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Tape bias
Tape bias
Tape bias
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Visualization of the magnetic field on a stereo cassette containing a 1 kHz audio tone. Individual high-frequency magnetic domains are visible.

Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Most contemporary tape recorders use AC bias.

When recording, magnetic tape has a nonlinear response as determined by its coercivity. Without bias, this response results in poor performance, especially at low signal levels. A recording signal that generates a magnetic field strength less than the tape's coercivity cannot magnetise the tape and produces little playback signal. Bias increases the signal quality of most audio recordings significantly by pushing the signal into more linear zones of the tape's magnetic transfer function.

History

[edit]

Magnetic recording was proposed as early as 1878 by Oberlin Smith, who on 4 October 1878 filed, with the U.S. patent office, a caveat regarding the magnetic recording of sound and who published his ideas on the subject in the 8 September 1888 issue of The Electrical World as "Some possible forms of phonograph".[1][2] By 1898, Valdemar Poulsen had demonstrated a magnetic recorder and proposed magnetic tape.[3] Fritz Pfleumer was granted a German patent for a non-magnetic "Sound recording carrier" with a magnetic coating, on 1 January 1928.[4] Years earlier, Joseph O'Neil had created a similar recording medium, yet had not made a working machine that could record sound.[5]

DC bias

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The earliest magnetic recording systems simply applied the unadulterated (baseband) input signal to a recording head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal, a DC bias, was found to reduce distortion by operating the tape substantially within its linear-response region. The principal disadvantage of DC bias was that it left the tape with a net magnetization, which generated significant noise on replay because of the grain of the tape particles. However: the earlier wire recorders were largely immune to the problem due to their high running speed and relatively large wire size. Some early DC-bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was replaced by AC bias but was later re-adopted by some very low-cost cassette recorders.[6][7][8][9]

AC bias

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The original patent for AC bias was filed by Wendell L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in a patent in 1927.[10] The value of AC bias was somewhat masked by the fact that wire recording gained little benefit from the technique and Carlson and Carpenter's achievement was largely ignored. The first rediscovery seems to have been by Dean Wooldridge at Bell Telephone Laboratories, around 1937, but their lawyers found the original patent, and Bell simply kept silent about their rediscovery of AC bias.[11]

Teiji Igarashi, Makoto Ishikawa, and Kenzo Nagai of Japan published a paper on AC biasing in 1938 and received a Japanese patent in 1940.[12] Marvin Camras (USA) also rediscovered high-frequency (AC) bias independently in 1941 and received a patent in 1944.[13]

The reduction in distortion and noise provided by AC bias was accidentally rediscovered in 1940 by Walter Weber while working at the Reichs-Rundfunk-Gesellschaft (RRG) when a DC-biased Magnetophon that he had been working on developed an 'unwanted' oscillation in its record circuitry.[14]

The last production DC biased Magnetophon machines had harmonic distortion in excess of 10 percent; a dynamic range of 40 dB and a frequency response of just 50 Hz to 6 kHz at a tape speed slightly in excess of 30 inches per second (76.8 cm/sec). The AC biased Magnetophon machines reduced the harmonic distortion to well under 3 percent; extended the dynamic range to 65 dB and the frequency response was now from 40 Hz to 15 kHz at the same tape speed. These AC biased magnetophons provided a fidelity of recording that outperformed any other recording system of the time.[9]

See also

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References

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Further reading

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Tape bias

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Tape bias refers to techniques used in analog recording to improve fidelity, including and the more common AC bias. AC bias involves superimposing a high-frequency (AC) signal, typically ranging from 40 to 150 kHz, onto the before it is applied to the recording head. This signal linearizes the non-linear magnetization curve of the , which exhibits and a "dead zone" near zero field where low-amplitude signals fail to properly magnetize the tape particles, thereby reducing distortion, improving , and extending the usable from low bass to high treble. The mechanism of tape bias operates by shifting the of the away from the non-linear regions of the tape's transfer characteristic—such as the coercive force threshold and saturation limits—onto more linear portions of the curve, ensuring that the recorded is proportional to the input signal . During playback, the signal is filtered out using a or "bias trap," restoring the original while the high-frequency , being inaudible and inefficiently reproduced by the playback head, does not interfere with the output. Optimal levels must be adjusted based on tape formulation—such as , chromium dioxide, or metal particle tapes—to avoid overbiasing, which can reduce high-frequency response, or underbiasing, which increases ; this is a standard procedure in professional analog recorders. The discovery of AC bias traces back to 1921, when researchers Wendell L. Carlson and Glenn L. Carpenter of the accidentally found that adding a high-frequency signal to the recording head dramatically improved sensitivity and reduced in early experiments. Although theoretical understanding evolved through the and at institutions like Bell Telephone Laboratories, practical implementation in commercial tape recorders occurred later; for instance, AEG's Magnetophon K1 in 1935 marked the start of the plastic tape era, with widespread adoption of AC by 1940, enabling high-fidelity recording by 1943. Tape bias remained essential throughout the analog era until the , influencing production, broadcasting, and consumer devices like cassette recorders, where it contributed to the medium's characteristic "warm" sound due to subtle harmonic saturation effects at higher levels. Today, while has largely supplanted analog tape, bias emulation in software plugins recreates these effects for modern music production.

Fundamentals of Tape Bias

Magnetic Hysteresis in Tape Recording

Magnetic tape serves as a recording medium composed of a thin plastic base, typically , coated with a layer of ferromagnetic particles such as (Fe₂O₃). These particles, often acicular in shape and measuring approximately 0.5 μm by 0.1 μm, are oriented longitudinally and bound in a matrix to ensure stability and minimize during transport. Audio signals are imprinted onto the tape by passing it over a recording head, where an generates a varying proportional to the signal's and , aligning the particles' magnetic domains to store the information as a pattern of magnetization. In ferromagnetic materials like those in , the relationship between the applied (H) and the resulting (B) is characterized by a loop, a nonlinear phenomenon arising from the material's tendency to retain . This loop, visualized as a closed, oval-shaped curve on a B-H graph, begins at the origin: as H increases in the positive direction, B rises sharply to reach saturation (B_s), the point where all domains are fully aligned and further field strength yields no additional . Upon reducing H to zero, B does not return to zero but remains at (B_r), the residual that persists without an external field. To erase this remanence and return B to zero requires applying a reverse field equal to the (H_c), the minimum strength needed to demagnetize the material; continuing in the negative direction saturates B at -B_s, and the cycle closes symmetrically. This "memory" effect stems from the energy barriers in domain wall motion and spin rotations within the particles. The nonlinear shape of the hysteresis loop leads to significant in tape recordings, as the response does not proportionally follow the input signal, particularly at low amplitudes where the curve's central "knee" region amplifies irregularities. This nonlinearity generates harmonic , predominantly odd-order harmonics (e.g., third and fifth), which add unwanted overtones and color the audio unfaithfully. Additionally, the effect compresses the by disproportionately distorting quieter signals, while altering —low frequencies suffer from poor reproduction due to self-bias, where the recorded signal's own remanent field interferes with subsequent , effectively creating an internal that unevenly influences bass signals. Without mitigation, these issues result in muffled lows and overall signal degradation. Historical observations from the early 1920s, during the pioneering work of inventors like Fritz Pfleumer on paper-based magnetic tapes, revealed high distortion levels in recordings, often rendering audio unintelligible or of severely compromised quality due to the unaddressed effects and lack of effective techniques. These early experiments, building on Valdemar Poulsen's 1898 wire recorder concepts, typically produced outputs with significant noise and distortion, far exceeding later standards like 1-3% (THD). To address such distortion from , tape bias acts as a technique by operating the material in a more responsive region of the curve.

Purpose and Benefits of Bias

Tape bias serves as a critical technique in analog magnetic tape recording to linearize the nonlinear response of the recording medium caused by . By superimposing a strong, inaudible high-frequency signal onto the audio input, bias shifts the operating point of the tape's to a more linear portion of the loop, where small variations in the produce proportionally larger and more faithful changes in . This overcomes the inherent nonlinearity that would otherwise cause severe , as the tape's magnetic particles retain residual from previous signals without such intervention, leading to inconsistent recording behavior. The primary benefit of bias is a dramatic reduction in , particularly and types that plague unbias-recorded signals. Without bias, low-level audio signals can exhibit (THD) levels of 10-20% or higher due to the steep, nonlinear sections of the curve, with third-order harmonics dominating and introducing harsh, unnatural tones. With proper bias application, this distortion drops to under 1% THD, enabling cleaner, more accurate reproduction across the audio spectrum and preserving the intended sonic character. Additionally, bias minimizes even-order harmonics, which contribute to a muddier sound, further enhancing overall clarity. Beyond distortion control, bias improves several key performance metrics of tape recording. It extends the frequency response to a flat characteristic from approximately 50 Hz to 15 kHz (±1 dB), allowing faithful capture of both bass and treble without excessive roll-off or emphasis in either range. The technique also lowers the noise floor by reducing artifacts from magnetic particle irregularities and prior magnetization traces, effectively "erasing" historical effects on the tape for each new recording. This results in a better dynamic range, typically up to 60 dB in consumer applications. These enhancements collectively make bias indispensable for professional audio fidelity, transforming what would be a highly distorted medium into a viable tool for high-quality sound capture.

Types of Bias

DC Bias

DC bias represents the earliest and simplest method of applying bias in recording, involving the superposition of a onto the fed to the recording head. This steady DC creates a constant that shifts the tape's operating point along its curve to a region of greater , reducing the compressive inherent in un-biased recording by utilizing only a portion of the magnetization curve. In practice, the tape is often pre-saturated with a strong DC field in one polarity via an erase head, after which the recording head applies an opposing current to return the average near zero; the then modulates this biased state, producing a remnant proportional to the input. This approach avoids the need for complex circuitry like oscillators, offering low cost and straightforward implementation in rudimentary . It proved effective for basic correction and mitigation in initial experiments during the . Despite these benefits, DC bias exhibits inherent limitations that restrict its utility for quality audio. The persistent DC field leaves the tape with net magnetization even in silent passages, resulting in elevated noise floors and poor signal-to-noise ratios; precise adjustment is critical, as deviations introduce high even-order harmonic distortion, such as second harmonics. Additionally, self-demagnetization effects cause significant high-frequency attenuation, while the constant field accelerates tape wear through increased frictional and magnetic stress. These factors render DC bias unsuitable for high-fidelity applications, where total harmonic distortion remains unacceptably high and overall frequency balance is compromised, particularly in low-frequency reproduction. DC bias found application in early prototypes, such as those developed at Bell Laboratories in the mid-1920s for magnetic wire and tape systems, but it was largely supplanted for consumer recording by due to these performance shortcomings.

AC Bias

AC bias, the predominant technique in analog magnetic tape recording, involves the injection of a high-frequency (AC) signal into the audio signal at the recording head. This bias signal, typically ranging from 50 to 150 kHz and operating at an amplitude 10 to 20 dB above the maximum audio level, creates rapid cycles of magnetization on the tape. These cycles effectively linearize the tape's hysteresis loop by continuously shifting the operating point along the steepest portion of the magnetization curve, allowing the audio signal to be recorded with reduced . Compared to , AC bias offers several key advantages that contributed to its widespread adoption as the industry standard. The absence of a net DC component prevents permanent magnetization of the recording head, thereby reducing wear and the need for frequent demagnetization. It also extends the high-frequency response up to 20 kHz by overcoming the tape's inherent low sensitivity at higher frequencies, minimizes through more uniform magnetization, and enables the use of tailored levels for different tape formulations, such as Type I (normal) versus Type II ( dioxide) in cassette systems. These improvements result in superior overall fidelity and . The bias oscillator must generate a clean sinusoidal waveform to avoid introducing additional distortion or noise into the recording process. Early systems relied on vacuum tubes, while later designs used transistors for more stable and efficient operation. Amplitude is precisely optimized: excessive bias (overbias) can erase the audio signal by fully saturating the tape, whereas insufficient bias (underbias) leads to residual hysteresis effects and increased distortion. Proper calibration ensures peak recording sensitivity without compromising audio integrity. AC bias was standardized in the 1930s by AEG in their systems, marking a pivotal advancement in recording. For instance, professional reel-to-reel recorders employing a 100 kHz bias signal achieved a signal-to-noise ratio of approximately 70 dB in full-track configurations, demonstrating the technique's effectiveness in broadcast and studio environments. This adoption solidified AC bias as the norm, supplanting earlier DC methods due to its enhanced performance.

Historical Development

Early Experiments with DC Bias

The invention of magnetic recording is credited to Danish engineer , who in 1898 developed the telegraphone, a device that recorded sound on steel wire by magnetizing it with an driven by the audio signal. This early system suffered from significant distortion due to the nonlinear of the steel wire, limiting its fidelity for audio applications. To address this issue, Poulsen and his collaborator Peder Oluf Pedersen experimented with biasing techniques in the early 1900s. In 1907, they patented the use of , which involved superimposing a steady DC signal on the audio input to shift the operating point on the curve toward a more linear region, thereby reducing and improving sensitivity. This U.S. No. 873,083 described applying the DC from batteries to the recording head during wire or tape , marking the first systematic application of in magnetic recording. Although effective for basic and dictation, the method still produced audible , particularly at low signal levels, and required relatively high tape speeds (around 7 feet per second) to minimize noise. In the pre-plastic magnetic tape era of the 1920s, researchers at Bell Laboratories extended these DC bias experiments to alternative media like steel wire and paper-backed steel tape for potential telephone recording uses. Clarence N. Hickman, working at Bell Labs, developed experimental steel tape recorders around 1919–1925, incorporating DC bias to enhance linearity and reduce harmonic distortion compared to un-biased recordings. These devices used continuous loops of thin steel tape (often 0.1–0.25 inches wide) pulled at speeds of 10–20 feet per second, with DC supplied via batteries or generators to the recording head, achieving better signal-to-noise ratios than Poulsen's wire systems but still plagued by mechanical issues like tape breakage and print-through. Western Electric, closely affiliated with Bell Labs, demonstrated such DC-biased steel tape systems at engineering conferences in 1927, showcasing their utility for voice logging but highlighting limitations in low-volume reproduction fidelity. Despite these advances, proved inadequate for high-fidelity audio, as it could not fully linearize the loop across the full , leading to residual and poor low-level performance on the rigid media. This prompted further research into alternative biasing methods by the late , setting the stage for the transition to more effective techniques.

Invention and Evolution of AC Bias

The technique of AC bias, which involves superimposing a high-frequency signal on the audio input to linearize the curve of recording media, emerged from multiple independent rediscoveries in the late and early , building on earlier theoretical work. Although W. L. Carlson and G. W. Carpenter first described the concept in a 1921 filing (granted in 1927) during experiments with for the U.S. , practical application for high-fidelity audio lagged due to limitations in media and . In the United States, Marvin Camras at the Armour Research Foundation independently rediscovered AC bias in 1940 while developing wire recorders, applying it to improve sensitivity and reduce in military applications; he filed a key in 1941 (issued 1944). Concurrently, in , engineer Walter Weber at AEG observed the effect serendipitously in 1939 during troubleshooting of an oscillating amplifier in the , leading to its implementation in 1940 with bias frequencies around 40-50 kHz. This German breakthrough, patented by AEG and co-inventor Hans-Joachim von Braunmühl, was refined for the K4 model in 1941. During , German engineers further evolved AC bias techniques, integrating them with BASF's improved gamma-ferric oxide tapes (introduced in 1939) to achieve a of approximately 60 dB and frequency response up to 10 kHz in systems used for radio broadcasts and recordings. These refinements, including optimized bias levels and erase signals, were kept secret but captured Allied attention; U.S. forces seized prototypes in 1945, accelerating postwar adoption. In the 1950s, AC bias became standard in consumer and professional equipment, notably in reel-to-reel recorders like the Model 300 (introduced 1948) and successors, which adopted a 100 kHz bias frequency as an industry benchmark for reduced distortion and extended high-frequency response. Key standardization efforts solidified AC bias's role, with the (IEC) publishing initial guidelines in 1952 (expanded in IEC 94 series by 1953) for bias signal characteristics and recording parameters in magnetic tape systems. The 1960s saw widespread consumer integration, exemplified by ' introduction of the Compact Cassette format in 1963, which employed AC bias at around 50-100 kHz tailored to ferric tapes; subsequent optimizations in the late 1960s and 1970s accommodated chrome dioxide tapes (first commercialized by in 1968) through adjustable high-bias settings to enhance saturation and minimize noise. These advancements enabled the explosive growth of the commercial audio industry, powering reel-to-reel, cassette, and eight-track formats; by the 1970s, AC bias was universally implemented in analog magnetic recording technologies worldwide.

Technical Implementation

Bias Signal Characteristics

The bias signal in analog tape recording is an (AC) waveform typically operating in the frequency range of 40 to 150 kHz, selected to lie well above the audible audio spectrum (up to 20 kHz) to minimize interference and ensure the signal does not imprint on the tape in a recoverable manner during playback. This range allows the bias to rapidly cycle the magnetic domains in the tape particles, effectively averaging their to a neutral state and enabling linear response to the superimposed . Higher frequencies within this band, such as around 120 kHz, are often employed for standard ferric tapes to optimize while accommodating the tape's characteristics. The of the signal is set significantly higher than the audio input to drive the tape into its linear region without causing excessive saturation or . This ensures the modulates the effectively for reduced and improved . The signal is mixed additively with the prior to application to the record head, allowing the combined to magnetize the tape in a manner that preserves audio fidelity. Generation of the bias signal has evolved from early oscillators, which produced stable high-frequency sine waves using circuits like Hartley or Colpitts configurations, to modern solid-state oscillators based on transistors or integrated circuits for greater efficiency and precision. Contemporary cassette and reel-to-reel decks often incorporate automatic calibration systems, particularly in units with , where test tones (e.g., 10 kHz sine waves) are recorded briefly, played back via a three-head setup, and analyzed to dynamically adjust for optimal response. Tape-specific adjustments are essential due to variations in formulation, , and thickness; type IV metal tapes require elevated bias levels (up to 250% of type I ferric) to compensate for their higher and achieve balanced . These optimizations, often guided by manufacturer specifications or tapes, ensure minimal distortion and maximum tailored to the tape's magnetic properties.

Impact on Recording Performance

The application of AC bias in magnetic tape recording significantly enhances by linearizing the process, enabling a broader audible range. With AC bias, typical systems achieve a of 40 Hz to 15 kHz, compared to 50 Hz to 6 kHz without bias (using ), where high-frequency output suffers due to nonlinear effects. In optimized setups, this extends to approximately 30 Hz to 20 kHz within ±3 dB, as the bias overcomes self-demagnetization and spacing losses at higher wavelengths. Bias also markedly reduces distortion and noise, improving overall fidelity. Total harmonic distortion (THD) drops to under 3% with AC bias at standard reference levels (e.g., 400 Hz at 6 dB above NAB reference), versus 5% or higher without it, where severe nonlinearity generates odd harmonics. Intermodulation distortion is similarly minimized through this linearization, preventing frequency-dependent compression. Signal-to-noise ratio (SNR) improves to 55-65 dB with bias (weighted or unweighted, depending on track configuration), compared to around 40 dB without, as the bias allows operation near the tape's maximum output while suppressing inherent oxide noise. Dynamic range expands substantially with proper bias settings, reaching 50-65 dB in practical analog audio systems, enabling capture of both quiet and loud passages without clipping or burial in hiss. Without bias, the range is limited to about 40 dB due to elevated and . For instance, optimal (set at maximum 1 kHz output) prevents over-magnetization, which otherwise leads to print-through—signal bleed between adjacent tape layers during storage—by maintaining even without residual DC fields. Measurement standards like NAB and IEC (including CCIR variants) incorporate bias-optimized equalization curves to ensure consistent performance across professional and consumer equipment. The NAB standard, prevalent in professional U.S. applications, specifies a response within ±2 dB from 30 Hz to 15 kHz at 7.5 ips (with tolerances up to ±3 dB), using 50 μs/3180 μs time constants for high-frequency pre-emphasis to compensate for tape losses under biased conditions. IEC/CCIR, common in , employs similar curves but with adjusted treble turnover (e.g., 35 μs at 15 ips) for modern tapes, yielding comparable SNR and distortion metrics while prioritizing low-frequency flatness.

Applications and Modern Context

Use in Analog Audio Formats

In reel-to-reel analog audio recording, AC was a standard technique applied to 1/4-inch , with professional machines like those from commonly employing a 100 kHz to linearize the recording process and achieve high-fidelity results. Three-head designs in these systems separated the record, playback, and erase heads, allowing the signal to be applied during recording while enabling real-time monitoring of the output without interrupting the process. The compact cassette format, invented by in 1963, utilized fixed AC tailored to specific tape formulations, incorporating equalization (EQ) curves to optimize . Normal tapes (Type I) employed a 120 μs EQ time for standard ferric oxide formulations, while chrome (Type II) and metal (Type IV) tapes required higher levels—approximately 150% of normal—and a 70 μs EQ curve to accommodate their improved high-frequency performance and reduced distortion. By the , advanced cassette decks incorporated auto- sensors that detected tape type via holes in the cassette shell, automatically adjusting and EQ for consistent playback and recording quality across formulations. 8-track and cartridge systems, popular in automotive applications during the and , adapted AC bias techniques similar to reel-to-reel formats to suit their continuous-loop, multi-channel design, ensuring reliable stereo playback in vehicle environments despite varying tape speeds and head configurations. In professional , such as 24-track setups using 2-inch tape, bias was calibrated to provide approximately +6 dB of headroom above the 0 VU operating level (typically 250 nWb/m fluxivity), allowing for greater in overdubbing workflows without saturation. High-speed dubbing in analog formats, often at 2x or 3x normal speed, necessitated specific bias adjustments to counteract high-frequency roll-off and preserve fidelity, with recorders like certain Pioneer and Sony models featuring dedicated high-speed modes that optimized bias amplitude for minimal distortion during duplication.

Legacy in Digital and Contemporary Audio

As digital audio technologies emerged in the 1980s, the principles of tape bias—particularly its role in linearizing nonlinear responses—influenced broader digital signal processing, with dithering serving as a direct analog to AC bias by adding low-level noise to randomize quantization errors and preserve low-amplitude details, much like bias pushes signals into a linear recording region on tape. Noise shaping in dither algorithms further echoes bias's high-frequency emphasis, shifting unwanted artifacts beyond the audible range to enhance perceived fidelity in digital workflows. In contemporary circles, tape persists through the 2020s revival of reel-to-reel recording, where boutique manufacturers like have reissued models such as the B77 MK III with refined AC circuits to optimize modern tape formulations for warmer, more dynamic analog sound. This resurgence, driven by demand for tangible analog experiences amid digital saturation, sees updated oscillators operating at frequencies up to 200 kHz to minimize on high-end tapes like RTM LPR 35. Niche applications extend to , where portable vintage analog machines—such as modified Portastudios—are employed by artists seeking the "warmth" of -induced saturation and subtle compression during on-location captures, often transferred digitally post-recording. Archival efforts, including those by the , rely on playback alignment techniques during the digitization of vintage analog tapes to counteract degradation effects like binder (addressed by oven baking the tape), ensuring accurate reproduction of high frequencies and reduced before conversion to formats like 96 kHz/24-bit . By the , tape bias became largely obsolete in mainstream production as solid-state eliminated the need for magnetic , supplanted by nonlinear processors and high-resolution ADCs. However, its legacy endures in digital tape emulation plugins, such as Universal Audio's or Waves J37, which model bias , overbias dulling, and frequency-dependent saturation to replicate analog "glue" in virtual mixes, allowing producers to invoke tape's character without physical media.

References

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