Audioclinic (May 1985)

Analog to CD Mastering

Q. I have recently purchased a CD player and some fine CDs. Two interesting questions came to mind, based on my collection of these discs. Most CBS Masterworks CDs have, at the right of their covers: "Digitally Mastered" and "Analog Recording." In side, the manufacturing process is described as being digital: "Mastered from the original recording at the CBS Recording Studios . . . on the CBS Dis Computer system." Is this process dig ital throughout, or did they cut a master analog tape? Also, why do I hear "hiss" on these recordings?

-Paul Spens, Indianapolis, Ind.

A. The phonograph record industry has been with us a long time. Tucked away in vaults are precious master recordings of great artists-and some not so great-made on various analog formats, tape or disc. If the CD is to succeed, there must be a source of music from which to build a library.

Many of these analog tape recordings of fine performances have excellent sound. Therefore, these tapes are of ten transferred to CDs. In order for this transfer to take place, an intermediate step is necessary. The original recordings must first be transferred to a digital master tape. This is not done to improve the sound; it is, however, a necessary step in the process of converting the original product to the Com pact Disc format. Any hiss on the original tape will be transferred onto the CD. The remastering process cannot, and should not, differentiate between desired program material and hiss.

FM and Dolby Noise Reduction

Q. Why does the high-frequency response increase when an FM program is being broadcast with Dolby NR, but the NR is not being decoded at the receiver?

My receiver has a 25-µS switch which is supposed to be used for Dolby reception. Is this the same as Dolby NR? If not, what is the difference? They both seem to increase the high frequencies.

-Edwin Cruz, Chicago, Ill.

A. When a signal is Dolby encoded, whether for broadcasting or recording, highs are boosted during quiet pas sages or whenever else there isn't enough high-frequency content to mask system noise. When it's decoded, in reception or playback, those boosted highs are cut back to normal, cut ting high-frequency noise in the pro cess. As a result, a Dolby-encoded signal always has emphasized high frequencies if heard without decoding.

To maintain compatibility with listeners not using Dolby NR, stations change their pre-emphasis curve from the usual 75 uS to 25 uS, cutting highs by just about as much as the Dolby system boosts them. For absolutely correct playback, you should therefore use both Dolby decoding and 25-4 de-emphasis, .counteracting the two changes made at the station and reducing noise. Using the 25-uS de-emphasis when listening to Dolby-en coded broadcasts without Dolby de coding, or when listening to non-Dolby broadcasts, will boost the highs unnaturally. However, switching in a Dolby decoder when listening to non-Dolby broadcasts should decrease the highs, not increase them.

The best way to find Dolby broad casts is to ask your local stations when they use the process. Failing that, try decoding broadcasts; if the sound is strange, with mysterious changes in "liveness," then the program is probably not broadcast with Dolby NR. (Incidentally, only the Dolby B NR system is used in broadcasting.)

Low Speaker Impedance

Q. I would like to operate my 8- and 4-ohm speakers at the same time, but I am leery of the demands that the re suiting low impedance of 2.6 ohms will place on my power amplifier. Would you envision any problems with using Y plugs to split the preamplifier's output for feeding it to two separate power amplifiers (one amplifier for each set of speakers)? How is this same task accomplished in professional installations?

-Jim Fenwood, Hot Springs, Ark.

A. I see no problem with using Y connectors to send the signal from your preamplifier to two power amplifiers. These inputs usually have much higher impedances than the minimum the preamplifier should drive. Thus, even with the impedance seen by the preamplifier cut in half, no problems will result. If, however, the cable runs are long, the cable capacitance may become large enough to cause some high-frequency loss.

In professional installations, such as recording studios, we do much the same as described above: We wire up some jacks on a patch bay and connect them all in parallel. (This arrangement is called a "mult.") The output of a tape player might be connected into one jack, with several recorders plugged into the other jacks. I have driven 10 or more recorder inputs in this manner without any audible degradation.

Defining Ambience

Q. What is "ambience"?

-Tom Wick, Huntington Station, N.Y.

A. "Ambience" refers to your surroundings, your environment. When related to sound, it has to do with the environment in which a given recording was made. Where there is no audible ambience because of "dry" acoustics, it is sometimes introduced artificially in the form of reverberation to simulate the ambience one might expect to encounter in a live performance. The producer often uses his imagination to create an ambience unlike anything we might find anywhere, but which nevertheless enhances the recording.

We can be our own recording engineers or producers, at least as far as ambience goes. There are many devices that can be attached to our sound systems to alter ambience by adding reverberation, delaying the sound before presenting it to loudspeakers, or feeding the left channel, out of phase, into the right (and vice versa).

Switched Outlets Again

Regarding "Switched Outlets" in July's "Audioclinic": Additional electrical stress will be placed on the preamp's power-switch contacts at the time of turn-on because of contact bounce. During the 1 or 2 mS following the initial ''make" of the contacts, each time the contacts close a small magnetic field will develop (in the load). When the contacts bounce open, the magnetic field will collapse, adding back EMF (from the load) to the applied electrical potential. This, in turn, can cause more damage to the switch (contact erosion) than would be experienced with a resistive load.

Putting a three-prong plug into a two-prong socket is another matter. In a quest to make everything "idiot-proof," our latest electrical code bans adaptors having green, pigtail grounding leads in favor of adaptors having fixed, metal tabs molded into them This is to prevent the "idiot" from plugging the pigtail into a live slot on a standard, duplex wall outlet. I would suggest that others who have such problems as described by Mr. Bower stock up on the discontinued adaptors with the pigtail leads, to facilitate grounding the third wire to the preamp chassis.

-G. Carroll, Culver City, Cal.

Digital and Analog Tuner Sensitivity

Q. In terms of the ability to "pull in" and "hold" a signal, how much better are digital tuners than analog tuners?

-Steve Mello, Lowell, Mass.

A. The difference between digital and analog tuners lies in how their tuning oscillators are adjusted, whether continuously (analog) or in discrete increments (digital). This has nothing whatever to do with the ability to "pull in" weak signals, which is governed by the gain and noise figure of the rest of the tuner's front-end.

VCR Timers and Power Outages

Q. I returned home one night, expecting to find a favorite TV program neatly recorded on our VCR. Obviously, while I was gone, there was a short power outage. Our digital-type alarm clock still had the correct time, but not the VCR.

When the timer in a $700 VCR is out performed by a $20 digital alarm clock, something is definitely wrong.

-Ken Knoles, Mason City, Ill.

A. A VCR is one of the most complex pieces of equipment that one can find in the home. It is amazing that such a device, with its built-in computer, servo systems, interrelated tape elevator, and loading and running motors can be priced so modestly.

You cannot compare a simple timer with a whole VCR. If you had told me that you had a $700 timer which did not work as well as your inexpensive clock, I would heartily agree with you. As it is, I cannot. Some VCRs have provisions for maintaining the clock's settings for a period of perhaps 8 hours. If your area suffers from even brief power outages, then one of the features you need to look for when buying a VCR is a back up system for its clock. Just as you checked to see if the picture was clean, or, perhaps, that the fast search was jitter-free, so, in your case, one feature you must consider is the back up for your timer.

(adapted from Audio magazine, May 1985; JOSEPH GIOVANELLI)

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