TAPE GUIDE (Jan. 1983)

Dolby B through Dolby C

Q. Can I play Dolby B-encoded cassette tapes through Dolby C decoding?

-Ward Audio; Kahului, Hawaii

A. I don't see why you would, since Dolby C decoders and tape decks with Dolby C tend to have Dolby B as well. If you try it, however, the main result of the mismatch will be a drop in treble response roughly equal to that produced when playing non-Dolby tapes through a Dolby B decoder.

A more common problem is having to play Dolby C tapes through a Dolby B decoder. This will emphasize the treble, much as playing Dolby B tapes with no decoder does.

Either way, there will also be other, mild response irregularities, because Dolby C processing covers a wider range than Dolby B. No harm will be done to either tape, tape deck or decoder, of course.

FM Hiss Frequency

Q. I have heard a lot about using FM interstation hiss to test the ability of a tape deck to give faithful audio reproduction. Approximately what frequency is this hiss?

-Rob Canaday; Northwood, Ohio

A. In a stereo tuner, FM hiss consists of all frequencies that the electronics are capable of reproducing up to nearly 19 kHz. On most tuners, a very sharp filter is used to remove the 19-kHz pilot signal, after it has served its purpose for stereo, so as not to interfere with operation of a tape deck.

Thus, there are an infinite number of frequencies from about 0 Hz to 19 kHz in the FM hiss you hear. However, these frequencies are not of equal amplitude. At the low end, below 20 Hz or so, tuners tend to be limited in their response, producing a drop-off. Between, say, 20 Hz and 19 kHz, the tuner's de-emphasis circuit produces declining amplitude as frequency rises. In the case of standard 75-uS de-emphasis, the decline begins (3 dB down) at 2,122 Hz and approaches a drop of 6 dB per octave. In the case of Dolby 25-µS de-emphasis, the decline begins at 6,366 Hz.

Thus, referring to 75-µS de-emphasis, there is essentially equal noise energy per unit bandwidth--say, 1-Hz bandwidth--from about 20 to 2,122 Hz, and declining energy per unit bandwidth above 2,122 Hz. However, as we go up the audio spectrum, there are twice as many unit bandwidths in each octave. Hence, the amount of noise energy doubles with each successive octave up to 2,122 Hz, with equal noise energy per octave thereafter. The total effect is to impart a hissy character to the FM noise since the lower octaves contain less noise energy. In the case of 25-µS de-emphasis, noise energy doubles with each octave up to 6,366 Hz so that FM noise becomes even brighter.

Premature Peaking

Q. My cassette deck has peak indicators in addition to VU meters for setting recording level. During recording, the peak indicators start flashing while the VU meters are registering a level of -4 dB. I checked this on FM interstation hiss. When / record music, the VU level is usually about -10 dB when the peak indicators start flashing. This makes the meters useless as far as I'm concerned. I obtain good results by using the peak indicators, so I conclude that the meters are improperly set. Can the meters be recalibrated so that they show 0 VU on peaks, and would it be possible for me to do this?

-Dan Zimmett; Saint Marys, Pa.

A. Peak-indicator LEDs read peak program levels; most tape-recorder meters (including all true VU meters) read average program levels, which are always lower. The difference between peak and average is greater on musical program material than on essentially steady-state test signals such as FM interstation noise. Therefore, there is no way you can get average- and peak-reading indicators to read identically on all types of program material, though you could have the meters recalibrated to match the LEDs on one specific type of signal.

The VU readings are by no means "useless"; recording engineers in this country used only VU indications, without supplementary peak indicators, for decades. However, it does put you in the unfortunate position of the man with two watches, who's never quite as sure what time it is as he was when he had only one.

Both indicators can be useful, though. If you want to aim for minimum distortion, use the peak-reading LEDs as your guide to maximum permissible recording levels, and your meters as a guide to permissible minimums. If you want to record for best signal-to-noise ratio, use your meters' zero indication as your permissible maximum under most conditions, recording a bit above that point on program material where the LEDs never flash and a bit below that point on material where the LEDs flash often.

Why Monitor?

Q. My preamplifier contains a switch for tape monitoring. What, then, are the advantages of a three-head cassette deck with monitoring features on the deck itself?

-Merrill German; Baltimore, Md.

A. True monitoring signifies that while a tape is being recorded, one can also hear the tape playback with only a fraction of a second delay, owing to the distance between the record and playback heads. This is possible only if the deck has separate record and playback head gaps and electronics. If the same head is used for both record and playback, it is impossible to record and play simultaneously.

In the case of a three-head deck, the tape monitor switch enables you to listen either to the signal fed into the deck for recording, or to the playback signal from the tape. Preamps generally offer a similar switching arrangement. Monitoring through your speakers requires that both the tape deck's and preamp's monitor switches be set to monitor position. Accessories which preempt the tape jacks, such as equalizers, frequently have their own monitor jacks and switches. With headphones plugged into the deck, you can monitor without using the preamplifier's monitor switch.

The purpose of tape monitoring is to assure the recordist that the quality of the signal on the tape is satisfactory, and to facilitate adjustments if it is not. Otherwise, one may discover only in playback, and all too late, that the recording is not satisfactory.

(Source: Audio magazine, Jan. 1983, HERMAN BURSTEIN)

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