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Emphasis (telecommunications)
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In signal processing, pre-emphasis is 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 called de-emphasis, making the output accurately reproduce the original input.
Emphasis is commonly used in many places ranging from FM broadcasting (preemphasis improvement) and vinyl (e.g. LP) records to PCI Express. For example, high-frequency signal components may be emphasized to produce a more equal modulation index for a transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range.
In audio signals
[edit]In processing electronic audio signals, pre-emphasis refers to a system process designed to increase (within a frequency 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 as attenuation distortion or saturation of 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 a pre-emphasis network which is essentially a calibrated filter. The frequency response is decided by special time constants. The cutoff frequency can be calculated from that value.
Pre-emphasis is commonly used in telecommunications, digital audio recording, record cutting, in FM broadcasting transmissions, and in displaying the spectrograms of speech signals. One example of this is the RIAA equalization curve on 33 rpm and 45 rpm vinyl records. Another is the Dolby noise-reduction system as used with magnetic tape.
Pre-emphasis is employed in frequency modulation or phase modulation transmitters to equalize the modulating signal drive power in terms of deviation ratio. The receiver demodulation process includes a reciprocal network, called a de-emphasis network, to restore the original signal power distribution.
De-emphasis
[edit]In telecommunications, de-emphasis is 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.
Special time constants dictate the frequency response curve, from which one can calculate the cutoff frequency.
Red Book audio
[edit]Although rarely used, there exists the capability for standardized emphasis in Red Book CD mastering. As CD players were originally implemented with affordable 14-bit converters, a specification for pre-emphasis was included to compensate for quantization 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.[1]
In digital transmission
[edit]In serial data transmission, emphasis is used to improve signal quality at the output of a communication 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 [2]: 6 (effectively a low pass filter), so emphasis needs to invert this effect (functioning as a high pass filter).[2]: 8 This makes emphasis a form of equalization, 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.[2]: 9 Well-known serial data standards such as PCI Express, SATA and SAS require transmitted signals to use de-emphasis.
Effects of channel insertion loss
[edit]As a lossy channel becomes longer, high-frequency attenuation worsens and the signal will be increasingly distorted.
In the demonstration below, a 5 Gbps PRBS-9 test pattern is sent through PCB traces of various lengths on standard FR-4 material.
| 55 mm | 300 mm | |
|---|---|---|
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At some point, depending on the specifics of the channel, the transmitter, and the receiver, the signal will become too distorted for the receiver to correctly interpret it and the link will experience a high error rate or completely fail. Emphasis is one way to undo this distortion and enable communication to be successful over such a channel.
Analog R-C circuit
[edit]De-emphasis can be implemented by means of an analog high-pass filter circuit in parallel with an attenuator. This weakens the entire signal by a fixed amount, then allows extra energy to bypass the attenuator when the signal changes. The end result is a sharp spike at each transition followed by an exponential decay to the steady-state amplitude.
In the demonstration below, a 5 Gbps PRBS-9 test pattern is sent through a 300mm FR-4 channel with increasing levels of de-emphasis. Note that as the emphasis is increased, the signal amplitude is reduced.
| 0 dB | 6 dB | 12 dB | |
|---|---|---|---|
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Unlike the FIR architecture discussed in the next section, with analog emphasis the shape of the overshoot is *independent* of the signal bit rate. Thus, at lower data rates the entire bit's amplitude is not increased, only the edge. In the example below, a deliberately excessive level of emphasis is used to make the overshoot more visible.
| 1.25 Gbps | 5 Gbps | |
|---|---|---|
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3-tap FIR
[edit]One common implementation of emphasis in real SERDES [3] is 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).[2]: 10,24 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 removes ISI at 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,[2]: 16 with lossier channels requiring a larger tap value.[4] Higher numbers of taps are possible but increase circuit complexity and tend to result in diminishing returns [2]: 14 so are not commonly used.
The effects of emphasis on a signal can be clearly seen in the eye pattern. In the following demonstration, we consider a 10.3125 Gbps PRBS-31 test pattern with NRZ modulation, typical for testing 10-Gigabit Ethernet. The channel has an insertion loss of 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 excessive overshoot. Excessive equalization can worsen jitter, increase overshoot, and result in a less open eye than a properly equalized signal.[5]
| Baseline | Excessive pre cursor | Optimized | Excessive post cursor | |
|---|---|---|---|---|
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| Width | Worst | Better | Best | Better |
| Height | Good | Bad | Best | Bad |
| Jitter | Worst | Better | Best | Better |
References
[edit]- ^ IEC 60908:1999: Audio recording – Compact disc digital audio system. Geneva: International Electrotechnical Commission. 1999. pp. 29, 131.
- ^ a b c d e f "ECEN720 High Speed Links Circuits and Systems, Lecture 7: Equalization Intro & TX FIR EQ" (PDF).
- ^ "UltraScale Architecture GTY Transceivers User Guide". p. 167.
- ^ "ECEN720 High Speed Links Circuits and Systems, Lecture 8: RX FIR, CTLE, and DFE Equalization" (PDF). p. 4.
- ^ a b "Eye Doctor: Why Too Much Equalization is Bad for Your Serial Link Health" (PDF). Texas Instruments.
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).
External links
[edit]Emphasis (telecommunications)
View on GrokipediaFundamentals
Definition and Purpose
Emphasis in telecommunications is a signal processing technique that intentionally alters the frequency response of a signal to mitigate losses and noise introduced by the transmission channel. Pre-emphasis, the core component, boosts the amplitude of higher frequency components relative to lower ones before transmission, compensating for the disproportionate attenuation or noise affecting high frequencies in many communication channels. This distortion is reversed at the receiver through de-emphasis, restoring the original spectral balance.[5] The primary purpose of emphasis is to enhance the signal-to-noise ratio (SNR) at the receiver, achieving a flatter overall frequency response after de-emphasis while minimizing distortion from noise, which often increases with frequency in analog systems. By equalizing the impact of channel impairments without raising overall transmitted power, emphasis extends the effective bandwidth and improves perceived signal quality, particularly for audio and voice applications.[6][7] This approach originated in early 20th-century telephony, where frequency-dependent attenuation in long wire lines necessitated equalization to maintain signal integrity; pioneering work at Bell Laboratories, including O.J. Zobel's 1928 designs for lumped-element filters, addressed linear amplitude and phase distortions in telephone circuits. In broadcasting, emphasis was integrated into FM systems from the late 1930s onward to counter high-frequency noise and broaden usable spectrum without power increases, evolving with standards like those for stereo transmission in the 1950s.[8][9] Pre-emphasis functions as a simple high-pass filter, with its response curve gradually increasing gain at higher frequencies based on a defined time constant. In FM audio broadcasting in the United States, a typical 75 μs time constant is employed, providing about 6 dB per octave boost above approximately 2.1 kHz to emphasize highs and optimize noise performance.[9]Pre-emphasis and De-emphasis Principles
Pre-emphasis involves applying a high-pass filter at the transmitter to amplify higher frequency components of the signal, thereby enhancing the signal-to-noise ratio (SNR) in systems prone to frequency-dependent noise, such as frequency modulation (FM). The transfer function for this process is given by $ H_{\text{pre}}(f) = 1 + j 2 \pi f \tau $, where $ \tau $ represents the time constant of the filter and $ j $ is the imaginary unit.[10] De-emphasis, performed at the receiver, employs a complementary low-pass filter to attenuate the boosted high frequencies and restore the original signal spectrum. Its transfer function is the inverse of the pre-emphasis filter: $ H_{\text{de}}(f) = \frac{1}{1 + j 2 \pi f \tau} $.[10] This ensures that the overall system response approximates a flat frequency response when combined with the channel characteristics. The combined effect of pre-emphasis, channel transmission, and de-emphasis yields an overall transfer function $ H(f) = H_{\text{pre}}(f) \cdot H_{\text{channel}}(f) \cdot H_{\text{de}}(f) \approx 1 $ for frequencies within the passband, assuming ideal channel behavior and matched time constants.[10] This compensation maintains signal fidelity while targeting noise mitigation. The noise reduction mechanism operates by pre-emphasizing the signal such that channel-added noise, which often increases with frequency, has a reduced relative impact on the high-frequency components; subsequent de-emphasis then shifts this noise burden toward lower frequencies, where the filter provides greater attenuation.[11] The 3 dB point of the pre-emphasis filter, where the magnitude response reaches $ 1/\sqrt{2} $ (or power response doubles, marking the +3 dB point relative to low frequencies), occurs at $ f_{3\text{dB}} = \frac{1}{2 \pi \tau} $. To derive this, consider the magnitude of the transfer function: $ |H_{\text{pre}}(f)| = \sqrt{1 + (2 \pi f \tau)^2} $. Setting this equal to $ \sqrt{2} $ for the 3 dB point gives $ 1 + (2 \pi f \tau)^2 = 2 $, so $ (2 \pi f \tau)^2 = 1 $, and solving for $ f $ yields $ f_{3\text{dB}} = \frac{1}{2 \pi \tau} $. A similar analysis applies to the de-emphasis filter's cutoff frequency.[11]Applications in Analog Audio Systems
FM Radio Broadcasting
In FM radio broadcasting, pre-emphasis and de-emphasis are standardized techniques applied to the audio signal to improve the signal-to-noise ratio (SNR) by compensating for the inherent noise characteristics of frequency modulation. The pre-emphasis filter boosts higher audio frequencies in the baseband signal before it is applied to the FM modulator, while the complementary de-emphasis filter in the receiver attenuates those frequencies after demodulation to restore a flat frequency response. This process ensures that high-frequency components, which are more susceptible to noise, receive proportional enhancement during transmission.[12] The specific implementation uses a single time constant for the RC network defining the filter response, with regional variations to align with broadcast standards. In the Americas (and South Korea), the standard time constant is 75 μs, resulting in a 3 dB boost frequency of approximately 2122 Hz; in Europe, Japan, and most other regions, it is 50 μs, yielding a 3 dB frequency of about 3183 Hz. These values are defined in international recommendations to maintain compatibility across transmitters and receivers within each region. For stereophonic FM broadcasts, the pre-emphasis is applied identically to both the main (L+R) and sideband (L-R) signals before modulation.[12] The primary benefit of this emphasis in FM radio arises from the triangular noise spectrum produced during demodulation, where noise power increases linearly with frequency due to the phase noise in the FM discriminator. By pre-emphasizing high frequencies, the effective SNR for audio above the 3 dB point is improved, with a typical gain of around 13 dB at 15 kHz for the 50 μs standard, thereby reducing audible hiss and enhancing overall perceived audio quality without exceeding the allocated frequency deviation limits. This noise shaping is particularly effective in wideband FM systems using ±75 kHz deviation, common in most regions.[13][14] Historically, pre-emphasis was integrated into FM broadcasting standards from the technology's early commercial deployment in the 1940s, with the U.S. Federal Communications Commission (FCC) formalizing the 75 μs standard for American FM stations in 1961 alongside the approval of stereophonic transmission. Internationally, the International Telecommunication Union Radiocommunication Sector (ITU-R) has codified these practices in Recommendation BS.450, first issued in 1960 and revised through subsequent editions to accommodate global variations while promoting interoperability. In analog FM systems, de-emphasis is a fixed receiver function matched to the regional standard, without reliance on pilot tones for detection—unlike modern digital radio hybrids that may incorporate metadata for adaptive processing.[15]Analog Tape Recording
In analog tape recording, particularly for compact cassette systems, emphasis techniques are employed to compensate for inherent limitations in magnetic tape, such as high-frequency self-erasure due to demagnetization effects and oxide particle saturation that causes distortion at elevated signal levels.[16] These issues become more pronounced at the standard cassette speed of 1.875 inches per second (ips), where high frequencies experience greater attenuation compared to faster reel-to-reel speeds like 7.5 ips.[17] The International Electrotechnical Commission (IEC) standard IEC 60094-1 defines specific emphasis curves tailored to different tape types to optimize frequency response and signal-to-noise ratio. For Type I (normal bias, ferric oxide) tapes, the curve uses a time constant of 120 μs; Type II (high bias, chrome or equivalent) and Type IV (metal particle) employ 70 μs.[17] During the recording process, pre-emphasis is applied after the initial equalization stage, which follows NAB or IEC curves to shape the signal for the tape's characteristics, boosting high frequencies to counteract anticipated losses and improve overall fidelity.[17] This high-pass filtering approach ensures that the recorded signal maintains a flatter response upon playback, with the emphasis circuit typically implemented via simple RC networks in consumer decks. Variable bias levels further influence emphasis effectiveness; higher bias for Type II and IV tapes reduces distortion but can alter the optimal emphasis curve if not matched precisely, while lower bias in Type I setups exacerbates high-frequency roll-off without adequate pre-emphasis.[18] On playback, de-emphasis circuits match the recording curve to restore the original frequency balance, attenuating the boosted highs to flatten the response and minimize noise perception.[16] Tape speed variations, such as between cassette (1.875 ips) and quarter-track open-reel (7.5 ips), necessitate adjusted emphasis; slower speeds amplify self-erasure, requiring steeper pre-emphasis slopes, whereas faster speeds allow milder curves due to reduced losses.[17] The evolution of noise reduction systems like Dolby B built on these principles, introducing sliding-band companding that dynamically compresses high frequencies during quiet passages—distinct from fixed emphasis but historically tied to tape's emphasis heritage for hiss mitigation in cassettes.[19]Applications in Digital Audio and Transmission
Compact Disc Audio Standard
The Compact Disc Digital Audio standard, known as the Red Book, was jointly published by Philips and Sony in 1980 and later formalized as IEC 60908, specifying an optional pre-emphasis feature to mitigate high-frequency quantization noise in 16-bit digital audio encoding.[20][21] This technique boosts high frequencies during mastering to improve the signal-to-noise ratio for treble content, which is perceptually more sensitive to quantization artifacts in early digital systems. The pre-emphasis employs a bilinear transfer function with time constants of 50 μs and 15 μs, resulting in a frequency response that provides up to approximately 10 dB of gain at 20 kHz relative to midrange frequencies.[21][22] Implementation occurs digitally during the encoding process, where the emphasized audio is stored on the disc, and de-emphasis is applied in the player's digital-to-analog converter (DAC) to restore the original spectral balance. The presence of pre-emphasis is indicated by a flag in the subcode Channel Q, specifically bit 7 of the control field set to "1" for two-channel audio with 50/15 μs emphasis (e.g., control code 0 0 X 1).[21] CD players are required to automatically detect this flag and engage the de-emphasis filter; if the flag is absent (bit 7 = "0"), no de-emphasis is applied, assuming unemphasized audio. This metadata-driven approach ensures compatibility without manual intervention, though early players sometimes implemented de-emphasis via analog RC circuits post-DAC for simplicity.[21] Pre-emphasis was commonly applied in the 1980s, particularly to pop and rock recordings with prominent high-frequency content, to perceptually reduce audible quantization noise from limited dynamic range in initial ADC and DAC technologies.[23] Its usage declined sharply by the late 1980s and into the 1990s as oversampling DACs and improved mastering practices minimized noise floors to inaudible levels, rendering the feature unnecessary for most productions.[24] In modern remastering of legacy CDs, de-emphasis is simulated using digital signal processing software tools, such as EQ modules or command-line utilities like SoX, to accurately reverse the original curve without relying on outdated hardware filters.[25]Baseband Digital Transmission
In baseband digital transmission, pre-emphasis plays a crucial role by shaping the transmit signal spectrum to counteract the dispersive characteristics of the communication channel, thereby equalizing the received signal and mitigating intersymbol interference (ISI). This technique boosts higher-frequency components at the transmitter, compensating for the channel's frequency-dependent attenuation, which allows for higher data rates and extended transmission distances over media like twisted-pair cables or short-reach optical links.[26] Channel insertion loss in baseband systems often arises from phenomena such as the skin effect in conductive cables, where alternating currents tend to flow near the conductor surface, resulting in increased resistance and a pronounced roll-off of high-frequency components. This attenuation closes the eye diagram of the received signal, reducing the margin for reliable detection; pre-emphasis restores eye opening by pre-boosting these high frequencies, enabling error-free transmission over longer distances or at higher speeds. For instance, in copper-based links, skin-effect losses can exceed 20 dB at multi-GHz frequencies over hundreds of meters, but pre-emphasis can extend effective reach by 2-4 times compared to uncompensated signals.[26][27][28] Partial response signaling techniques, such as duobinary or class-4 partial response, incorporate emphasis-like precoding to intentionally introduce controlled ISI, which shapes the signal spectrum to fit within narrower bandwidths while maintaining detectability. In duobinary systems, this precoding correlates adjacent symbols to reduce the required channel bandwidth by approximately half compared to full-response NRZ signaling, with pre-emphasis taps adjusting for channel losses to preserve signal integrity.[29] Practical applications of pre-emphasis in baseband digital transmission include Ethernet standards like 10BASE-T, where it enhances signal integrity over unshielded twisted-pair cables up to 100 meters by applying internal wave-shaping and high-frequency boosting in the physical layer transceiver.[30] More recent implementations appear in high-speed baseband links for 5G fronthaul, where pre-emphasis via transmitter feed-forward equalization (FFE) compensates for fiber or copper dispersion in PAM4-modulated signals, enabling 25-50 Gb/s rates over short reaches. As referenced in the fundamentals, this approach improves SNR by emphasizing frequencies less affected by channel noise.[31]Implementation Techniques
Analog RC Circuits
Analog RC circuits implement emphasis in telecommunications through simple resistor-capacitor (RC) networks, which form the basis for pre-emphasis and de-emphasis filters in early FM audio systems. These passive or active configurations exploit the frequency-dependent behavior of capacitors to boost or attenuate high-frequency components, compensating for noise characteristics in analog transmission channels.[32] The basic pre-emphasis circuit uses a configuration providing a high-shelf filter response, such as a passive network with a series resistor and a shunt parallel resistor-capacitor combination, or more commonly implemented actively with op-amps. The transfer function for this filter is given byDigital FIR Filters
Digital finite impulse response (FIR) filters provide a discrete-time implementation of emphasis techniques in telecommunications, enabling precise compensation for channel distortions through programmable tap coefficients processed in digital signal processors or hardware accelerators. These filters approximate the inverse frequency response of the transmission channel, boosting high-frequency components in pre-emphasis or attenuating them in de-emphasis to mitigate intersymbol interference (ISI) and noise. Unlike continuous-time analog methods, FIR filters operate on sampled signals, allowing for linear phase response and stability without feedback loops.[37] A common configuration for pre-emphasis is the 3-tap FIR filter with coefficients , where to is selected based on the degree of channel loss to achieve a simple high-pass characteristic that enhances signal transitions. This symmetric structure maintains a DC gain of 1 while shaping the response to counteract low-pass effects in media like cables or backplanes. For instance, with , the coefficients are approximately , reducing the main cursor relative to pre- and post-cursors to emphasize edges.[38][39] The design of these FIR filters typically involves estimating the channel's impulse response and deriving coefficients that approximate its inverse, often using optimization techniques like least squares to minimize residual ISI. The filter's transfer function is expressed in the z-domain aswhere , , and correspond to the tap coefficients, enabling straightforward implementation via convolution in digital hardware.[37] In practical applications, 3-tap FIR pre-emphasis is widely deployed in digital signal processors within modems for baseband transmission and in FPGAs for high-speed serial interfaces such as 10G Ethernet, where it compensates for PCB trace losses. Adaptive variants employ the least mean squares (LMS) algorithm to iteratively update coefficients based on error signals from the receiver, improving performance in varying channel conditions without manual intervention.[37][40] Digital FIR filters offer advantages over analog RC circuits, including superior precision in coefficient values (limited only by word length) and reconfigurability via software or firmware updates, facilitating adaptation to different standards or environments. For example, in USB 3.0, a 3-tap FIR configuration provides de-emphasis of 3.5 dB to equalize cable attenuation up to 5 Gbps, while similar implementations in HDMI support dynamic compensation for varying cable lengths.[41] Extensions to multi-tap FIR filters, such as 5-7 taps, address the demands of higher-speed SerDes in 2020s standards like PCIe 6.0, enabling more granular control over ISI cancellation in PAM4 signaling at 64 GT/s; for instance, PCIe 6.0 specifies a 4-tap transmitter FFE, but implementations often scale to 5+ taps for enhanced performance in lossy channels.[42]