Disc Recording Equalization Demystified

Disc Recording Equalization Demystified By Gary A. Galo The subject of disc-recording equaliza- and much more. You can obtain mem- a record is known as the recording char- tion has generated much confusion bership information from Peter Sham- acteristic, and a typical explanation for over the years. Many knowledgeable barger, Executive Director, ARSC, PO the 33-1/3-rpm LP record is illustrated in collectors and audio professionals Box 543, Annapolis, MD 21404-0543. Fig. 1. The recording curve shows the have been content with conventional In this article, italicized terms are bass rolled off (attenuated) and the treble explanations. Transfer engineers and defined in the Glossary on p. 52. boosted, with a flatter region in the mid- collectors are well aware that electri- dle of the curve. In order to obtain a flat cally recorded discs require a bass frequency response in playback, a com- boost, and sometimes a treble cut, in INTRODUCTION plementary equalization is necessary. playback. isc-recording equalization is The playback curve shows the bass They often assume that the playback often misunderstood by boosted and the treble attenuated. correction, or equalization, compen- audio professionals and hob- This equalization is normally accom- sates only for the method by which the Dbyists alike. Even the most ca- plished in the preamplifier, which also actual recording was made. If the bass sual collector of 33-1/3-rpm long-playing provides sufficient amplification of the is attenuated during the recording (LP) records has probably encountered relatively weak signal from the phono process, it must be boosted in playback; the term RIAA equalization. Most serious cartridge (which was known in the early similarly, if the treble is boosted when collectors involved with the playback of days of electrical playback as the the record is cut, it must be attenuated 78 rpm recordings are familiar with pickup, a term still used by the British). in playback. Close examination shows terms such as bass turnover and treble If the playback equalization curve is an that the recording and playback turnover, since any audio system suit- exact mirror of the recording curve, a process is more complex. able for the playback of “historic” potentially flat response will result. Selecting playback equalization recordings must have provisions for ad- Closer investigation shows that this must take account not only of the justing these parameters. explanation is an over-simplification, at recording characteristics, but also Currently available preamplifiers with best. The “record” curve of Fig. 1 is, in those of the playback cartridge. This tu- suitable adjustment capabilities include fact, not the recording curve at all, at torial will explain the methods of cut- the veteran Owl 1, distributed by Audio least not in terms of recorded amplitude. ting disc records, the characteristics of 78, the Resolution Series high-end pre- Rather, it is the frequency response of magnetic phono cartridges, and how amps from the Swiss firm FM Acoustics, the record when played back with a their combined response determines and the Esoteric Sound Re-Equalizer, magnetic phono cartridge. the required playback equalization. which is intended to correct a modern To add to the confusion, some sources This article is based on a paper I pre- preamp’s RIAA response to match older label the rising curve constant ampli- sented at the May 1996 conference of recording characteristics. tude, and the flatter region in the middle the Association for Recorded Sound Some collectors use vintage tube constant velocity, which, on the face of Collections (ARSC) in Kansas City, and units from the 1950s, either in stock or it, makes little sense.1 These terms de- was first published in the Fall 1996 modified form, since many preampli- scribe the two basic cutting methods, issue of the ARSC Journal. The Associa- fiers from that period offer flexible but the recording curve is a result of the tion is a nonprofit organization serv- equalization settings that reflect the lack disc cutting and the response of the mag- ing librarians, scholars, sound of standards at the time they were man- netic cartridge used in playback. The archivists, dealers, private collectors, ufactured. The best of these audio “clas- principles of disc recording and playback discographers, and reviewers. The sics” include the McIntosh C-8 and the equalization are the same whether the biannual ARSC Journal is devoted to re- Marantz Audio Consolette. record is lateral, vertical, or 45/45 stereo. search on sound-recording history, As most of you are probably aware, preservation and restoration of sound disc records are not cut with a flat fre- TWO CUTTING METHODS recordings, record and book reviews, quency response. The method of cutting Constant-amplitude (Fig. 2) is the cut- 44 The LP Is Back! Magnetic transducers are velocity-sen- sitive devices—they produce a flat frequency response only when the recorded velocity remains constant as the frequency rises. Understanding the behav- ior of magnetic phono cartridges is the key to the mystery of disc recording and playback equalization. The horizontal line at the bottom of Fig. 4 shows the output of a magnetic cartridge playing a constant-velocity recording. The cartridge output is flat across the entire recorded spectrum. The slanting line in Fig. 2 shows the cartridge’s output playing a constant-amplitude recording. Here, the cartridge’s output increases as the frequency rises, at a rate of 6dB/octave. In these illustrations, the straight lines not only illustrate the playback cartridge’s relative output, but FIGURE 1: Typical graph illustrating the RIAA recording characteristic for 33¹⁄₃-rpm also show relative recorded velocity. long-playing records. The record curve shows the bass rolled off and the treble boosted. A complementary equalization during playback restores a flat response. BASIC REQUIREMENTS The cutting of a phonograph record involves two seemingly contradictory re- ting method easiest to understand. If the speed in tracing each of the waveforms. quirements. First, the record must be cut signal being recorded is at the same level Figure 4 shows a constant-velocity at a level higher than its own residual for all frequencies, the recorded ampli- recording characteristic, with the ampli- surface noise, particularly in the high fre- tude will also be the same. Constant ve- tude progressively decreasing as the frequencies. This at first appears easy—sim- locity is somewhat more difficult to quency rises. ply cut the record at a very high level, grasp. The velocity of the playback stylus and any surface noise will be virtually in- is the speed with which it moves while MAGNETIC TRANSDUCERS audible. Unfortunately, doing so would tracing the record groove, and is directly Disc playback normally involves the use violate the second requirement—play- related to the physical distance the stylus of magnetic phonograph cartridges, back tracking and tracing ability. travels during a given time period. whether the playback system is modest or If a record is cut at too high a level, Referring to Fig. 2, if the stylus is trac- “state of the art.” A phonograph cartridge the playback cartridge and stylus will be ing at a very low frequency, say 20Hz, it is a transducer, since it converts mechani- unable to track the record. At both low must move back and forth 20 times each cal energy into electricity (a transducer is and high frequencies, the cartridge will second. As the frequency rises, the num- a device that converts one form of energy be unable to cope with excessively ber of times the stylus must move back into another). There are several variations wide groove excursions. The excursion and forth in one second also increases. on the magnetic-cartridge theme, includ- is the physical distance the stylus must At 10kHz, for example, the stylus must ing moving-magnet, moving-iron, and travel from the center of the groove move back and forth 10,000 times each moving-coil. In principle, all function the modulation (called the zero crossing) second. If the amplitude of the signal re- same way—a magnetic field is in motion to either peak. mains constant, it stands to reason that relative to a coil of wire. At high frequencies, there is an addi- the stylus velocity must increase as the frequency rises. In order to keep the stylus velocity FIGURE 2: Constant- constant at all frequencies, it is necessary amplitude recording to reduce the amplitude of the recorded characteristic. The signal as the frequency increases. Figure recording amplitude is 3 illustrates this concept. The large held constant as the waveform is one cycle at an arbitrary fre- frequency increases. quency—the exact frequency does not The diagonal line matter for the purposes of illustration. shows the relative Each time the frequency doubles, the output of a magnetic amplitude must be cut in half to keep cartridge, and also illustrates the increase the velocity constant. in velocity as the fre- If each of the three waveforms in quency rises. Fig. 3 were made with a piece of string, the lengths of all three pieces would be identical. The stylus travels the same physical distance during the time period, and thus moves at the same rate of • THEORY AND CARE • The LP Is Back! 45 problems. The physical mass of the sty- FIGURE 3: Conceptual lus assembly limits the velocity at which illustration of con- the stylus can travel. stant-velocity recording. In order for the HYBRID APPROACH playback stylus to Maxfield and Harrison opted for a hybrid travel the same physical distance in a recording characteristic that used both given time period, constant-amplitude and constant-velocity the amplitude must cutting to best advantage.

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