high-quality cassette decks, such as Nakamichi's, do not have Dolby HX Pro?

-Daniel H. Larie, Madison, Wisc.

A. The idea behind HX Pro is that the treble content of the program material behaves like bias, calling for an increase or decrease in oscillator bias as the treble content decreases or increases. The question is whether changes in oscillator bias are of the correct amount from moment to moment.

Apparently, Nakamichi and others feel it is not worthwhile to follow the HX Pro approach of sensing the bias effects of the program material and altering oscillator bias correspondingly.

Nakamichi and some other companies have chosen different ways of preserving treble response, instead of using HX Pro. Such approaches include the use of separate high-quality record heads which do not easily saturate, and fine playback heads with very narrow magnetic gaps that provide excellent treble response. Skillfully engineered record and playback electronics with very low noise reduce the need to push recording level to the utmost in order to achieve a satisfactory S/N ratio. Thus, the danger of tape saturation, with consequent treble loss and distortion, is lowered-in other words, headroom is increased. Use of a separate head for playback increases signal output and thereby improves S/N ratio, again reducing the need to push recording level to the utmost. With the use of Dolby NR, particularly Dolby C NR, S/N is greatly improved. Furthermore, the Dolby C NR encoding characteristic limits the amount of treble boost in the extreme treble region, thereby limiting the danger of tape saturation and resultant treble loss. For this reason, treble response at high recording levels tends to be better with Dolby C NR than with Dolby B NR, or even without noise reduction.

(Adapted from: Audio magazine, Feb. 1989, HERMAN BURSTEIN)

= = = =