Disc Recording Equalization Demystified
By Gary A. Galo
The subject of disc-recording equalization has generated much confusion over the years. Many knowledgeable collectors and audio professionals have been content with conventional explanations. Transfer engineers and collectors are well aware that electrically recorded discs require a bass boost, and sometimes a treble cut, in playback.
They often assume that the playback correction, or equalization, compensates only for the method by which the actual recording was made. If the bass is attenuated during the recording process, it must be boosted in playback; similarly, if the treble is boosted when the record is cut, it must be attenuated in playback. Close examination shows that the recording and playback process is more complex.
Selecting playback equalization must take account not only of the recording characteristics, but also those of the playback cartridge. This tutorial will explain the methods of cutting disc records, the characteristics of magnetic phono cartridges, and how their combined response determines the required playback equalization.
This article is based on a paper I presented at the May 1996 conference of the Association for Recorded Sound Collections (ARSC) in Kansas City, and was first published in the Fall 1996 issue of the ARSC Journal. The Association is a nonprofit organization serving librarians, scholars, sound archivists, dealers, private collectors, discographers, and reviewers. The biannual ARSC Journal is devoted to research on sound-recording history, preservation and restoration of sound recordings, record and book reviews, and much more. You can obtain membership information from Peter Shambarger, Executive Director, ARSC, PO Box 543, Annapolis, MD 21404-0543. In this article, italicized terms are defined HERE.
Disc-recording equalization is often misunderstood by audio professionals and hobbyists alike. Even the most casual collector of 33 1/3 rpm long-playing (LP) records has probably encountered the term RIAA equalization. Most serious collectors involved with the playback of 78 rpm recordings are familiar with terms such as bass turnover and treble turnover, since any audio system suitable for the playback of "historic" recordings must have provisions for adjusting these parameters.
Currently available preamplifiers with suitable adjustment capabilities include the veteran Owl 1, distributed by Audio 78, the Resolution Series high-end preamps from the Swiss firm FM Acoustics, and the Esoteric Sound Re-Equalizer, which is intended to correct a modem preamp's RIAA response to match older recording characteristics.
Some collectors use vintage tube units from the 1950s, either in stock or modified form, since many preamplifiers from that period offer flexible equalization settings that reflect the lack of standards at the time they were manufactured. The best of these audio "classics" include the McIntosh C-8 and the Marantz Audio Consolette.
As most of you are probably aware, disc records are not cut with a flat frequency response. The method of cutting a record is known as the recording characteristic, and a typical explanation for the 331/3 rpm LP record is illustrated in Fig.1. The recording curve shows the bass rolled off (attenuated) and the treble boosted, with a flatter region in the middle of the curve. In order to obtain a flat frequency response in playback, a complementary equalization is necessary. The playback curve shows the bass boosted and the treble attenuated.
FIGURE 1: Typical graph illustrating the RIAA recording characteristic for 331/3-rpm long-playing records. The record curve shows the bass rolled off and the treble boosted. A complementary equalization during playback restores a flat response.
This equalization is normally accomplished in the preamplifier, which also provides sufficient amplification of the relatively weak signal from the phono cartridge (which was known in the early days of electrical playback as the pickup, a term still used by the British). If the playback equalization curve is an exact mirror of the recording curve, a potentially flat response will result.
Closer investigation shows that this explanation is an over-simplification, at best. The "record" curve of Fig. 1 is, in fact, not the recording curve at all, at least not in terms of recorded amplitude. Rather, it is the frequency response of the record when played back with a magnetic phono cartridge.
To add to the confusion, some sources label the rising curve constant amplitude, and the flatter region in the middle constant velocity, which, on the face of it, makes little sense.1 These terms describe the two basic cutting methods, but the recording curve is a result of the disc cutting and the response of the magnetic cartridge used in playback. The principles of disc recording and playback equalization are the same whether the record is lateral, vertical, or 45/45 stereo.
TWO CUTTING METHODS
Constant-amplitude (Fig. 2) is the cutting method easiest to understand. If the signal being recorded is at the same level for all frequencies, the recorded amplitude will also be the same. Constant velocity is somewhat more difficult to grasp. The velocity of the playback stylus is the speed with which it moves while tracing the record groove, and is directly related to the physical distance the stylus travels during a given time period.
FIGURE 2: Constant amplitude recording characteristic The recording amplitude is held constant as the frequency increases. The diagonal line shows the relative output of a magnetic cartridge and also illustrates the increase in velocity as the frequency rises.
Referring to Fig. 2, if the stylus is tracing at a very low frequency, say 20Hz, it must move back and forth 20 times each second. As the frequency rises, the number of times the stylus must move back and forth in one second also increases. At 10kHz, for example, the stylus must move back and forth 10,000 times each second. If the amplitude of the signal remains constant, it stands to reason that the stylus velocity must increase as the frequency rises.
In order to keep the stylus velocity constant at all frequencies, it is necessary to reduce the amplitude of the recorded signal as the frequency increases. Figure 3 illustrates this concept. The large waveform is one cycle at an arbitrary frequency-the exact frequency does not matter for the purposes of illustration. Each time the frequency doubles, the amplitude must be cut in half to keep the velocity constant.
If each of the three waveforms in 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 speed in tracing each of the waveforms. Figure 4 shows a constant-velocity recording characteristic, with the amplitude progressively decreasing as the frequency rises.
FIGURE 3: Conceptual illustration of constant-velocity recording. In order for the playback stylus to travel the same physical distance in a given time period. The amplitude must be cut in half each time the frequency is doubled.
Disc playback normally involves the use of magnetic phonograph cartridges, whether the playback system is modest or "state of the art." A phonograph cartridge is a transducer, since it converts mechanical energy into electricity (a transducer is a device that converts one form of energy into another). There are several variations on the magnetic cartridge theme, including moving-magnet, moving-iron, and moving-coil. In principle, all function the same way---a magnetic field is in motion relative to a coil of wire.
Magnetic transducers are velocity-sensitive devices---they produce a flat frequency response only when the recorded velocity remains constant as the frequency rises. Understanding the behavior 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 also show relative recorded velocity.
FIGURE 4: Constant velocity recording characteristic. The recorded amplitude must decrease as the frequency increases. The straight line shows the relative output of a magnetic cartridge, and also shows the velocity being held constant as the frequency rises.
The cutting of a phonograph record involves two seemingly contradictory requirements. First, the record must be cut at a level higher than its own residual surface noise, particularly in the high frequencies. This at first appears easy---simply cut the record at a very high level, and any surface noise will be virtually inaudible. Unfortunately, doing so would violate the second requirement-playback tracking and tracing ability.
If a record is cut at too high a level, the playback cartridge and stylus will be unable to track the record. At both low and high frequencies, the cartridge will be unable to cope with excessively wide groove excursions. The excursion is the physical distance the stylus must travel from the center of the groove modulation (called the zero crossing) to either peak.
At high frequencies, there is an additional problem, technically called the radius of curvature, that is directly related to the physical size of the playback stylus. As you can see in Fig. 2, the playback stylus must change direction as it passes through each peak of the groove modulation. At low frequencies, the turn is far less sharp than it is at high frequencies; i.e., the radius of the curve is longer.
As the frequency increases, however, the radius becomes shorter and shorter, and a point is reached where the radius of the playback stylus is actually greater than that of the curve. This results in tracing distortion. If the tracking force is heavy enough, the playback stylus will cut its own path through the vinyl or shellac, destroying the original recorded vibrations. These problems were fully understood by Maxfield and Harrison,2 the inventors of electrical disc recording.
Neither a constant-amplitude nor a constant-velocity recording characteristic can meet all of the requirements, yet both have unique advantages. If a record is cut with a constant-velocity characteristic, playback equalization is unnecessary, since the magnetic cartridge has a flat frequency response playing this type of recording. However, constant-velocity recording has two inherent problems.
First, since the recorded amplitude rises as the frequency decreases, groove excursions at low frequencies become too large. This not only makes the record difficult or impossible for the playback stylus to track, but it also limits the playing time on the record.
Playing time is directly related to the groove pitch, which, in this context, does not refer to high or low pitch (i.e., frequency), but instead to the spacing of the record grooves. Normally, groove pitch is defined as the number of lines per inch across the radius of the record surface. (Adjacent grooves are called lines, since a record technically has only one groove---a continuous spiral from the beginning to the end of the side.) If the excursions are extremely wide, the spacing must be increased to prevent bridging of adjacent grooves, which would decrease the number of lines and thus reduce the playing time.
The second problem with constant-velocity recording is that relative to the low frequencies, the highs are recorded at an extremely low level, potentially lower than the surface noise of the record. If a wide frequency response is possible, constant-velocity cutting is not desirable at extremely high frequencies, since the surface noise will mask the treble region. This was not a concern during the early years of electrical recording, since it was not possible to achieve a wide frequency response at that time.
The particular advantage of constant amplitude cutting is that it works well at low frequencies. It holds groove excursions to a reasonable level, since the recorded amplitude does not increase as the frequency drops. It also minimizes the high-frequency noise problem, because the recorded amplitude will always be higher than the surface noise of the record.
However, there is no free lunch here, either. Recording high frequencies at these levels will also cause stylus-tracing distortion, in part due to the radius-of-curvature problem described above. Wide excursions at high frequencies also cause cartridge/stylus tracking problems. The physical mass of the stylus assembly limits the velocity at which the stylus can travel.
Maxfield and Harrison opted for a hybrid recording characteristic that used both constant-amplitude and constant-velocity cutting to best advantage. Figure 5 shows the electrical-recording characteristic used during the early years of the 78-rpm era. The lower frequencies used a constant-amplitude characteristic, while that of the treble region was a constant-velocity type. The point where the transition from a constant-amplitude to a constant-velocity characteristic occurs is known as the bass turnover frequency, or simply the turnover frequency.
FIGURE 5: Recording characteristic for early 78-rpm electrical recordings. At the bass turnover frequency, the characteristic changes from constant amplitude to constant velocity. The solid line shows the relative velocity and a magnetic cartridge's output. The dashed line shows the playback equalization required to produce the flat response shown by the dotted line
The constant-amplitude characteristic limits the groove excursions in the bass region, thus minimizing stylus-tracking problems and maximizing playing time. The constant-velocity characteristic minimizes high-frequency tracking and tracing problems by limiting groove excursion and preventing an excessively narrow radius of curvature. A comparison of Figs 2 and 4 shows that reducing the amplitude at a given frequency increases the radius of curvature.
At high recorded amplitudes, the lines connecting the peaks of the waveform are steeper; consequently, the radius at the peaks is smaller. Maxfield and Harrison were not concerned about high-frequency surface noise, since the high-frequency response of the first electrical recordings was extremely limited.
Maxfield and Harrison initially set the bass turnover frequency at 200Hz. From 200 to 4kHz, they employed a constant velocity characteristic. In their 1926 paper, they describe using an approximate constant-acceleration characteristic between 4k and 6kHz.2 For the purpose of this discussion, all that need be said about constant acceleration is that, as the frequency rises, the amplitude decays even more rapidly than with constant ve1ocity---6dB/octave faster, to be specific.1 They used constant acceleration to further minimize tracing problems in the treble region due to an excessively short radius of curvature.
EARLY PHONOGRAPH DESIGN
Maxfield and Harrison initially designed an acoustic phonograph for the playback of electrical recordings. They sold the exclusive rights to this design to the Victor Talking Machine Company, which then marketed it as the Orthophonic Victrola.
Like previous acoustic phonographs, this machine used large steel needles and a heavy, metal sound box. The only way to prevent destruction of the high-frequency information during playback was to attenuate the response above 4kHz hence the need to reduce the amplitude even more rapidly above this frequency.
Bell Laboratories subsequently modified the cutting apparatus to allow a constant-velocity characteristic up to 5.5kHz. Recordings made with the modified characteristic were best suited for electrical playback.3
With the introduction of electrical phonographs having magnetic cartridges, it was possible to equalize the signal below the turnover frequency to restore a flat response in the bass region. The ability to do so was an advantage of electrical playback that Maxfield and Harrison recognized.
The solid line in Fig. 5 shows the output of a magnetic cartridge playing a 78-rpm electrical recording. In the constant amplitude region, beginning at the lowest recorded frequency, the cartridge response rises up to the turnover frequency. Above the turnover frequency, in the constant-velocity region, the response is flat up to the highest recorded frequency. (The graphs in Figs. 5, 6, and 7 are asymptotic curves. The transition points from constant amplitude to constant-velocity are gradual, as you can see in Fig. 1. An asymptotic curve simply removes the bends in the graph for the purposes of illustration.)
Relative to the constant-velocity region, the bass frequencies are attenuated. The dashed line in Fig. 5 shows the playback equalization necessary to achieve a flat response; the bass is boosted below the turnover frequency. The desired flat response is shown by the dotted line. The term flat response is a theoretical ideal. Due to variety of mechanical and electrical limitations, this ideal was never realized during the 78rpm or LP era. Even with modern, sophisticated disc recording and playback equipment, a perfectly flat response is rarely achieved.
NONSTANDARD TURNOVER FREQUENCIES
One problem faced by those who play 78-rpm records is the lack of a standard turnover frequency. During the course of the 78-rpm era, turnover frequencies varied widely, anywhere from 250Hz to as high as 1kHz. Even a given record label may be inconsistent from year to year, or from one recording session to the next.
In order to achieve a flat response in the bass region, an electrical 78 must be equalized with the same turnover frequency with which it was recorded. If the playback turnover frequency is set too high, excessive, "boomy" bass will result; if it is too low, the bass region will sound "thin." In order to properly reproduce 78-rpm recordings, a preamplifier with adjustable bass turnover is absolutely necessary (see "Demos" sidebar below).
TREBLE TRANSITION TO CONSTANT AMPLITUDE
As electrical recording advanced, it became possible to extend the high-frequency response of the recording well beyond Maxfield and Harrison's 6kHz limit. In order to prevent surface noise from burying the extended high frequencies, later electrical 78s were cut with the characteristic shown in Fig. 6. Here, the constant-velocity characteristic was not used up to the highest recorded frequency. At a predetermined transition point, the treble transition (or treble turnover frequency), the cutting characteristic became, once again, constant amplitude.
FIGURE 6: Later 78-rpm electrical recording characteristic. A constant-amplitude characteristic is used above the treble transition frequency. The solid line shows the relative velocity and a magnetic cartridge's output. The dashed line shows the playback equalization required to produce the flat response shown by the dotted line.
This switch back to a constant-amplitude characteristic is often referred to as treble preemphasis, but this term is misleading. The treble region is boosted only in terms of recorded velocity---the amplitude is still lower than it is below the treble transition frequency.
Since the amplitude in the treble region is substantially lower than in the bass, high-frequency tracking and tracing problems are minimal. By the late 1930s, phono cartridges possessed improved high-frequency tracking abilities that made possible a constant-amplitude characteristic, provided the amplitude was held at a reasonable level.
The constant-velocity characteristic between the bass and treble turnover points then functioned as a transitional region between the higher amplitude in the bass and the lower amplitude of the treble. The solid line in Fig. 6 shows the response of a magnetic cartridge, differing from Fig. 5 in that the cartridge output rises above the treble transition frequency.
The dashed line shows the required playback equalization---the treble is now attenuated above the treble transition frequency. The dotted line shows the resulting flat response
TREBLE TRANSITION FREQUENCY ADJUSTMENT
Playback of these recordings is problematic, since the treble transition frequency was not standardized, but was typically somewhere between 2k and 3kHz. A preamplifier suitable for playback of these old recordings must allow the insertion of a treble transition frequency, if needed, and must also provide for the variation of that frequency to match the characteristics of each recording. If the playback transition frequency is set too high, the playback will be excessively bright in the treble region; if it is set too low, the high frequencies will be dull.
Every preamplifier with adjustable high-frequency equalization is calibrated in decibels of attenuation at 10kHz, with the 0dB reference set at 1kHz. In fact, what you are adjusting is the treble turnover frequency. Each setting on the preamplifier produces a 6dB/octave rolloff, beginning at a specific turnover frequency.
It is only the turnover frequency, not the rate of rolloff, that you are adjusting. The lower the turnover frequency, the greater the 10kHz attenuation (Table 1)
(This turnover frequency is really the 3dB point for the equalization circuit. At the turnover frequency, the response will be either 3dB down or 3dB up, depending on whether a cut or a boost is involved. This is why the transitions are gradual, as mentioned previously.)
Determining the exact dates when various record companies began using treble preemphasis is difficult, and beyond the scope of this work. R. C. Moyer's article3 sheds considerable light on the evolution of Victor's recording curves.
Many Victor electrical 78s recorded from the mid-'20s through the late '30s sound bright in the treble region, even though Victor was using a constant-velocity characteristic up to the highest recorded frequency. The brightness more likely has its source in the microphones or microphone preamps. Early Victor electrics used Western Electric condenser microphones, which had an elevated response in the higher frequencies. Frayne, in his interview with Sutheim, notes that "The condenser had a peak about 5 or 6dB at 3.5kHz.." 4
In 1932, Victor began using ribbon microphones. The conversion to these was gradual, though, and some recordings made through the mid '30s still used the older condenser mikes. Ribbon microphones had a much flatter high-frequency response, and consequently few people liked them as well as the condensers-many missed the presence and brilliance of the condenser mikes. In order to achieve the same recorded brightness levels, Victor engineers added equalization to the ribbon-mike preamplifiers.3 This electronic high-frequency boost was called voice effort equalization.4
A NEW RECORDING-CHAIN DESIGN
In 1938, RCA Victor redesigned the entire recording chain. They removed the high-frequency equalization from the microphone preamplifiers and added it to the disc-cutting equipment after the recording bus.3 At this point, the high frequency boost, or pre emphasis, became part of the actual disc-recording characteristic.
Some authors and engineers consider the recording characteristic to be the combined response of the actual disc-cutting equipment, the microphones, and any equalization applied in the recording bus, which includes the mike preamps. But this does not appear to have been the official position at RCA Victor.
On Oct. 30, 1935, in response to an inquiry from J.M. Kaar of Menlo Park, CA, E.C. Forman, of Victor's Recording and Record Sales Division, described the Victor recording characteristic as follows:
"From 300 cycles down to 30 cycles, the recording is made at constant amplitude and can be represented on the curve as a straight line down 20dB at 30 cycles. From 300 cycles to 5,000 cycles, the curve is flat at zero level within 11/2dB. At 6,000 cycles, the curve is up 11/2dB, with zero level again reached at approximately 6,500 cycles. The curve then trails down to -30dB at approximately 7,500 cycles."
"The above should enable you to apply the recording curve, and we believe that if you compensate your equipment in line with the above curve, you will have the counterpart of our recorder. However, you should bear in mind that instrument placement during recording plays almost as important a part as the actual recorder characteristic, and that even with proper compensation, certain records may have too many or too few lows or highs. This particular situation can be corrected only by the use of adjustable low and high-frequency tone controls to give the desired balance."
VICTOR'S OFFICIAL POSITION
The above letter is extremely significant in that it clearly states RCA Victor's official position on what constituted the recording characteristic---it is the characteristic of the disc-cutter head and its associated electronics. The 1 1/2dB peak at 6kHz is a cutter-head resonance, but the frequency-response characteristics of microphones and mike preamplifiers are not described as part of the actual recording characteristic. Interestingly, Mr. Fonnan does not discuss microphones and mike preamps at all, but he clearly recognizes that factors other than the recording characteristic can affect the tonal balance of a recording, which may make additional compensation desirable.
It is also possible that some record manufacturers introduced a moderate high-frequency boost into the recording process in order to compensate for the high-frequency rolloff that can occur at the inner grooves of a disc record. This was called diameter equalization.
Moyer also notes that additional preemphasis was applied when lacquer replaced wax as the disc-mastering medium. This was necessary to compensate for high-frequency losses that occurred when lacquer was cut with a cold stylus (a heated stylus was not used until 1950).3 Moyer does not give a specific date for the introduction of lacquer, but Powell implies that it was around 1937.5
On modern wideband playback equipment, a variety of factors can make a record sound too bright. There is nothing fundamentally incorrect about using a preamp's treble rolloff switches to correct these problems. However, it is important to understand what is being corrected. The actual disc recording characteristic should not be confused with other factors, that can cause brightness in playback.
During the late '40s and early '50s there were several attempts to arrive at a standardized recording characteristic. In 1950, the Audio Engineering Society (AES) endorsed a recording characteristic specifying a 400Hz bass turnover and a 2.5kHz treble transition frequency. The AES standard was originally proposed as a compromise playback curve that would yield satisfactory results with a variety of recording characteristics. Some very late 78s as well as many early LPs were recorded using the AES bass and treble turnover frequencies.6
THE MODERN LONG-PLAYING RECORD
Columbia's introduction in 1948 of the modern long-playing record (331/3 rpm) brought another turnover frequency into the equation. Now there were three such frequencies, and no industry standards to govern them. In 1953, RCA Victor introduced the recording characteristic shown in Fig. 7 as the New Orthophonic curve. This characteristic is similar to that shown in Fig. 6, with the addition of the third turnover frequency in the low bass region.
FIGURE 7: Modem RIAA recording characteristic for 331/3-rpm long-playing records. Bass turnover and treble transition frequencies are standardized at 500Hz and 2.122kHz. A constant-velocity characteristic is used below 50Hz. The solid line shows the relative velocity and a magnetic cartridge's output The dashed line shows the playback equalization required to produce the flat response shown by the dotted line.
Below 50Hz (actually 50.05), the characteristic is constant velocity. The bass and treble turnover frequencies have been set at 500Hz (actually 500.5) and 2.122kHz. By 1956, the Recording Industry Association of America (RIAA) had adopted the RCA Victor standard, and the entire industry followed suit.
The reason for the switch to constant velocity below 50Hz is related to low frequency noise in the playback process. As the turntable speed progressively decreases, from 78 to 33 1/3 rpm, turntable rumble and record-warp noise also become lower in frequency.
The late-78 recording characteristic, with its playback bass equalization extending down to the lowest recorded frequency, also boosts turntable rumble and record-warp noise. If the low-frequency noise is severe enough, the excessive boost can cause amplifiers and loudspeakers to operate in a nonlinear manner, producing an unacceptable amount of distortion. In the worst case, damage to these components, especially loudspeakers, is possible.
If a constant-velocity characteristic is used below 50Hz, the output of a magnetic cartridge is flat up to this frequency, as shown by the solid line in Fig. 7. Consequently, no bass boost below this frequency is needed in playback, as shown by the dashed line. This minimizes low-frequency noise problems, including turntable rumble, record-warp noise and AC power-line hum.
The flat response resulting from the playback equalization is shown by the dotted line. Columbia had set the low bass turnover at 100Hz for the first long playing records, which was even more effective at minimizing low-frequency noise problems in playback. RCA Victor decided that this frequency was too high, since it could result in excessively wide groove excursions at the lowest recorded frequencies. Hence, a frequency of 50Hz was accepted as a sensible compromise between reasonable groove excursions and noise levels.
MODIFIED NAB CURVE
The original Columbia LP record also used a bass turnover frequency of 500Hz, but its treble transition frequency was 1.6kHz. The Columbia recording characteristic was a modification of the National Association of Broadcasters (NAB) standard, a curve in use for 331/3 rpm broadcast transcriptions since 1942.
The NAB curve used the same bass and treble turnover frequencies as the Columbia LP curve, but the NAB characteristic did not have the low bass transition to constant velocity described above. The 100Hz low bass turnover was added by Columbia.5,7
Both Victor and AES found the high frequency amplitude to be too high with a 1.6kHz treble turnover, since it might cause mistracking and tracing distortion on wideband high-fidelity recordings. By 1950 improved microphones, electronics, and disc-cutting equipment made it possible to record frequencies up to an unprecedented 15kHz.6
The Columbia/NAB curve had originally been optimized for 16" discs. The radius-of-curvature problem is not nearly as severe on a 16" record, since most of the playing surface has a high linear recording speed and consequently longer physical wavelengths---than a 12" record. At any given frequency, the physical wavelengths will be shortest near the center of the record, where the linear recording speed is slowest.
The RIAA, AES, NAB and Columbia IP playback characteristics are unacceptable for nearly all 78-rpm recordings. Playback of most electrically recorded 78s with any of these curves invariably results in dull, muffled sound in the upper midrange and treble region. Early electric discs also sound bass-heavy, since the turnover frequency is too high for many of these recordings.
At least one source, Tremaine, has erroneously listed the RIAA bass turnover frequency as 1kHz. The front panel of one of FM Acoustics' Resolution Series preamps is also labeled this way in an ad in Stereophile (May 1996, p. 80). This is incorrect; 1kHz is simply a 0dB reference point to which the record and playback equalization levels at all other frequencies are compared; it is not the turnover frequency.
The first commercial electrical recordings appeared in 1925. Before then, all disc records and cylinders were made with the acoustical recording process. Although there was no electrical means by which record companies could adjust the recording characteristic, the acoustical process did possess an inherent mechanical equalization. Even the best acoustic records were limited to a frequency range of 150-4kHz, and most were even narrower.
Within their limited frequency range, acoustical recordings exhibited a constant-velocity characteristic. This characteristic yields a flat frequency response from a magnetic cartridge, but that response is flat over a very narrow range. Only those who adhere to a strictly scientific approach to restoration of historic recordings use a completely flat playback.
(Due to horn resonances and other mechanical limitations, acoustical recordings have many irregularities. These prevent them from having a perfect constant-velocity characteristic-and hence a true flat response---when played with a magnetic cartridge.)
Most listeners, including myself, find that acoustical recordings sound extremely thin in the bass when played back without any bass equalization. One solution is to set the phono preamplifier for an unequalized, flat response, and then apply a discreet amount of boost in the 100-200Hz region with an external equalizer, preferably parametric, rather than graphic. This can "warm up" the upper bass/lower midrange region, resulting in a more realistic sonic presentation, assuming there is any information on the record at these frequencies.
Another approach is to set the preamplifier to a low turnover frequency, 250-300Hz; the bass boost will add some warmth to the recording.8 This approach can be problematic, however, since the preamplifier's bass equalization will also boost low-frequency noise and rumble well below the usable range of the recording. It makes little sense to boost the low frequencies down to 20Hz if the record contains virtually no musical information below 150Hz. One restoration equipment dealer actually recommends using a turnover frequency of 1kHz for acoustic records.
Electrical recordings are not cut with a flat frequency response, but convention, all descriptions of the recording characteristics used for 78-rpm and long-playing records are often misleading. Disc records are not cut with the bass attenuated and the treble boosted, at least not in terms of recorded amplitude. Electrical recordings are made using a combination of constant-amplitude and constant-velocity cutting.
Early electrical 78s were cut with a constant-amplitude characteristic in the bass region, and a constant-velocity characteristic in the treble. Later electrical 78s and all modern 331/3 rpm LPs were cut with a constant-amplitude characteristic in the treble region, as well.
Glossary of terms and descriptions used in this article.
A CD-R of the eight recorded examples cited in the text is available for $10 in the US, $12 in Canada, $15 for all other countries. Prices include postage (air mail outside the USA). Total time on the CD is 6:45.
Send payment in US funds to: Gary A. Galo, 72 Waverly St., Potsdam, NY 13676 - USA
This copyrighted article is reprinted with the kind permission of AUDIO AMATEUR, INC., Petersborough, NH, and is taken from the book "The LP is Back"
This fascinating book is a real bargain at US $7.95 and is of interest to anyone who loves phonograph recordings. Audio Amateur (Old Colony Sound Lab) is also the publisher of AudioXpress magazine and other audio magazines.
Printable version of this article in PDF format.
Back to SMART Main page