RIAA equalization
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RIAA equalization is a specification for the correct playback of gramophone records, established by the Recording Industry Association of America (RIAA). The purpose of the equalization is to permit greater playback times, improve sound quality, and to limit the vinyl damages that would otherwise arise from recording analog records without such equalization.
The RIAA equalization curve has operated as a de facto global industry standard for the recording and playback of vinyl records since 1954. Before then, especially from 1940, each record company applied its own equalization; there were over 100 combinations of turnover and rolloff frequencies 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.
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[edit] The RIAA curve
RIAA equalization is a form of preemphasis on recording, and deemphasis on playback. A record is cut with the low frequencies reduced and the high frequencies boosted, and on playback the opposite occurs. The result is a flat frequency response, but with noise such as hiss and clicks arising from the surface of the medium itself much attenuated. The other main benefit of the system is that low frequencies, which would otherwise cause the cutter to make large excursions when cutting a groove, are much reduced, so grooves are smaller and more can be fitted in a given surface area, yielding longer playback times. This also has the benefit of eliminating physical stresses on the playback stylus which might otherwise be hard to cope with, or cause unpleasant distortion.
A potential drawback of the system is that rumble from the playback turntable's drive mechanism is greatly amplified, which means that players have to be carefully designed to avoid this.
RIAA equalization is not a simple low-pass filter. It carefully defines transition points in three places - 75 µs, 318 µs and 3180 µs, which correspond to 2122 Hz, 500 Hz and 50 Hz. Implementing this characteristic is not especially difficult, but more involved than a simple amplifier. Most hi-fi amplifiers have a built-in phono preamplifier with the RIAA characteristic, though it is often omitted in modern designs, due to the gradual obsolescence of vinyl records. A solution in this case is to purchase an outboard preamplifier with the RIAA equalization curve, which adapts a magnetic cartridge to a standard line-level input. Some modern turntables feature built-in preamplification to the RIAA standard. Special preamplifiers are also available for the various equalization curves used on pre-1954 records.
Digital audio editors often feature the ability to equalize audio samples using standard and custom equalization curves, removing the need for a dedicated hardware preamplifier when capturing audio with a computer. However, this can add an extra step in processing a sample, and may amplify audio quality issues of the sound card being used to capture the signal.
[edit] History
[edit] Origins of pre-emphasis
Equalization practice for electrical recordings dates to the beginning of the art. In 1926 it was disclosed by Joseph P. Maxwell and Henry C. Harrison from Bell Telephone Laboratories that the recording pattern of the Western Electric "rubber line" magnetic disc cutter had a constant velocity characteristic. This meant that as frequency increased in the treble, recording amplitude decreased. Conversely, in the bass as frequency decreased, recording amplitude increased. Therefore, it was necessary to attenuate the bass frequencies below about 250 Hz, the bass turnover point, in the amplified microphone signal fed to the recording head. Otherwise, bass modulation became excessive and overcutting took place into the next record groove. When played back electrically with a magnetic pickup having a smooth response in the bass region, a complementary boost in amplitude at the bass turnover point was necessary. 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. This meant that the electrical recording characteristics of Western Electric licensees such as Columbia Records and Victor Talking Machine Company had a higher amplitude in the midrange region. Brilliance such as this compensated for dullness in many early magnetic pickups having drooping midrange and treble response. As a result, this practice was the empirical beginning of using pre-emphasis above 1,000 Hz in 78 and 33 1/3 rpm records, some 29 years before the RIAA curve.
Over the years a variety of record equalization practices emerged and there was no industry standard. For example, in Europe,for many years recordings required playback with a bass turnover setting of 250 to 300 Hz and a treble rolloff at 10,000 Hz ranging from 0 to -5 dB, or more. In the United States there were more varied practices and a tendency to use higher bass turnover frequencies, such as 500 Hz, as well as a greater treble rolloff like -8.5 dB, and more. The purpose was to record higher modulation levels on the record.
[edit] Standardization
Evidence from the early technical literature concerning electrical recording suggests that it was not until the 1942-1949 period that there were serious efforts to standardize recording characteristics within an industry. Heretofore, electrical recording technology from company to company was considered a proprietary art all the way back to the 1925 Western Electric licensed method first used by Columbia and Victor. For example, what Brunswick-Balke-Collender (Brunswick Corporation) did was different from the practices of Victor.
Broadcasters were faced with having to adapt daily to the varied recording characteristics of many sources: various makers of "home recordings" readily available to the public, European recordings, lateral cut transcriptions, and vertical cut transcriptions. Efforts were started in 1942 to standardize within the National Association of Broadcasters (NAB), later known as the National Association of Radio and Television Broadcasters (NARTB). The NAB, among other items, issued recording standards in 1949 for laterally and vertically cut records, principally transcriptions. A number of 78 rpm record producers as well as early LP makers also cut their records to the NAB/NARTB lateral standard.
The lateral cut NAB curve was remarkably similar to the NBC Orthacoustic curve which evolved from practices within the National Broadcasting Company since the mid-1930s. Empirically, and not by any formula, it was learned that the bass end of the audio spectrum below 100 Hz could be boosted somewhat to override system hum and turntable rumble noises. Likewise at the treble end beginning at 1,000 Hz, if audio frequencies were boosted by 16 dB at 10,000 Hz the delicate sibilant sounds of speech and high overtones of musical instruments could survive the noise level of cellulose acetate, lacquer/aluminum, and vinyl disc media. 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 33 1/3 rpm, microgroove, long playing record. 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 150 Hz. The authors disclosed electrical network characteristics for the Columbia LP curve. This was the first such curve based on formulae.
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. This was intended for use by hi-fi amplifier manufacturers. If records were engineered to sound good on hi-fi amplifiers using the AES curve, this would be a worthy goal towards standardization. This curve was defined by the time constants of audio filters and had a bass turnover of 400 Hz and a 10,000 Hz rolloff of -12 dB.
RCA Victor and Columbia were in a "market war" concerning which recorded format was going to win: the Columbia LP versus the RCA Victor 45 rpm disc (released in February 1949). Besides also being a battle of disc size and record speed, there was a technical difference in the recording characteristics. RCA Victor was using "New Orthophonic" whereas Columbia was using the LP curve.
Ultimately the New Orthophonic curve was disclosed in a publication by R. C. Moyer of RCA Victor in 1953. He traced RCA Victor characteristics back to the Western Electric "rubber line" recorder in 1925 up to the early 1950s laying claim to long-held recording practices and reasons for major changes in the intervening years. The RCA Victor New Orthophonic curve was within the tolerances for the NAB/NARTB, Columbia LP, and AES curves. It eventually became the technical predecessor to the RIAA curve and superseded all other curves. By the time of the stereo LP in 1958, the RIAA curve, identical to the RCA Victor New Orthophonic curve, became standard throughout the national and international record markets.
[edit] IEC RIAA curve
An improved version of the curve was proposed to the International Electrotechnical Commission with an extra high-pass filter at 20 Hz (7950 µs). The justification was that DC coupling was becoming more common, which meant that turntable rumble would become a greater problem. However, the proposal did not achieve traction, as manufacturers considered that turntables, arm and cartridge combinations should be of sufficient quality for the problem not to arise.
[edit] References
- Powell, James R., Jr. The Audiophile's Technical Guide to 78 RPM, Transcription, and Microgroove Recordings. 1992; Gramophone Adventures, Portage, MI. ISBN 0-9634921-2-8
- Powell, James R., Jr. Broadcast Transcription Discs. 2001; Gramophone Adventures, Portage, MI. ISBN 0-9634921-4-4
- Powell, James R., Jr. and Randall G. Stehle. Playback Equalizer Settings for 78 RPM Recordings. Second Edition. 1993, 2001; Gramophone Adventures, Portage, MI. ISBN 0-9634921-3-6
- Description and diagrams of RIAA equalization curve
