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SB3231-E1 データシート(PDF) 8 Page - ON Semiconductor |
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SB3231-E1 データシート(HTML) 8 Page - ON Semiconductor |
8 / 18 page RHYTHM SB3231 www.onsemi.com 8 • High level gain (HLGAIN) • Upper threshold (UTH) • Compression ratio (CR) During the Parameter Map creation, constraints are applied to the compression parameters to ensure that the I/O characteristics are continuous. Parameter adjustments support two popular styles of compression ratio adjustment: • The compression region of the I/O curve pivots about the upper threshold. As the compression ratio trimmer is adjusted, high−level gain remains constant while the low−level gain changes. • The compression region of the I/O curve pivots about the lower threshold. Low−level gain remains constant as the compression ratio trimmer is adjusted. The squelch region within each channel implements a low level noise reduction scheme (1:3 expansion) for listener comfort. This scheme operates in quiet listening environments (programmable threshold) to reduce the gain at very low levels. Automatic Telecoil The automatic telecoil feature in Rhythm SB3231 is to be used with memory D programmed with the telecoil or MIC + TCOIL front end configuration. The feature enables the part to transition to memory D upon the closing of a switch connected to MS2. With the feature enabled and a reed switch connected to MS2, the static magnetic field of a telephone handset will close the switch whenever the handset is brought close to the device, causing the hybrid to change to memory D. The part will transition back to the initial memory once the switch is deemed opened after proper debouncing. A debounce algorithm with a programmable debounce period is used to prevent needless switching in and out of memory D due to physical switch bounces when MS2 is configured for automatic telecoil. Upon detecting a close to open switch transition, the debounce algorithm monitors the switch status. The debounce algorithm switches the device out of memory D only once the switch signal has been continuously sampled open over the specified debounce period. Adaptive Feedback Canceller The Adaptive Feedback Canceller (AFC) reduces acoustic feedback by forming an estimate of the hearing aid feedback signal and then subtracting this estimate from the hearing aid input. The forward path of the hearing aid is not affected. Unlike adaptive notch filter approaches, Rhythm SB3231’s AFC does not reduce the hearing aid’s gain. The AFC is based on a time−domain model of the feedback path. The third−generation AFC (see Figure 6) allows for an increase in the stable gain1 of the hearing instrument while minimizing artefacts for music and tonal input signals. As with previous products, the feedback canceller provides completely automatic operation. 1. Added stable gain will vary based on hearing aid style and acoustic setup. Please refer to the Adaptive Feedback Cancellation Information note for more details. Figure 6. Adaptive Feedback Canceller (AFC) Block Diagram G Σ H’ H + − Feedback path Estimated feedback Feedback Path Measurement Tool The Feedback Path Measurement Tool uses the onboard feedback cancellation algorithm and noise generator to measure the acoustic feedback path of the device. The noise generator is used to create an acoustic output signal from the hearing aid, some of which leaks back to the microphone via the feedback path. The feedback canceller algorithm automatically calculates the feedback path impulse response by analyzing the input and output signals. Following a suitable adaptation period, the feedback canceller coefficients can be read out of the device and used as an estimate of the feedback−path impulse response. Adaptive Noise Reduction The noise reduction algorithm is built upon a high resolution 64−band filter bank (32 bands at 16 kHz sampling) enabling precise removal of noise. The algorithm monitors the signal and noise activities in these bands, and imposes a carefully calculated attenuation gain independently in each of the 64 bands. The noise reduction gain applied to a given band is determined by a combination of three factors: • Signal−to−Noise Ratio (SNR) • Masking threshold • Dynamics of the SNR per band The SNR in each band determines the maximum amount of attenuation to be applied to the band − the poorer the SNR, the greater the amount of attenuation. Simultaneously, in each band, the masking threshold variations resulting from the energy in other adjacent bands is taken into account. Finally, the noise reduction gain is also adjusted to take advantage of the natural masking of ‘noisy’ bands by speech bands over time. Based on this approach, only enough attenuation is applied to bring the energy in each ‘noisy’ band to just below the masking threshold. This prevents excessive amounts of attenuation from being applied and thereby reduces unwanted artifacts and audio distortion. The Noise |
同様の部品番号 - SB3231-E1 |
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同様の説明 - SB3231-E1 |
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