Key terms in phono EQ curves — turnover, time constants, curve notation, and more

Glossary

Definitions of technical terms used throughout this site.

Constant amplitude and constant velocity

The signal cut into a record groove exhibits two fundamental behaviors.

• Constant amplitude: The groove excursion (amplitude) remains the same regardless of frequency. At lower frequencies, the cutter stylus moves more slowly, but the amplitude stays the same
• Constant velocity: The cutter stylus moves at the same speed regardless of frequency. At higher frequencies, the amplitude becomes smaller

Why does this distinction matter? If low frequencies were recorded at constant amplitude, the groove excursion would become so large that adjacent grooves would interfere with each other. The solution is to reduce the amplitude of low frequencies during recording (constant velocity) and boost them back during playback. This is the starting point of phono equalization.

→ What is phono equalization?

→ Pt.1 — the physics of constant velocity and constant amplitude

Turnover frequency

The frequency at which the transition from constant amplitude (low frequencies) to constant velocity occurs.

For example, in the RIAA standard the turnover is set at 500 Hz. This means that frequencies below 500 Hz are recorded with reduced amplitude, and the corresponding boost is applied during playback.

Historically, the turnover frequency varied by label and era:

• Earliest electrical recording (c. 1925): around 250 Hz
• Early 1930s onward: Columbia and RCA Victor raised it to around 500 Hz
• AES (1951): 400 Hz
• RIAA (1954): 500 Hz (equivalent to a time constant of 318 μs)

→ Why wasn't there a unified standard from the start? (In a Nutshell Part 1)

→ What EQ curves existed before RIAA?

High-frequency pre-emphasis

A technique in which the high frequencies are boosted during recording, then cut by the same amount during playback to restore the original signal.

Record surfaces are always accompanied by surface noise — the "hiss" caused by vinyl grain and the pressing process. This noise is concentrated in the high frequencies, so attenuating the highs during playback reduces noise. But doing so without compensation would also lose the high-frequency musical content.

The solution: boost the highs during recording (pre-emphasis), then cut them during playback (de-emphasis). The musical signal is restored to its original level, while the noise is reduced.

This concept dates back to the 1930s Bell Labs–Stokowski experimental recordings (vertical-cut transcription discs). It was adopted for consumer records from the 1940s onward, and the amount of pre-emphasis (how many dB of boost at 10 kHz) varied by label and standard. The RIAA standard specifies +13.7 dB at 10 kHz (time constant of 75 μs).

→ Stokowski and Bell Labs: the experimental recordings

→ Why wasn't there a unified standard from the start? (In a Nutshell Part 1)

Bass shelf (low-frequency shelving)

A technique in which the very low frequencies are boosted during recording and attenuated during playback. This is applied to a frequency range below that of the turnover (typically below 50–100 Hz).

The purpose of the bass shelf is to reduce turntable rumble (low-frequency vibration from the motor and bearing mechanism) and the effects of record warping during playback.

The concept was introduced in the late 1930s, when 33⅓ rpm transcription discs came into use in the broadcast industry. At a slower rotation speed than 78 rpm, turntable vibrations interfered with the groove at lower frequencies, making this countermeasure necessary. The 1942 NAB standard included a bass shelf.

However, not all curves included a bass shelf. The AES curve (400N-12) and RCA Victor's Old Orthophonic curve (500N-12) had no bass shelf — which is why they are marked "N" in the curve notation system.

→ What EQ curves existed before RIAA?

Time constant

A value used to describe the characteristics of an EQ curve, expressed in microseconds (μs).

Time constants and frequencies are related by the following formula:

Frequency (Hz) = 1,000,000 ÷ (2π × time constant in μs)

For example, the three time constants of the RIAA standard are:

For historical reasons, EQ curves are sometimes expressed in frequency and sometimes in time constants. In earlier documentation, it was also common to describe the characteristic in dB values, such as "+X dB at 10 kHz."

Curve notation (how to read "500C-16")

The curve tables on this site use the following notation format:

[Turnover frequency][Bass shelf type]-[High-frequency pre-emphasis amount]

Each component:

• Number (500, 400, 630, etc.): Turnover frequency (Hz)
• Letter:
  • N = No bass shelf
  • B = NAB-type bass shelf (time constant 3,180 μs = 50 Hz; derived from the NAB lateral curve)
  • C = Columbia-type bass shelf (time constant 1,590 μs = 100 Hz)
  • R = RCA/RIAA-type bass shelf (time constant 3,180 μs = 50 Hz; identical to NAB type)
• Number after the hyphen (16, 13.7, 12, 10.5, etc.): High-frequency pre-emphasis at 10 kHz (dB)

Examples:

→ What EQ curves existed before RIAA?

Recording curve and playback curve

The recording curve and the playback curve are mirror images of each other.

• Recording curve: The frequency response intentionally applied during cutting. Low frequencies are attenuated; high frequencies are boosted
• Playback curve: The inverse of the recording curve. Low frequencies are boosted; high frequencies are attenuated, restoring the original signal

What a phono equalizer does is apply the playback curve.

An important caveat: in historical documents, the word "curve" is sometimes ambiguous as to whether it refers to the recording side or the playback side. For example, "AES curve" refers to the playback curve defined by the AES, but it is sometimes used to refer to the corresponding recording curve as well. Context matters.

→ What is phono equalization?

Microgroove and widegroove

A classification based on the width of the record groove.

• Widegroove (coarse groove): The wide groove used on 78 rpm shellac records. Groove width was approximately 6 mil (about 0.15 mm), with a standard playback stylus tip radius of 3 mil (about 0.076 mm). Roughly 85–100 grooves per inch were cut
• Microgroove (fine groove): The narrow groove introduced by Columbia with the LP in 1948. Groove width was approximately 2.5 mil (about 0.064 mm), with a playback stylus tip radius of 1 mil (about 0.025 mm). This allowed roughly 224–300 grooves per inch

By making the groove narrower, far more grooves could be packed onto the same disc surface, dramatically extending playing time. However, several technological prerequisites had to be met:

1. Light tracking force — Playing a fine groove with a heavy stylus would destroy the groove. The 5 g tracking force that Pierce & Hunt achieved in the laboratory in 1938 finally reached consumers with the 1948 LP player
2. Vinyl material — Shellac was hard and brittle, unsuitable for microgrooves. A flexible material — Vinylite — was needed
3. Hot stylus technology — A technique for cutting stable fine grooves with a heated cutter stylus

This chain of technologies is covered in detail in the playback technology evolution FAQ.

→ How did record playback technology evolve?

→ Who invented the LP?

← Back to FAQ