TAPE GUIDE (Jan. 1989)

Tape-to-Deck Matching

Q. I recently purchased an Onkyo cassette deck with Accubias. The manufacturer recommends Maxell tape, but I previously was using TDK. Will Accubias correct for the differences between the two tape brands?

-Ray Maken, Rochester, N.Y.

A. It is likely, but not certain, that your deck will perform best with the tape recommended by the manufacturer. If you use other tapes, Accubias should enable you to get good performance with them.

Lest there be any misunderstanding, let me add that Accubias can do nothing for you in playback. Bias is employed only in recording (to achieve low distortion and high recording level on the tape). When playing your old tapes, which were recorded on another deck, there is the possibility of deficient treble response owing to a difference in azimuth between the head employed in recording and the head employed in playback. Your audio system's treble control--or better yet, a built-in or stand-alone equalizer--may be able to compensate fairly satisfactorily.

Gilding the Lily

Q. I have just purchased a Hi-Fi VCR without any built-in noise-reduction system. Would adding an external noise-reduction unit make any difference?

-Stuart Cohen, Austin, Tex.

A. I doubt that a noise-reduction unit would produce significant audible benefit. All such units have some side effects, albeit usually unnoticeable.

Risking these possible side effects in view of the minimal benefits does not appear worthwhile.

Those VCRs which contain noise reduction apply it to the linear (edge) audio track, which normally has an S/N ratio of something like 45 dB. With Dolby B NR, S/N rises beyond 50 dB. In contrast, the FM recording system of a Hi-Fi VCR achieves an S/N ratio of 80 dB or better.

Magnetic Field Effect

Q. Because of limited space, I am forced to use my speakers as "shelves" to hold my audio components. As a result, my cassette deck is directly on top of one of the speakers.

Does the speaker's magnetic field affect the quality of my cassettes in recording or playback? (The woofer is mounted very near the top of the speaker enclosure, and it has a very strong magnet which can distort a TV picture when the TV is positioned where I now have the cassette deck.) Can I shield the deck by placing it on a sheet of metal, or must I move it further away from the speaker, even though this is impractical for me?

-Robert F. Zurn, White Bear Lake, Minn.

A. A pretty strong magnetic field is needed to cause significant erasure of a cassette tape, especially with Type II (ferri-cobalt or chromium dioxide) and even more so with Type IV (metal) tape. Ordinarily, the cassette has to be brought within 3 inches of the magnetic source in order for the recording to be affected.

I suggest that you experiment by recording a cassette and bringing it as close as possible to the speaker after you have completed a few minutes of recording. Then play the tape, preferably comparing it with the source-such as an album. Careful listening should allow you to ascertain whether there is any impairment.

If you believe that the speaker is affecting the tape, try using Styrofoam or some other material to raise the deck a few inches above the speaker.

If you are stacking components, you might put another component between the speaker and the deck. This Solution, however, raises the possibility that the deck's playback head will pick up hum from the transformer of the component on which it rests (a problem that occurs with some components and not others). I doubt that ordinary metal will shield the cassette deck. Mumetal, if you can get hold of some, might, but this material is expensive. (Editor's Note: The last source we knew of was Magnetic Metals, 21st and Hayes Streets, Camden, N.J. 08105. -E.P.)

Signal Processing for the Recordist

Q. I tape on an open-reel deck and sometimes use an equalizer and a Dolby noise-reduction unit. Should I be doing so? On which sources should I use them and on which not?

-George Maraudas; Upper Marlboro, Md.

A. If your open-reel deck is sufficiently quiet without Dolby NR, taking into account the levels at which you customarily listen, you probably should not use any NR. If you feel that you need a significant improvement in S/N ratio, use of an NR device-Dolby or dbx-is indicated. In the case of Dolby NR, Dolby C is preferable to Dolby B, in order to achieve a greater reduction in tape system noise and to better avoid tape saturation in the treble range.

If you decide that your tape system isn't sufficiently noise-free, use NR with all sources. Of course, an encode-decode system (such as Dolby or dbx) will do nothing about noise already present in a signal source. A dynamic noise-reduction unit can help, more or less, in the case of noisy signal sources.

Preferably, EQ should be used mainly or entirely in playback. Using it in recording may risk distortion if you boost treble and/or bass substantially.

However, some source material may demand EQ in recording, if you wish to obtain tapes that make for pleasurable listening when playback occurs on another audio system.

The equalizer is useful in compensating for deficiencies in either your audio system (for example, speakers which are bass-shy or shrill) or the program source. In the first case, you would leave the desired EQ on whenever reproducing tapes or other sources. In the second case, you would use EQ only as necessary. If system EQ is also needed, as in the first case, you would modify the system EQ as the program source required, and afterward return to the system EQ settings.

If you need the equalizer for recording as well as playback, it should be in the tape loop. Equalizers usually incorporate a switch so that, when thus situated, they can apply EQ either in recording or playing any source. If the equalizer is not intended for use in recording, a desirable location is between the preamp and the power amp (or in the case of receivers and integrated amps, between the preamp and power amp sections, if feasible).

Audio on Video Recorders

Reader Brent Jessee of Hoffman Estates, Illinois has a number of illuminating remarks on the subject of high-fidelity recording on a VCR: I have been recording with a Hi-Fi VCR and a PCM processor for three years. The one serious drawback to recording on a Hi-Fi VCR that you and Joseph Giovanelli failed to mention in the September 1988 issue is tape dropouts. Even the best grades of videotape have them. Deep dropouts can produce an audible tick or pop. Frequent use of a tape can increase the number of dropouts. While there seems to be no method of dropout compensation with a Hi-Fi VCR, there is one in PCM. In the 14-bit EIAJ Standard, an error of up to 32 lines can be concealed-that is, TV horizontal lines, as this is a video-encoded PCM. Thirty-two lines is a big dropout!

Recording in a slow-speed mode with a Hi-Fi VCR aggravates the problem. Dropouts then pass the heads more slowly and wreak more havoc.

Recording with a PCM converter gives excellent error correction, the ability to make digital dubs directly, and, at least in Sony units, the ability to decode and encode at the same time.

This last feature enables me to "bounce" tracks by decoding the digital signal, adding signal processing in the analog loop, and returning to another tape in the digital domain.

Use of a Hi-Fi VCR and PCM gives me four high-quality tracks: Two in analog form on Hi-Fi and two in digital form (via the PCM) on video. For two track work, I can use the Hi-Fi analog tracks as a backup. This can be an asset, when recording live, if either the video or Hi-Fi heads clog. Then I have a much better chance of obtaining at least one good set of stereo tracks.

For Hi-Fi VCR and/or PCM recording, I suggest using only the fastest speed on the VCR and employing the very best grade of tape. When recording live, be aware that the very low noise floor of the Hi-Fi and PCM recordings can be marred by hiss from consumer-grade microphones and mixers. Stick with professional mikes, and buy the best you can afford.

In conclusion, both Hi-Fi VCR and PCM recording produce very good results. I prefer PCM, but because PCM processors are becoming harder to find, I hope DAT will soon become widely available.

Automatic Bias Problem

Q. My cassette deck has a feature which, with the aid of internal test tones and a microprocessor, enables me to obtain optimum bias for whatever tape I use. This feature requires that I manually turn a knob until a red light appears, indicating that bias is correct.

However, the knob setting is seldom the same, even though I use the same brand and kind of tape. Sometimes, the setting even varies on the same cassette. Is there a problem with my tapes or cassette deck?

-Len Lochmiller; North Hollywood, Calif.

A. One explanation for why the bias setting changes is that varying voltage conditions in your location may cause the amount of bias at a given setting to change somewhat. Another explanation is as follows: The decks that allow you to adjust bias to an optimum point operate on the principle of equating the response of a high-frequency tone, such as 10 kHz, with the response of a low-frequency tone, such as 300 or 400 Hz. Response of the high-frequency tone depends not only on bias but also on azimuth; azimuth is correct when the head gap is exactly at right angles to the length of the tape. But there may be enough play in the cassette shell, or perhaps in the deck, to allow azimuth to change slightly, so that the best bias setting also changes.

A third explanation is simply that something is wrong in your deck and requires servicing.

The real test is in what you hear. If, despite the varying optimum bias settings, the audible results are satisfactory, then this is what counts.

Double Noise Reduction

Q. My home tape deck has only Dolby B noise reduction. All my recorded tapes have been made on this machine with Dolby B NR. The audio system in my new car provides DNR (Dynamic Noise Reduction). Are Dolby B NR and DNR compatible? Most new home decks have Dolby B and C as well as dbx NR. Which would be most compatible with the DNR in my car's audio system?

-Harry Schwartz; Diamond Bar, Cal.

A. A fundamental rule in audio is that one should not tamper with the signal any more than is strictly necessary. While Dolby (either B or C) and dbx NR are not incompatible with DNR, it is inadvisable to use DNR unless you find that noise is excessive.

It is possible to use DNR together with Dolby B or C NR, because the DNR and Dolby systems work in totally different ways. With Dolby B NR, which only achieves signal-to-noise ratios in the range of 60 to 69 dB, it is possible that you would find system noise excessive when playing tapes at a loud level in a quiet home. Using DNR (if home decks had it) with Dolby B NR would reduce this noise. In the far less quiet environment of a moving car, I think it unlikely that using the two together would add to your enjoyment: However, there is no harm in trying.

With Dolby C NR, which achieves S/N ratios of 70 dB or more, it is quite unlikely that you would find tape noise excessive in a car, so use of DNR is all the more contraindicated. On the other hand, if you are playing a tape that contains a serious amount of noise--due to the program source or under-recording--DNR might then prove a valuable aid.

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

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