SSM2000 HUSH Stereo Noise Reduction System with Adaptive Threshold
Document overview
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Technical content
REV. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. (See page 16.) a HUSH Stereo Noise Reduction System with Adaptive Threshold SSM2000 © Analog Devices, Inc., 1996 Tel: 617/329-4700 Fax: 617/326-8703 PIN CONFIGURATION 24-Lead Plastic DIP 24-Lead SOIC TOP VIEW (Not to Scale) NC = NO CONNECT L IN R VCF C2 R VCF C1 R OUT L OUT R IN L VCF C1 L VCF C2 NC DIGITAL GND V–V+ ACOM VCA PORT VCF DET IN SUM OUT VCA DET IN AUTO THRESHOLD CAP DEFEAT MUTE VCF DET CAP VCA DET CAP DEFAULT THRESHOLD NC SSM2000
FEATURES
Up to 25 dB of Noise Reduction from Virtually Any Audio Source without Sonic Artifacts “Single-Ended” Operation Eliminates Need for Encode- Decode Process Adaptive Threshold Dynamically Adjusts to Changing Nominal Signal Levels Effectively Decodes Dolby B ® Encoded Sources Direct VCA Control Port Access for Additional Level Control Functionality Logic-Controllable Bypass and Muting 100 dB Dynamic Range (Noise Reduction OFF) 0.02% Typical THD+N (@ 1 kHz, Noise Reduction OFF) +7 V to +18 V Operation No Royalty Requirements
APPLICATIONS
Auto Radio Sound Processing Multimedia PC Sound Cards Television Sound Processing Cassette Tape Players AM/FM Receivers Telephone & Wireless Links Professional Audio L IN R VCF CAP 2 R VCF CAP 1 R OUT L OUT R IN L VCF CAP 1 L VCF CAP 2 DO NOT CONNECT 1 DIGITAL GND V–V+ ANALOG COMMON (V+/2) VCA CONTROL PORT VCF DET IN SUM OUT VCA DET IN AUTO THRESHOLD CAP DEFEAT MUTE VCF DET TC VCA DET TC DEFAULT THRESHOLD 2 1kΩ 2nF 500Ω 20nF 3µF 20nF 1.0µF 3.3µF 6.8µF 6.8µF 1nF 0.22µF DO NOT CONNECT 1 NOTES: 1MAKE NO CONNECTION TO PINS 13 AND 18 2DEFAULT THRESHOLD. NORMALLY CONNECTED TO ANALOG COMMON 1nF SSM2000 Figure 1. Typical Basic Application ing, compression, etc. with minimal external circuitry. Dolby B is a registered trademark of Dolby Laboratories, Inc. HUSH is a registered trademark of Rocktron Corporation.
REV. 0 –3– PIN DESCRIPTION Pin # Name Function and Explanation 1 L IN Left Audio Input. 2 R IN Right Audio Input. 3 L VCF C1 Left VCF Filter Cap Port. 4 L VCF C2 Left VCF Filter Cap Port. 5 V+ Positive Supply. 6 ACOM Analog Common Voltage. In- ternal Circuit Ground Point. Must be held to a voltage ap- proximately halfway between V+ and V–. Should be a clean low impedance voltage source capable of at least 4 mA, such as from a buffer driven off a supply-splitting voltage divider. 7 VCA PORT Input to VCA Control Port. Allows external adjustment of attenuation with a +22 mV/dB slope. Zero volts relative to ACOM gives zero additional attenuation. Should be con- nected to ACOM if function not required. 8 VCF DET IN Input to VCF Detector.
9 SUM OUT Combined Left Plus Right
Output. 10 VCA DET IN Input to VCA Detector. 11 VCF DET CAP Voltage Controlled Filter. Time Constant Capacitor Port. 12 VCA DET CAP Voltage Controlled Amplifier. Time Constant Capacitor Port. WARNING! ESD SENSITIVE DEVICE CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the SSM2000 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. ABSOLUTE MAXIMUM RATINGS Junction Temperature (T ESD RATINGS ORDERING GUIDE Temperature Package Package Model Range Description Option SSM2000P –40 °C to +85°C 24-Pin Plastic DIP N-24 SSM2000S –40 °C to +85°C 24-Pin SOIC SOL-24
PACKAGE INFORMATION
24-Pin Plastic DIP (P) 54 27 °C/W 24-Pin SOIC (S) 72 24 °C/W NOTE 1θJA is specified for the worst case conditions, i.e., θJA is specified for device in socket for P-DIP packages; θJA is specified for device soldered onto a circuit board for surface mount packages.. Pin # Name Function and Explanation
13 NC Make No Connection to this
Pin.
14 DEFAULT Default Threshold Adjust-
THRESHOLD ment Port. Allows reduction of noise reduction action if signal source is relatively clean such as CD sources. Normally connected to Analog Common. –1.2 V is recom- mended for CDs; –2 V com- pletely defeats noise reduction.
15 AUTO THRESHOLD
CAP Automatic Threshold Detec- tor Capacitor Port.
16 DEFEAT Logic High: Defeats Noise
Reduction. Logic Low: Normal Action. Connect to Ground if not used. 17 MUTE Logic High: Mute. Logic Low: Normal Action. Connect to Ground if not used.
18 NC Make no connection to this
pin.
19 DIGITAL GND Digital Logic Reference
(Logic Zero). 20 V– Negative Supply. 21 R VCF C2 Right VCF Capacitor Port. 22 R VCF C1 Right VCF Capacitor Port. 23 R OUT Right Audio Output. 24 L OUT Left Audio Output.
audible frequencies and is therefore not very useful. Single Supply” for more information. ACOM is the internal ground reference for the audio circuitry. low impedance potential that is one half the positive supply. SSM2000—Single Supply” for more information. As shown in Figure 5, this port connects into the VCA circuitry. the SUM OUT (Pin 9) and VCF IN (Pin 8). plitude and frequency content. Figure 29. The Internal Circuit for SUM OUT artificial noise threshold using this pin. reduce the responsiveness of HUSH. capacitor is required to hold the charge.
MUTE and DEFEAT sections for more information. the negative power rail in dual-supply applications. exceed 2 kΩ or 300 pF additional buffering may be necessary.
- DC Volume Control—one dc level controls both audio chan- nels without routing audio signals to the front panel.
- Automatic Volume Leveling—constrains long term average volume levels to within a few dB, avoiding constant resetting of volume as one tuner through various stations or switches from radio to tape, etc.
- Compression—reduces dynamic range to improve audibility of softer sounds in noisy environments such as when playing CDs in cars.
- Volume vs. Road Speed—a speedometer signal can be used to increase audio levels as road speed increases.
- Compression vs. Road Speed—see above. Automatic Leveling Automatic leveling is similar to compression, however instead of attempting to attenuate all large amplitude audio signals, the leveling circuit attenuates audio signal when the overall signal amplitude has increased for a considerable amount of time, >1 sec. Attenuating loud TV commercials whose average vol- ume levels are significantly above the program material is an ap- plication were this feature is particularly useful. The circuit shown in Figure 30 is an example of HUSH automatic leveling. Automatic leveling is a signal amplitude operation, therefore, the input of the circuit is derived from Pin 12, VCA TC. This input voltage is gained and level shifted. The volume control is “counter intuitive,” because the higher the gain the lower the re- sulting volume, as the VCA control port attenuates when posi- tive voltage is applied. The level threshold potentiometer sets the volume level, which is a gain range of 1.5 dB to –80 dB. After the op amp, the signal passes through a positive peak detector, and is averaged by a long time constant RC. The sig- nal is now representative of the long term average amplitude of the original audio that is applied to the L/R input of the SSM2000. This signal is then passed to the VCA control port. Audio Compression Audio compression can be thought of as fast automatic leveling. The purpose of audio compression is to attenuate larger ampli- tude signals, such that signals above the adaptive threshold will be scaled by 0.2 dB/dB, while the signals below the externally set level threshold will remain essentially unchanged. A shorter time constant and slight gain change are the only differences be- tween the audio compression circuit and the automatic leveling circuit. Figure 30 illustrates the audio compression circuit. The OP275 is an excellent audio amplifier and is suggested for this application when the power supply rails are greater than or equal to ± 5 V. For applications where the power supplies are separated by less than 10 V, then a single-supply op amp like the OP279 is recommended. 3kΩ 100kΩ 7.5kΩ 27kΩ +5V 20kΩ +5V –5V 5kΩ POT VOLUME CONTROL 10kΩ POT LEVEL THRESHOLD OP2750.1µF OFF LEVEL COMPRESS3.3µF 10M Ω SSM2000 +5V –5V SP3T OFF LEVEL COMPRESS
Figure 30. An Automatic Leveling/Compression Circuit
REV. 0–16– VCF—minimum bandwidth) to their fully open positions. If the VCA and VCF are fully opened then at least 1.5 seconds of no signal is required for the VCA and VCF detectors to return to the closed position. A series of 10 kHz pulses interspersed with 1.5 seconds of no signal will accomplish this requirement. The system’s attack and release times will be dominated by VCA since the attack and release times of the VCA are slower than the VCF’s. In addition, monitoring Pins 11 (VCF) and 12 (VCA) will ensure that the VCA and VCF are either fully opened or fully closed. As stated before, the most common cause of a failure to reduce noise is fixed-frequency tones that are being detected by the output. A solution for this condition is detailed in the “Precau- tions, Limitations, and Observations” section. In single-supply applications, ACOM should be driven with a well regulated reference capable of both sinking and souring current. Zener-resistor references are unacceptable, because of their inability to sink significant amounts of current. By following these simple suggestions, the common pitfalls to a successful implementation of the HUSH noise reduction system can be avoided. C2130–10–5/96PRINTED IN U.S.A. OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 24-Pin Plastic DIP (N-24) 11 2 13 0.280 (7.11) 0.240 (6.10) PIN 1 1.275 (32.30) 1.125 (28.60) 0.150 (3.81) MIN0.200 (5.05) 0.125 (3.18) SEATING PLANE 0.022 (0.558) 0.014 (0.356) 0.060 (1.52) 0.015 (0.38) 0.210 (5.33) MAX 0.070 (1.77) 0.045 (1.15) 0.100 (2.54) BSC 0.325 (8.25) 0.300 (7.62) 0.015 (0.381) 0.008 (0.204) 0.195 (4.95) 0.115 (2.93)
24 Pin SOIC
(SOL-24) 24 13 121 0.6141 (15.60) 0.5985 (15.20) 0.4193 (10.65) 0.3937 (10.00) 0.2992 (7.60) 0.2914 (7.40) PIN 1 SEATING PLANE 0.0118 (0.30) 0.0040 (0.10) 0.0192 (0.49) 0.0138 (0.35) 0.1043 (2.65) 0.0926 (2.35) 0.0500 (1.27) BSC 0.0125 (0.32) 0.0091 (0.23) 0.0500 (1.27) 0.0157 (0.40) 0.0291 (0.74) 0.0098 (0.25)x 45° promotional and descriptive literature. Rocktron Corp. reserves the right to approve use of the HUSH logo and may require submission of a schematic illustrating the use of the SSM2000; a physical sample of the end product may also be required. Applications for approval of the use of the HUSH name and logo may be obtained by contacting ADI. IMPORTANT NOTICE—LICENSING The SSM2000 is manufactured under a license granted to Analog Devices, Inc., by Rocktron Corp. and embodies technology covered by the following patents: 4696044, 4893099, 5124657, 5263091, 5402498, and 5493617. For- eign patents are pending. Rocktron Corp. grants a royalty-free license to purchasers of the SSM2000 for use in end equipment, and no further rights are implied. The use of the HUSH logo is encouraged on products, packaging,