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AUDIO _ _

Basically Speaking

Audio concepts and terms explained by Michael Riggs

v°:

The Stereo Recording/ Playback Chain

Last month I talked a linie aboui fre-quency responsc and its importance as a performance specification. li is only one of many. of course: they form the tech-nical foundation on which audio buying decisions are madę. and I shall be con-sidering others in futurę columns.

Sueh specifications wouldn’t be nec-essary if there were sorne way to catch the sound of a performance whole in a bonie to take home and uncork at your leisure. Unfortunately. that isn't pos-sible. so weTe forced to usc an elaborate and rather circuitous procedurę that is. in concept, analo-gous to writing down words '■ that reprcsent thoughts for other people to reconstruct by reading the symbols. If the writing (or the reading) is poor. the thoughts will not be accu-rately conveyed.

The same is true in audio. If anything goes wrong. whether at the recording (writing) or playback (reading) stage. some aspect of the sound of the origi-nal performance will be lost or corrupted. The fault could be a frequency-response error. or ii could be something else. These other possibilities and the specifications that pertain to them will be explored in Corning months. But first. so we don't get lost in a sea of abstractions. let's take a short tour of the stages involved in bringing musie from the concert hall or studio to your listening room.

The first step is to translate the sound waves created by the musicians into electrical waves. This is done bv a

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microphone. whose main element is a smali diaphragm (usually less than an inch in diameter) that acts much like your eardrum: When struck by sound waves. it vibrates in sympathy. The diaphragm is part of (or is attached to part of) an electrical generating system whose output is an alternating voltagc at the same frequency as the sound wave with an amplitudę proportional to that of the original vibrations.

This signal is then amplified and-in somecases— equalized (i.e.. its frequency response is deliberately altered, either to compensate fordeficiencies in the microphone or its placement. or to achieve some special effect).

From here. the signal goes into a tape recorder. which imprints the electrical waveform as a pattern of magnetiza-tion on a thin plastic tape that has been specially coated with particles of a suit-ably permeable materiał like iron oxide. For stereo recording at least two micro-phones are necessary. and in most classi-cal and almost all pop and jazz record-ings many morę than that are actually used; it is not unusual to see live mikes placed around a set of drums. Usually. the output from each microphone goes onto a separate track on the tape. These tracks. often sixteen or thirtv-two in

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number. run parallel to each other along the length of the tape.

Mowever. there are only two chan-nels in a standard (that is. not quadri-phonic) stereo system. No matter how many tape tracks a producer starts out with. he must eventua!ly reduce them. or mix them down. to just two. Using a de-vice called a mixer. the producer eom-bines the various tracks in whatever pro-portions he ńnds most pleasing and transfers the finał mixdown through an-other recorder onto two-track tape. Ad-ditional equalization or other signal processing may also be applied at this stage.

This tape—or. morę often. a copy of it is the source from which records are madę. The two-track master tape is played baek and its output fed to a disc-mastering lathe. which converts the vari-ations of the electrical signal (derived from the tapes magnetic pattern) into minutę squiggles in a groove as it is created by the cutterhead in a lacquer master disc. Some additional modifications may be madę to the signal at this point to adjust it to the limilations of dises. In all cases. for example. the high frequen-cies are boosted (to drown out surface hiss) and the Iow frequencies are rolled off (to reduce space requirements and. therefore. inerease playing time) accord-ing to the industry-standard RIAA equalizalion curve. The completed lac-quer master goes through a series of plating and duplication steps to produce a set of metal stampers. which are used to press the vinyl LPs you buv in shops.

To play these records back. you first need a lumtable to spin the disc at the correct speed as the Stylus of a phono cartridge follows the record’s spiral groove guided by a tonearm. The cartridges liny diamond Stylus vibrates in response to the undulations cut in the rec-ord groove. A tubular canti-lever attached to the Stylus iransmits these vibrations back to a generating element within the body of the cartridge. which produces an electrical voltage whose frequency and amplitudę correspond to those of the groove modulations.

The signal passes to an j-y amplifier. which reverses the RIAA equalization built into the recording and boosts the pickup*s liny voItage to a level great enough to power a loudspeaker. The amplifier (or receiver. w hich includes an amplifier) has a variety of Controls that enable you to adjust volume. frequency balancc. and other sonie characteristics to your taste.

The finał link in the chain. the loudspeaker is essentially an enlarged mirror image of a microphone. The power from the amplifier causes the speaker’s diaphragm (or. in the case of a multiple-driver system, diaphragms) to vibrate in the air and thereby to create sound. In a perfect world. you would hear just what the microphones heard. Unfortunately. nene of the many links that con-nect the recording studio to your living room is flawless. Each in its own way adds to or subtracts from the sound of the original.

Next month. we’ll look at twoof the most basie (and annoying) gremlins: noise and distortion.    HF