Betoning (signal)

For alternative betydninger, se Betoning (flertydig). (Se også artikler, som begynder med Betoning)

Inden for signalbehandling er forbetoning^[1] en teknik til at beskytte mod forventet støj. Ideen er at forstærke (og dermed forvrænge) det frekvensområde, der er mest modtageligt for støj på forhånd, så der efter en støjende proces (transmission over kabel, båndoptagelse, radiokommunikation...) kan bibeholdes mere information fra det frekvensområde. Fjernelse af forvrængning forårsaget af forbetoning kaldes efterbetoning, hvilket får outputtet til at gengive det originale input nøjagtigt.^[2]^[3]^[4] Efterbetoning bliver også kaldt korrektion eller udligning.

Forbetoning og efterbetoning kaldes under et for signalbetoning eller betoning.

Standardbaserede signalbetoning er almindeligt anvendt i FM-udsendelser og vinylplader (fx RIAA-forstærker). Fx kan højfrekvente signalkomponenter fremhæves for at frembringe et mere lige FM-modulationsindeks for et transmitteret frekvensspektrum og derfor et bedre signal-støj-forhold for hele frekvensområdet.

Anvendte parametre ved forbetoning:

		Dæmpning af bas		Fremhævning af diskant
		Tidskonstant	Overgangsfrekvens	Tidskonstant	Overgangsfrekvens
FM-radio¹				Europa: τ₁ = 50 µs^[5] Nordamerika: τ₁ = 75 µs	Europa: 3,18 kHz Nordamerika: 2,12 kHz
Grammofonplader		τ₂ = 318 µs τ₃ = 3180 µs (Fremhævelse af den laveste bas)	0,50 kHz 0,05 kHz	τ₁ = 75 µs	2,12 kHz
CD'ere²				τ₁ = 50 µs τ₂ = 15 µs (Dæmpning af den øverste diskant)	3,18 kHz 10,61 kHz
Kassettebåndoptagere		τ₂ = 3180 µs (Fremhævelse af den laveste bas)	0,05 kHz	Dæmpning: IEC I-bånd (Fe₃O₄; ferro): τ₁ = 120 µs IEC II/III/IV-bånd (hhv. CrO₂; chrome, ferro-chrome, metal): τ₁ = 70 µs	1,33 kHz 2,27 kHz
Spolebåndoptagere	Båndhastighed 4,76 cm/s	τ₂ = 3180 µs (Fremhævelse af den laveste bas; Hjemmestudioapparater)	0,05 kHz	Dæmpning: IEC I-bånd: τ₁ = 120 µs	1,33 kHz
	Båndhastighed 9,5 cm/s	τ₂ = 3180 µs (Fremhævelse af den laveste bas; Hjemmestudioapparater)	0,05 kHz	Dæmpning: IEC I-bånd: τ₁ = 90 µs IEC II-bånd: τ₁ = 50 µs	1,77 kHz 3,18 kHz
	Båndhastighed 19 cm/s	τ₂ = 3180 µs (Hjemmestudioapparat; Fremhævelse af den laveste bas); uden fremhævelse (Studioapparater)	0,05 kHz	Dæmpning: IEC I-bånd: τ₁ = 50 eller 70 µs IEC II-bånd: τ₁ = 35 µs	3,18 eller 2,27 kHz 4,55 kHz

Bemærkninger:

Den anvendte ITU-R (verden incl. Europa og Danmark, excl. Nordamerika og Japan) tidskonstant gør at signal-støj-forholdet forbedres 13 dB.
Der er en statusbit som indikerer om der skal anvendes efterbetoning. På de fleste CD'eres lyd, er der ikke anvendt forbetoning.

Se også

RIAA-betoning

Referencer

^ denstoredanske.lex.dk: forbetoning, backup
^ Betoningseksempel; vinylplader: Apr 2, 2009, stereophile.com: Cut and Thrust: RIAA LP Equalization Citat: "...To obviate this and produce the constant-amplitude cut we desire, electronic equalization (called preemphasis) must be applied to the signal fed to the cutter. Ideally, this preemphasis needs to double the signal voltage for every doubling of frequency, equivalent to a 6dB/octave rise throughout the audible frequency range. This is illustrated as the red line in fig.2 alongside the actual RIAA preemphasis curve (blue), which doesn't deliver an exact constant-amplitude cut but a fair approximation of it....to restore a flat frequency response, replay equalization (deemphasis) that is the inverse of the record equalization must be applied. This is done in the disc input stages of the replay system's preamplifier (footnote 1). The RIAA pre- and deemphasis curves are defined by three time constants (T), each of which is related to a corner frequency (f) in the amplitude response by the equation:...These time constants (3180, 318, and 75 microseconds) and their respective corner frequencies (50.05, 500.5, and 2,122Hz) are depicted in the red trace of fig.3, which shows the straight-line approximation of the RIAA deemphasis (ie, replay) curve, alongside the actual curve (blue). Note that the latter is a mirror image of the preemphasis curve in fig.2, so that the two are exactly complementary: applying the deemphasis on replay cancels the effect of the preemphasis applied during cutting, to leave a flat response overall (footnote 2)..."
^ Betoningseksempel; FM-radio: daenotes.com: Pre-Emphasis And De-Emphasis Citat: "...Pre-emphasis refers to boosting the relative amplitudes of the modulating voltage for higher audio frequencies from 2 to approximately 15 KHz...De-emphasis means attenuating those frequencies by the amount by which they are boosted...", backup
^ Betoningseksempel; FM-radio: rfwireless-world.com: Pre-emphasis vs De-emphasis Citat: "...As we know noise usually will have higher amplitude and higher frequency components. This high frequency noise will lead to frequency distortion when its amplitude is higher than the components present in the modulating signal. To overcome this situation most of the FM circuits use techniques known as Pre-emphasis at the transmitter and De-emphasis at the receiver...The Pre-emphasis helps amplify high frequency signal components such that they will have magnitude higher than noise components. This lead to improvement in the Signal to Noise Ratio i.e. SNR...The De-emphasis does exactly reverse of the Pre-emphasis counterpart. It is used at the receiver part...", backup
^ Brechmann, Dzieia, Hörnemann, Hübscher, Jagla, Petersen: Elektrotechnik Tabellen Kommunikationstechnik. 3. udgave. Westermann, Braunschweig 2001, ISBN 3-14-225037-9.

Eksterne henvisninger

[1] storedanske.lex.dk: forbetoning, backup

[2] Betoningseksempel; vinylplader: Apr 2, 2009, stereophile.com: Cut and Thrust: RIAA LP Equalization Citat: "...To obviate this and produce the constant-amplitude cut we desire, electronic equalization (called preemphasis) must be applied to the signal fed to the cutter. Ideally, this preemphasis needs to double the signal voltage for every doubling of frequency, equivalent to a 6dB/octave rise throughout the audible frequency range. This is illustrated as the red line in fig.2 alongside the actual RIAA preemphasis curve (blue), which doesn't deliver an exact constant-amplitude cut but a fair approximation of it....to restore a flat frequency response, replay equalization (deemphasis) that is the inverse of the record equalization must be applied. This is done in the disc input stages of the replay system's preamplifier (footnote 1). The RIAA pre- and deemphasis curves are defined by three time constants (T), each of which is related to a corner frequency (f) in the amplitude response by the equation:...These time constants (3180, 318, and 75 microseconds) and their respective corner frequencies (50.05, 500.5, and 2,122Hz) are depicted in the red trace of fig.3, which shows the straight-line approximation of the RIAA deemphasis (ie, replay) curve, alongside the actual curve (blue). Note that the latter is a mirror image of the preemphasis curve in fig.2, so that the two are exactly complementary: applying the deemphasis on replay cancels the effect of the preemphasis applied during cutting, to leave a flat response overall (footnote 2)..."

[3] Betoningseksempel; FM-radio: daenotes.com: Pre-Emphasis And De-Emphasis Citat: "...Pre-emphasis refers to boosting the relative amplitudes of the modulating voltage for higher audio frequencies from 2 to approximately 15 KHz...De-emphasis means attenuating those frequencies by the amount by which they are boosted...", backup

[4] Betoningseksempel; FM-radio: rfwireless-world.com: Pre-emphasis vs De-emphasis Citat: "...As we know noise usually will have higher amplitude and higher frequency components. This high frequency noise will lead to frequency distortion when its amplitude is higher than the components present in the modulating signal. To overcome this situation most of the FM circuits use techniques known as Pre-emphasis at the transmitter and De-emphasis at the receiver...The Pre-emphasis helps amplify high frequency signal components such that they will have magnitude higher than noise components. This lead to improvement in the Signal to Noise Ratio i.e. SNR...The De-emphasis does exactly reverse of the Pre-emphasis counterpart. It is used at the receiver part...", backup

[5] Brechmann, Dzieia, Hörnemann, Hübscher, Jagla, Petersen: Elektrotechnik Tabellen Kommunikationstechnik. 3. udgave. Westermann, Braunschweig 2001, ISBN 3-14-225037-9.

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