Pre-emphasis

The pre-emphasis (accentuation), engl. preemphasis and deemphasis , means increasing the high frequencies and lowering the low frequencies during recording or transmission ( linear pre-emphasis ). It is then reversed during playback or when receiving ( equalization , de-emphasis , de-emphasis, engl. Deemphasis ), so that overall a faithful transfer takes place. This approach reduces the noise that occurs more intensely at high frequencies.

The cutting characteristic (blue) and its counterpart, the RIAA playback characteristic (red) as de-emphasis for vinyl records

application

The emphasis process is used in radio and audio tape transmission, vinyl cutting, many early CDs of the 1980s and occasionally in digital transmission. There must be standards with corresponding time constants for this forward and reverse equalization , from which the associated transition or limit frequencies can be calculated using the following formula: ${\ displaystyle \ tau}$ ${\ displaystyle f_ {c}}$

{\ displaystyle \ tau = {\ frac {1} {2 \ pi \ cdot f_ {c}}}}

The advantage of this method is the low circuit complexity. The disadvantage is that the frequencies raised during the pre-distortion can no longer be picked up or transmitted at the maximum level .

The method can be used because a large part of the acoustic events relevant for humans takes place in the frequency range up to about 6 k Hz , above which almost only overtones with low amplitudes occur.

Used parameters

		Lowering the bass		Raising the treble
		Time constant	Crossover frequency	Time constant	Crossover frequency
VHF radio ¹				Europe: τ ₁ = 50 µs North America: τ ₁ = 75 µs	Europe: 3.18 kHz North America: 2.12 kHz
Records		τ ₂ = 318 µs τ ₃ = 3180 µs ( increase of the deep bass)	0.50 kHz 0.05 kHz	τ ₁ = 75 µs	2.12 kHz
CDs ²				τ ₁ = 50 µs τ ₂ = 15 µs ( lowering of the uppermost heights)	3.18 kHz 10.61 kHz
Cassette recorder		τ ₂ = 3180 µs ( increase of the deep bass)	0.05 kHz	Reduction : IEC I bands: τ ₁ = 120 µs IEC II / III / IV bands: τ ₁ = 70 µs	1.33 kHz 2.27 kHz
Reel tape recorders	Belt speed 4.76 cm / s	τ ₂ = 3180 µs ( increase of the deep bass; home studio devices)	0.05 kHz	Reduction : IEC I bands: τ ₁ = 120 µs	1.33 kHz
	Belt speed 9.5 cm / s	τ ₂ = 3180 µs ( increase of the deep bass; home studio equipment)	0.05 kHz	Reduction : IEC I bands: τ ₁ = 90 µs IEC II bands: τ ₁ = 50 µs	1.77 kHz 3.18 kHz
	Belt speed 19 cm / s	τ ₂ = 3180 µs (home studio equipment; increase of the deep bass); without increase ( studio devices)	0.05 kHz	Reduction : IEC I bands: τ ₁ = 50 or 70 µs IEC II bands: τ ₁ = 35 µs	3.18 or 2.27 kHz 4.55 kHz

Notes:
1) The time constant used in European VHF broadcasting improves the signal-to-noise ratio by around 13 dB .

The different time constant in VHF broadcasting in North America means that VHF radio sets that were produced for Europe have a somewhat poorer signal-to-noise ratio when operated in North America and reproduce higher tones too loud, as the pre-distortion in the The transmitter does not match the equalization in the receiver. Conversely, American VHF radios in Europe have a somewhat duller sound, with a likewise poorer signal-to-noise ratio.

These differences in the emphasis parameters only have a significant effect on high-quality receivers. With simple VHF radios such as portable radios , these differences are hardly noticeable, if only because of poor speaker systems.

2) Pre- / de-emphasis are provided for in the CD standard , but were only used on a few CDs in practice. In this case, a corresponding status bit indicates to the CD player that it has to activate the de-emphasis.

literature

Thomas Görne: Sound engineering. 2nd edition, Hanser, Munich 2008, ISBN 978-3-446-41591-1 .
Hubert Henle: the recording studio manual. 5th edition, GC Carstensen Verlag, Munich 2001, ISBN 3-910098-19-3 .
Andreas Friesecke: The audio encyclopedia. A reference work for sound engineers, KG Saur Verlag, Munich 2007, ISBN 978-3-598-11774-9 .
Stefan Weinzierl (Ed.): Handbook of audio technology. Springer Verlag, Berlin 2008, ISBN 978-3-540-34300-4 .
Li Deng, Douglas O'Shaughnessy: Speech Processing . A Dynamic and Optimization-Oriented Approach, Marcel Dekker Inc, New York 2003, ISBN 0-8247-4040-8 .

Web links

Individual evidence

↑ Brechmann, Dzieia, Hörnemann, Pretty, Jagla, Petersen: Electrical Tables communication technology . 3. Edition. Westermann, Braunschweig 2001, ISBN 3-14-225037-9 .

[1] Brechmann, Dzieia, Hörnemann, Pretty, Jagla, Petersen: Electrical Tables communication technology . 3. Edition. Westermann, Braunschweig 2001, ISBN 3-14-225037-9 .