Emphasis (telecommunications)

RIAA equalization curve forvinyl records

Insignal processing,pre-emphasisis a technique to protect against anticipated noise and loss. The idea is to boost (and hence distort) the frequency range that is most susceptible to noise and loss beforehand, so that after a noisy and lossy process (transmission over cable, tape recording...) more information can be recovered from that frequency range. Removal of the distortion caused by pre-emphasis is calledde-emphasis,making the output accurately reproduce the original input.

Emphasis is commonly used in many places ranging fromFM broadcasting(preemphasis improvement) andvinyl(e.g.LP) records toPCI Express.For example, high-frequency signal components may be emphasized to produce a more equalmodulation indexfor a transmitted frequency spectrum, and therefore a bettersignal-to-noise ratiofor the entirefrequency range.

In waveform signals

In processing electronicaudio signals,pre-emphasis refers to a system process designed to increase (within afrequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena asattenuation distortionorsaturationof recording media in subsequent parts of the system. The mirror operation is called de-emphasis, and the system as a whole is called emphasis.

Pre-emphasis is achieved with apre-emphasis networkwhich is essentially a calibratedfilter.Thefrequency responseis decided by specialtime constants.Thecutoff frequencycan be calculated from that value.

Pre-emphasis is commonly used intelecommunications,digital audiorecording,recordcutting, inFM broadcastingtransmissions,and in displaying thespectrogramsof speech signals. One example of this is theRIAA equalizationcurve on 33 rpm and 45 rpmvinyl records.Another is theDolby noise-reduction systemas used with magnetic tape.

Pre-emphasis is employed infrequency modulationorphase modulationtransmitters to equalize the modulatingsignaldrivepowerin terms ofdeviation ratio.The receiverdemodulationprocess includes a reciprocal network, called a de-emphasis network, to restore the original signal power distribution.

De-emphasis

In telecommunication,de-emphasisis the complement of pre-emphasis, in the antinoise system called emphasis. De-emphasis is a system process designed to decrease, (within a band of frequencies), the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.

Specialtime constantsdictate thefrequency responsecurve, from which one can calculate thecutoff frequency.

Red Book audio

Although rarely used, there exists the capability for standardized emphasis inRed Book CDmastering. As CD players were originally implemented with affordable 14-bit converters, a specification for pre-emphasis was included to compensate forquantization noise.After economies of scale eventually allowed full 16 bits, quantization noise became less of a concern, but emphasis remained an option. The pre-emphasis is described as a first-order filter with a gain of 10 dB (at 20 dB/decade) and time constants 50 μs and 15 μs.

In digital transmission

Inserial data transmission,emphasis is used to improvesignal qualityat the output of acommunication channel.In transmitting signals at high data rates, the transmission medium may introduce distortions, so emphasis is used to distort the transmitted signal to correct for this distortion. When done properly this produces a received signal that more closely resembles the original or desired signal, allowing the use of higher data rates or producing fewer bit errors. Most real world channels have loss that increases with frequency(effectively a low pass filter), so emphasis needs to invert this effect (functioning as a high pass filter).This makes emphasis a form ofequalization,implemented at the transmit side of the channel.

Emphasis can be implemented either by boosting high frequencies (pre-emphasis, increasing the amplitude of transition bits) or attenuating low frequencies (de-emphasis, reducing the amplitude of non-transition bits). Both have the same net effect of producing a flatter system frequency response; de-emphasis is typically more convenient to do in real circuits since it only requires attenuation rather than amplification.Well-known serial data standards such asPCI Express,SATAandSASrequire transmitted signals to use de-emphasis.

One common implementation of emphasis in real SERDESis a 3-tap feed-forward equalizer (FFE): rather than driving the output pin with the desired output voltage directly, the actual output voltage is a weighted sum of the desired bit value (main cursor), the previous bit (post cursor), and the next bit to be transmitted (pre cursor).The main cursor coefficient controls the nominal amplitude of the bit and is always positive (as a negative coefficient would invert the bit value). The pre cursor coefficient removesISIat the receiver caused by bits which have not yet arrived (e.g. fields coupling across meanders in a delay-matched trace) and is typically zero or a very small negative value, as this is often not a major contribution to total ISI. The post cursor coefficient removes ISI at the receiver caused by the immediately preceding bit and is typically a larger negative value,with lossier channels requiring a larger tap value.Higher numbers of taps are possible but increase circuit complexity and tend to result in diminishing returnsso are not commonly used.

The effects of emphasis on a signal can be clearly seen in theeye pattern.In the following demonstration, we consider a 10.3125 GbpsPRBS-31test pattern withNRZmodulation, typical for testing10-Gigabit Ethernet.The channel has aninsertion lossof roughly 2 dB at the fundamental, 3 dB at the 2nd harmonic, and 4 dB at the 3rd. The goal is to achieve a well-equalized channel response in which the eye is maximally open without excessiveovershoot.Excessive equalization can worsen jitter, increase overshoot, and result in a less open eye than a properly equalized signal.

	Baseline	Excessive pre cursor	Optimized	Excessive post cursor
	Baseline signal with no emphasis. Transition bits are clearly weaker than non-transition bits and the signal is touching the mask (fail).	Excessive pre-cursor emphasis. The eye is more closed than the baseline, indicating minimal pre-cursor ISI was present and the emphasis is doing more harm than good.	Well-tuned post-cursor emphasis. The eye is open and transition and non-transition bits are well matched in amplitude, indicating a correct level of equalization. The signal is passing the mask test.	Excessive post-cursor emphasis. The eye is starting to close and transition bits have significant overshoot, indicating excessive equalization. The "double banding" artifact visible in the eye indicates the presence of significant ISIcaused by the excessive emphasis.
Width	Worst	Better	Best	Better
Height	Good	Bad	Best	Bad
Jitter	Worst	Better	Best	Better

References

• This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.(in support ofMIL-STD-188).

External links

• Emphasis – Frequency response and equalization EQ – Conversion: time constant to cut-off frequency and vice versa
• Deemphasis – Frequency response and equalization EQ – Conversion: time constant to cut-off frequency and vice versa