Time constants and playback curves

Vinyl records, magnetic tape and FM radio all use deliberate, standardised frequency-response curves. The signal is shaped when it is recorded and corrected with the opposite curve on playback. The curves are described by time constants.

Why record with a curve at all

Each medium has a limit:

• Vinyl: big bass amplitudes would make the groove too wide, and strong treble would cut it distorted. Reducing bass and boosting treble on cutting fits the signal within the limits.
• Tape: the high frequencies are weak and the tape's own noise is strong. Boosting highs on record and cutting them on playback reduces the noise as well.
• FM radio: the noise spectrum of FM rises with frequency, so the treble is boosted at the transmitter and cut in the receiver.

The playback filter reverses the record curve, so the end result is flat, and the noise added in between is reduced in the treble.

Time constants

A first-order filter is described by its time constant τ, related to the corner frequency by

f = 1 / (2π · τ)

Convert with the time constant tool.

RIAA

The record industry standardised one curve in the 1950s. It has three time constants, and the playback equaliser (in a phono preamplifier) boosts the bass and attenuates the treble: relative to 1 kHz about +19 dB at 20 Hz and −20 dB at 20 kHz. Before the RIAA curve, record companies used their own curves, which explains why old records can sound wrong when played with the RIAA curve.

Tape

Tape equalisation standards define time constants that depend on the tape speed. The two best-known systems are the American NAB curve and the European IEC (formerly CCIR) curve. They differ at high frequencies; at 15 in/s (38 cm/s) NAB uses 50 µs and IEC 35 µs. The correct curve depends on the machine and the tape, so check the documentation.

Why it matters today

• Phono inputs have an RIAA equaliser. A line input does not, which is why a turntable connected to a line input sounds thin and with no bass.
• A digital copy of an old tape or record needs the correct playback curve.
• The same idea appears in modern noise reduction and in pre-emphasis and de-emphasis in some digital formats.

Moving between filters and time constants

A first-order low-pass filter built with a resistor and a capacitor has τ = R · C. A 10 kΩ resistor with 100 nF gives 1 ms, which is 159 Hz. See the RC filter tool.

Draft: written by an AI assistant, awaiting technical review.