Los Senderos Studio
Emphasis and Equalization Curves
 Issue 57 June 2014 

Words from the Glossary

We continue our journey through the terms from our glossary by taking a look at emphasis and equalization curves.

Emphasis and Equalization Curves

This month's terms: emphasis (including de-emphasis and pre-emphasis), equalization curve, Academy curve, RIAA curve, X curve, NAB curve, CCIR curve, IEC curve,

The RIAA Curve
The RIAA Curve

Have you ever used software to duplicate a CD and it asked you if you wanted to apply pre-emphasis? If so, maybe you were as baffled as I was the first time I encountered the term.

So what is pre-emphasis? To understand the concept, we need to go back to the early days of talking movies. I'm sure you have watched an old-time movie and heard the annoying crackling in the background of the soundtrack. The sound on a movie soundtrack consists of a strip through which light is projected, with the varying amount amount of light being converted into sound. As a film was played over and over, the sound track would accumulate dust, dirt, and scratches that audibly affected the sound—the aforementioned crackling.

Engineers decided something needed to be done to correct the problem. So they began recording soundtracks while emphasizing the upper frequencies, those most affected by the dust and scratches. Upon playback, they decreased the same frequencies by an equal amount, which lowered the response back to normal, but also decreased the amount of noise. The entire process was called emphasis. The process of boosting the high frequencies during recording is called pre-emphasis and the decoding process is called de-emphasis.

In 1938, they standardized the process by using a standardized equalization curve called the Academy curve, named after the Academy of Motion Picture Arts and Sciences (AMPAS). A complimentary curve is applied during playback to restore the frequency response to normal. In the 1970s, the Academy curve was replaced with an updated version called the X-curve. Of course, today most motion picture sound is digital and requires no noise reduction system.

The same process was later used on LP records. Dust and scratches accumulated in the grooves, which led to pops and noise. So they boosted the highs during recording so that the noise could be decreased. However, they also had another problem with the lows. A big heavy bass beat on a record would sometimes cause the stylus (needle) to jump completely out of the groove. To correct this problem, they decreased the bass frequencies during recording. This process was standardized with the RIAA curve (named for the Record Industries Association of America) shown above right. As you can see the diagram shows the curve used during recording (pre-emphasis) and the one used during playback (de-emphasis) which results in a normal response curve.

A similar process was later used on professional tape recorders. Because magnetic recording tape has an inherent hiss, they boosted the high frequencies during recording and de-emphasized them during playback, greatly reducing the hiss problem. In the US, the NAB curve is used for this process. However, in England and most of Europe, they use either the CCIR or IEC curve. Slightly different equalizations that accomplish the same task.

So why would you use pre-emphasis while duplicating a CD? No reason I can think of.

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