Academic journal article ARSC Journal

The Columbia Lp Equalization Curve

Academic journal article ARSC Journal

The Columbia Lp Equalization Curve

Article excerpt

The RIAA recording characteristic, which has been in widespread use since it became an industry standard in 1956, uses a hybrid of constant-amplitude and constant-velocity cutting (Figure 1). Three turnover, or transition, frequencies are involved in this curve, which we'll call F1, F2 and F3. (1) The low-bass turnover, F1, is set at 50.05Hz (this is sometimes call the low-bass shelf, since it flattens the response at the lowest frequencies). The bass turnover, F2, is at 500.5Hz, and the treble transition is 2122Hz. These three frequencies correspond to time constants of 3180[micro]S (T1), 318[micro]S (T2), and 75[micro]S (T3). The RIAA curve is actually defined by the three time constants; those numbers are essential for the design of accurate playback equalization circuits.

Below F1 the record is cut with a constant-velocity characteristic. Between F1 and F2 the characteristic is constant-amplitude. Between F2 and F3, the characteristic reverts to constant-velocity. Above F3 the characteristic is constant amplitude and remains so up to the top of the audible spectrum. Phonograph records are generally played back with magnetic cartridges (pickups), which are velocity-sensitive devices. When playing a constant-velocity recording, magnetic cartridges produce a flat frequency response; a constant-amplitude characteristic produces an output that rises with frequency at the rate of 6dB/octave. Hence, the hybrid recording characteristic shown in the upper portion of Figure 1, played with a magnetic cartridge, produces the frequency response shown by the solid line at the bottom of Figure 1. The curve shown by the solid line is often referred to as the recording characteristic, but it really represents two different pieces of information. As a graph of the recording characteristic, it is a graph of recorded velocity, not recorded amplitude. It becomes a graph of recorded amplitude when the record is played with a velocity-sensitive magnetic pickup. The complementary playback response, provided by the phono preamplifier, is shown by the dashed line; the resulting flat playback response is shown by the dotted line across the center.

The treble portion of the curve is frequently specified in terms of the number of decibels of velocity boost at 10kHz in recording process--which becomes an amplitude boost when the record is played with a magnetic pickup--and the corresponding cut required at that frequency during playback. The RIAA curve is [+ or -] 13.73dB at 10kHz: 13.73dB of boost in the recording process, and a complementary 13.73dB of roll-off in playback. Note that the treble portion of the curve can be specified three different ways:

1. +dB (record) or -dB (playback) at 10kHz

2. The actual transition frequency ([+ or -] 3dB frequency)

3. The time constant associated with the transition frequency

The formula for converting the playback attenuation (-dB) at 10kHz to the actual transition ([+ or -]3dB) frequency is given in the sidebar accompanying this article. Table 1 gives the transition frequencies for common 10kHz playback attenuation levels. For a detailed discussion of disc recording equalization, readers are referred to the paper cited in Reference 1 (a .PDF version is available for download).

Conflicting Opinions

Prior to the standardization of the RIAA curve, each record label had its own, and preamplifiers for disc playback manufactured in the 1940s and 1950s often had adjustable equalization to accommodate the myriad curves in use at the time. The Columbia [Lp.sup.2] equalization curve pre-dated industry-wide adoption of RIAA by eight years. The year after Columbia introduced the long-playing record, its inventors Peter C. Goldmark, Rene Snepvangers and William S. Bachman published a technical article on the new recording system, and included the graph shown in Figure 2 (Reference 2). Since Rene Snepvangers did most of the work on the Columbia Lp record, and has traditionally received the least amount of credit, I will refer to Figure 2 as the "Snepvangers Graph" throughout the remainder of this paper. …

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