전자부품 데이터시트 검색엔진 |
|
SSM2000S 데이터시트(PDF) 7 Page - Analog Devices |
|
SSM2000S 데이터시트(HTML) 7 Page - Analog Devices |
7 / 16 page SSM2000 REV. 0 –7– APPLICATIONS INFORMATION Introduction The SSM2000 is a dual channel audio noise reduction IC which reduces noise through a combination of variable filtering and downward expansion in conjunction with a unique adaptive noise threshold detector. These two techniques yield an overall noise reduction of up to 25 dB on AM and FM radio, open reel and cassette tape, CD, Dolby B encoded programming, broad- cast studio-transmitter links, telephone lines, and other audio sources without the need for any additional manual adjustment. The HUSH Noise Reduction System as implemented in the SSM2000 has been shown to substantially reduce noise in PC multimedia, intercom systems, teleconferencing systems, mobile communications, automotive audio, home stereos and televi- sions, and other consumer and professional audio applications while preserving full signal fidelity and transparency. A brief explanation of the nature of audio signals is useful before discussing the details of the SSM2000’s operation. First, audio signals have both amplitude and frequency content. Music and voice are created by changing both the amplitude and frequency of sound waves as shown in Figure 17. Second, the highest audio signal amplitudes typically occur at low frequencies (100 Hz–1 kHz) and taper off exponentially as frequency in- creases. Third, when an audio signal is recorded, audio noise is also generated. This noise is the white noise “hiss” or waterfall sound that is easily heard on taped material. Audio noise for a given source and bandwidth does not, in general, change with frequency or amplitude. However, noise levels do change be- tween different sources, because of differences in recording equipment, media, and the surrounding environment. Fourth, psychoacoustic effects mask noise that occurs at or near the fre- quency of the audio signal. Finally, audio noise is usually con- sidered to be most objectionable in the 3 kHz–8 kHz bandwidth region. NOISE MUSIC'S FREQUENCY AND AMPLITUDE ENVELOPE NORMAL AUDIO SYSTEM FREQUENCY AND AMPLITUDE ENVELOPE FREQUENCY Figure 17. A Frequency-Amplitude Plot of the Audio Envelope An analog noise reduction system must first distinguish between the desired source material and the undesirable noise. It must then attenuate the noise while leaving the source material unaf- fected. One approach to noise reduction is to assume that signal below a predetermined amplitude is noise, and to then attenuate the noise by using a Voltage Controlled Amplifier (VCA). A variation of this noise reduction method is found in Dolby B cassette tape systems. This method achieves about 10 dB of improvement in signal-to-noise ratio (SNR). This system incor- porates a high-frequency compressor on the recording side, and a high frequency expander during playback. Another noise reduction technique senses and reduces noise by measuring the frequency content of the audio signal and then filters noise that occurs above the highest signal frequency. This noise reduction method utilizes a Voltage Controlled Filter (VCF) and is the basic method of operation in the DNR ® system, which provides about 10 dB of noise reduction. The HUSH system shown in Figure 34 combines elements of both of these techniques to achieve 25 dB of noise reduction, and also has significant improvements. x3 PRE-DETECTOR SIGNAL CONDITIONING VCF DETECTOR NOISE THRESHOLD DETECTOR SSM2000 VCA DETECTOR L/R IN L/R OUT VCA EXTERNAL CONTROL PORT MUTE VCF FILTER CAP HUSH DEFEAT 1,2 3,22 4,21 7 17 24,23 16 OUT AMP 8 9 10 VCF VCA Figure 18. SSM2000 Block Diagram (One Audio Channel Shown) Recalling that the noise floor changes with different audio sources due to recording equipment, media, and the environ- ment, we realize that fixed threshold approaches cannot yield optimal results. The HUSH Noise Reduction System incorpo- rates an automatic noise threshold detector that senses these changes and adapts the VCA and VCF to become more or less aggressive depending on the amplitude of the noise floor. To determine the amplitude of the noise floor the SSM2000 as- sumes that the averaged amplitude during short periods of no audio is equal to the noise floor. This assumption works well in audio applications, but does have implications for evaluation and testing that are discussed in the “Evaluating the SSM2000” section. Variable Low-Pass Filtering The audio signal is first passed through a single-pole low-pass Voltage Controlled Filter (VCF). Both the Left and Right VCFs are controlled by a detector which places their cutoff fre- quencies just beyond the highest audio signal frequency. Since the highest audio signal frequency constantly changes, the VCF’s cutoff frequency must also change in concert with the audio signal to avoid attenuating desired signal. For example, with signal levels below the filter threshold (pre- sumed to be mostly noise), the VCF shuts down to about 1 kHz, providing noise reduction in the critical 3 kHz– 8 kHz band, as Figure 19 illustrates. The VCF progressively “opens up” as higher frequency amplitudes are detected at the inputs. The VCF’s cutoff reaches 20 kHz when the high frequency signal amplitude is 30 dB above the threshold. At this point the VCF is acoustically transparent. The VCF’s cutoff frequency range is between 1 kHz to 35 kHz. The minimum range of the VCF is limited to 1 kHz for two reasons: one, to avoid high frequency loss at the leading edge of transients, because the lower the minimum cutoff the longer it takes the VCF to slew “open,” and two, noise is most objectionable at mid and high range frequen- cies. Once again, if the VCF characteristics are to be measured, then please refer to the section on “Evaluating the SSM2000.” DNR is a registered trademark of National Semiconductor Corporation. |
유사한 부품 번호 - SSM2000S |
|
유사한 설명 - SSM2000S |
|
|
링크 URL |
개인정보취급방침 |
ALLDATASHEET.CO.KR |
ALLDATASHEET 가 귀하에 도움이 되셨나요? [ DONATE ] |
Alldatasheet는? | 광고문의 | 운영자에게 연락하기 | 개인정보취급방침 | 링크교환 | 제조사별 검색 All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |