RIAA equalization
The RIAA equalization curve was intended to operate as a de facto global industry standard for records since 1954, but when the change actually took place is difficult to determine.
[1] Before then, especially from 1940, each record company applied its own equalization; over 100 combinations of turnover and rolloff frequencies were in use, the main ones being Columbia-78, Decca-U.S., European (various), Victor-78 (various), Associated, BBC, NAB, Orthacoustic, World, Columbia LP, FFRR-78 and microgroove, and AES.
The net result is a flat frequency response, but with attenuation of high-frequency noise such as hiss and clicks that arise from the recording medium.
However, this can add an extra step in processing a sample, and may amplify or reduce audio quality deficiencies of the sound card being used to capture the signal.
In 1926, Joseph P. Maxwell and Henry C. Harrison from Bell Telephone Laboratories disclosed that the recording pattern of the Western Electric "rubber line" magnetic disc cutter had a constant-velocity characteristic.
G. H. Miller in 1934 reported that when complementary boost at the turnover point was used in radio broadcasts of records, the reproduction was more realistic and many of the musical instruments stood out in their true form.
West in 1930 and later P. G. H. Voight (1940) showed that the early Wente-style condenser microphones contributed to a 4- to 6-dB midrange brilliance or pre-emphasis in the recording chain.
Brilliance such as this compensated for dullness in many early magnetic pickups having drooping midrange and treble response[citation needed].
As a result, this practice was the empirical beginning of using pre-emphasis above 1,000 Hz in 78 and 331⁄3 rpm records, some 29 years before the RIAA curve.
In the United States, practices varied and a tendency arose to use higher bass turnover frequencies, such as 500 Hz, as well as a greater treble rolloff such as −8.5 dB, and more.
Empirically, and not by any formula, the bass end of the audio spectrum below 100 Hz could be boosted somewhat to override system hum and turntable rumble noises.
When the record was played back using a complementary inverse curve (de-emphasis), signal-to-noise ratio was improved and the programming sounded more lifelike.
In a related area, around 1940 treble pre-emphasis similar to that used in the NBC Orthacoustic recording curve was first employed by Edwin Howard Armstrong in his system of frequency modulation (FM) radio broadcasting.
FM radio receivers using Armstrong circuits and treble de-emphasis would render high-quality, wide-range audio output with low noise levels.
When the Columbia LP was released in June 1948, the developers subsequently published technical information about the 331⁄3 rpm, microgroove, long-playing record.
[5] Columbia disclosed a recording characteristic showing that it was like the NAB curve in the treble, but had more bass boost or pre-emphasis below about 150 Hz.
In 1951, at the beginning of the post-World War II high fidelity (hi-fi) popularity, the Audio Engineering Society (AES) developed a standard playback curve.
If records were engineered to sound good on hi-fi amplifiers using the AES curve, this would be a worthy goal towards standardization.
Ultimately, the New Orthophonic curve was disclosed in a publication by R. C. Moyer of RCA Victor in 1953;[7] additional background information about this evolution can also be found in another article of the same author, published in 1957.
As a result, some audio manufacturers today produce phono equalizers with selectable EQ curves, including options for Columbia LP, Decca, CCIR, and TELDEC's Direct Metal Mastering.
In 1995, an unqualified source erroneously suggested that Neumann cutting amplifiers applied a single high-frequency zero at 3.18 μs (about 50 kHz) and that a complementary zero should therefore be included upon playback.
This so-called IEC amendment to the RIAA curve is not universally seen as desirable, as it introduces considerable amplitude and—of more concern—phase errors into the low-frequency response during playback.
The simple first-order roll-off also provides only very mild reduction of rumble,[11] and many manufacturers consider that turntables, arm, and cartridge combinations should be of sufficient quality for problems not to arise.
