RIAA EQUALIZATION CURVE
FOR PHONOGRAPH RECORDS.
By Don Hoglund
"Curve? We don't need no stinkin' curves." The simplest form of disk-cutter consists of an amplifier, similar to that used to drive a loudspeaker, connected to a cutting-head having a stylus connected to a coil, which is placed in the field from a strong magnet (or, more usually in later designs, a magnet within a coil). When the signal is applied to the coil, the stylus moves and engraves a groove in the blank disk. This is, of course, the simple explanation. (See photo of Westrex 3D cutter head at work above.)
However, because the cutter head's movements translate the amplitude swings of the original signal into velocity - the rate at which the stylus moves during its swings - low-frequency signals would be recorded with a much larger swing than high-frequency signals of the same original amplitude. So, the low frequency grooves would be much wider than the grooves on an equalized disk. Wider grooves take up more room which reduces the available recording time. They are also much harder for the cartridge to track which increases distortion. The solution is to reduce the amplitude of low frequencies during disk cutting and then boost them with a reverse curve during playback.
Another problem is distortion and signal-to-noise ratios in the high frequencies. Early disc recording equipment did not have the extended high frequency capabilities of today's modern equipment. However, as disk cutters improved during the 1940's through the 1960's the need to address the high frequencies increased. The solution was to boost the high frequencies during cutting and then reduce them during playback. Now there was a high and low curve with a "knee" frequency.
Each 3 dB difference in the curve is double the volume. Some of the curves boosted or reduced certain frequencies by 18 dB or 64 times. The "knee" or "cutoff frequencies" ranged from 250 to 500 Hz in the lower end and 6KHz to 15KHz in the upper end. With that much difference in the volume and cutoff points, the curve could not be ignored during playback. Tone controls normally operate at the extremes and don't reach the knee frequency. So, you do need a playback decoding curve.
"Which curve is best?" Well, there's always Gena Davis's curves on Phil's 10ft. HDTV. Or Columbia, Decca, N.A.B., R.C.A., B.B.C., E.M.I., C.C.I.R., A.E.S., or maybe Orthacoustic to name but a few? The experts argued for decades about which curve to use. There were compromises with every option. Each curve traded and balanced signal-to-noise, distortion, trackability, rumble, disk space, and other sonic qualities against each other. The critics complained about "tracking", "muddiness", "smearing", "overmodulation by second & third harmonics of the soprano voice", "cymbals and brass instruments overload the system", and more. While each manufacturer adopted the curve that suited them, the individual disk mastering engineers altered the "company curve" to suit their own preferences. The equipment manufacturers tried to keep up with all the different equalization curves and build playback circuits with curves that would make their own equipment sound "good". Audio equipment in the 1950's had a separate knob for the playback curves. The user had anywhere from 3 to maybe 5 or more choices for the desired playback curve. This meant checking each album cover for the recommended curve or keeping a log nearby for quick reference. Users who stacked their records on automatic changers had to pick a single curve for the whole stack or run to the record changer at every change. Many 78 rpm acoustic recordings had no curve, so their tremendous sonic potential is absolutely ruined by any playback curve. Some record makers didn't give the recommended curve, so the user had to experiment. Others even lied about which curve they used. Imagine your stereo system is located in a busy airline terminal and everyone who passes turns the bass and treble tone controls to different extreme positions. Now imagine you are blind folded and trying to make your system sound "right". That was life before the RIAA standard. In a word, there was CHAOS!!!!
R.I.A.A. to the rescue. In 1955 the Recording Industry Association of America published a new standard equalization curve that was adopted by the entire industry of record and audio equipment makers. Eventually all equipment had only the one playback curve and the EQ knob disappeared. In 1965 the National Association of Broadcasters created the NAB Test Record which "constitutes the only test record made in the U.S.A. certified by any organization which writes and issues standards." Finally there was one curve and one standard by which to check it. The NAB Test Record has individual frequencies recorded on it at the exact correct level. By playing this disk on your stereo system and checking the system output with a decibel meter, you can see if your system is correctly decoding the RIAA Curve. (See photo of RIAA encoding and decoding curves below.)
"But, why do I need to even check it? Surely the equipment makers today have adopted the RIAA standard and built the correct playback curve into their gear?" The short answer is, probably they have. But, the problem is not in the amplifier circuit. The problem is in the cartridge loading. If the phono cartridge is not properly loaded with the correct resistance and impedance, it will not play back flat. It will send the wrong curve to the amplifier for decoding. In other words, garbage in - garbage out. Many modern preamps have user adjustable cartridge loading just for this purpose. (Ahem, like our Granite Audio preamp.) The RIAA feedback loop in the amp assumes that it is getting the correct curve to begin with. I've lost count of the number of systems that I've checked and found the frequency response off by 6 to 10 dB due to improper cartridge loading. That means the volume at certain frequencies was off by 4 to 10 times! Many times a 10 cent resistor is making an $800.00 cartridge squeal like a ruptured canary. Since most high end audio equipment is sans tone controls, the listener is stuck with some bad or harsh sounds.
"How do I check my RIAA curve?" The first step is to check the amplifier. Plug an audio frequency generator into the amp's phono input. Or use the Granite Audio Model #CD-101 Phono Burn-In & RIAA Test CD in your CD player plugged into your phono input and use tracks 1-22. Plug a decibel meter into the output. Feed the RIAA test frequencies in at the recommended volume for each. The meter should give a constant reading of 0 dB. Now do the same test with the NAB Test Record supplying the input signal via the turntable. If your response is way off, you will need a knowledgeable tech to modify the loading. Now, spin some vinyl.
References: Radiotron Designer's Handbook, Fourth Edition; NAB Engineering Handbook, Fifth Edition; Gotham Audio Corp., Engineering Bulletin 26.1.77; NAB Test Record #12-5-98.