LM832 Dynamic Noise Reduction System DNRÉ General Description Features The LM832 is a stereo noise reduction circuit for use with audio playback systems. The DNR system is noncomplementary, meaning it does not require encoded source material. The system is compatible with virtually all prerecorded tapes and FM broadcasts. Psychoacoustic masking, and an adaptive bandwidth scheme allow the DNR to achieve 10 dB of noise reduction. DNR can save circuit board space and cost because of the few additional components required. The LM832 is optimized for low voltage operation with input levels around 30 mVrms. For higher input levels use the LM1894. Y Y Y Y Y Y Y Y Y Applications Y DNRÉ is a registered trademark of National Semiconductor Corporation. The DNRÉ system is licensed to National Semiconductor Corp. under U.S. patent 3,678,416 and 3,753,159. A trademark and licensing agreement is required for the use of this product. Low voltage battery operation Non-complementary noise reduction, ‘‘single ended’’ Low cost external components, no critical matching Compatible with all prerecorded tapes and FM 10 dB effective tape noise reduction CCIR/ARM weighted Wide supply range, 1.5V to 9V 150 mVrms input overload No royalty requirements Cascade connection for 17 dB noise reduction Y Y Y Headphone stereo Microcassette players Radio cassette players Automotive radio/tape players Order Number LM832M See NS Package M14A Order Number LM832N See NS Package N14A Application Circuit FIGURE 1. Component Hook-up for Stereo DNR System C1995 National Semiconductor Corporation TL/H/5176 TL/H/5176 – 1 RRD-B30M115/Printed in U. S. A. LM832 Dynamic Noise Reduction System DNR August 1989 Absolute Maximum Ratings Soldering Information Y Dual-In-Line Package If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/Distributors for availability and specifications. Supply Voltage Power Dissipation (Note 1) Input Voltage Storage Temperature Soldering (10 seconds) 260§ C Small Outline Package Vapor Phase (60 seconds) 215§ C Infrared (15 seconds) 220§ C See AN-450 ‘‘Surface Mounting Methods and Their Effects on Products Reliability’’ for other methods of soldering surface mount devices.’’ 10V 1.2W 1.7 Vpp Y b 65 to a 150§ C Operating Temperature (Note 1) b 40 to a 85§ DC Electrical Characteristics TA e 25§ C VCC e 3.0V Symbol Parameter Conditions Min Typ Max 1.5 3.0 9.0 V 2.5 4.0 mA 5.0 8.0 mA 0.36 0.5 V 0.8 V VOP Operating Voltage Supply Voltage for Normal Operation ICC(1) Supply Current (1) Pin 9 to GND 0.1 mF, BW e Min, Note 2 ICC(2) Supply Current (2) DC GND Pin 9 with 2k, BW e Max, Note 2 VIN(1) Input Voltage (1) Pin 2, Pin 13 0.20 VIN(2) Input Voltage (2) Pin 6 0.50 0.65 Units VIN(3) Input Voltage (3) Pin 9 0.50 0.65 0.8 V VOUT(1) Output Voltage (1) Pin 4, Pin 11 0.20 0.35 0.50 V VOUT(2) Output Voltage (2) Pin 5 Stereo Mode 0.15 0.28 0.40 V VOUT(3) Output Voltage (3) Pin 5 Monaural Mode, DC Ground Pin 14 0.10 0.20 0.30 V VOUT(4) Output Voltage (4) Pin 8 0.25 0.40 0.60 V VOUT(5) Output Voltage (5) Pin 10 BW e Max, Note 2 1.00 1.27 1.50 V VOUT(6) Output Voltage (6) Pin 10 BW e Min, Note 2 0.50 0.65 0.75 V VOS Output DC Shift Pin 4, PIN 11; Change BW Min to Max 1.0 3.0 mV AC Electrical Characteristics Symbol Parameter Conditions Min Typ Max Units MAIN SIGNAL PATH (Note 3) AV Voltage Gain VIN e 30 mVrms, f e 1 kHz, BW e Max, Note 2 b 1.0 0.0 a 1.0 dB C.B. Channel Balance VIN e 30 mVrms, f e 1 kHz, BW e Max, Note 2 b 1.0 0 a 1.0 dB fMIN Min Bandwidth 0.1 mF between Pin 9 - GND 600 1000 1500 Hz fMAX Max Bandwidth DC Ground Pin 9 with 2k 24 30 46 kHz THD Distortion VIN e 30 mVrms, f e 1 kHz, BW e Max, Note 2 0.07 0.5 MVIN Max Input Voltage THD e 3%, f e 1 kHz, BW e Max Note 2 120 150 S/N Signal to Noise REF e 30 mVrms, BW e Max, CCIR/ARM 60 68 ZIN Input Impedance Pin 2, Pin 13 14 20 C.S. Channel Separation Ref e 30 mVrms, f e 1 kHz, BW e Max, Note 2 40 68 dB PSRR PSRR VRIPPLE e 50 mVrms, f e 100 Hz 40 55 dB % mVrms dB 26 kX CONTROL PATH AVsum(1) Summing Amp Gain (1) VIN e 30 mVrms at R and L, f e 1 kHz b 3.0 b 1.5 0.0 dB AVsum(2) Summing Amp Gain (2) DC Ground Pin 14, f e 1 kHz b 9.0 b 6.0 b 3.0 dB AV 1st Gain Amp Gain Pin 6 to Pin 8 25 30 35 dB ZIN 1st Input Impedance Pin 6 28 40 52 kX AVPKD Peak Detector Gain AC In, DC Out; Pin 9 to Pin 10 25 30 35 V/V ZINPKD Input Impedance Pin 9 500 800 1100 X VRPKD Output DC Change Pin 10, Change BW Min to Max 0.5 0.62 0.8 V Note 1: For operation in ambient temperature above 25§ C, the device must be derated based on a 150§ C maximum junction temperature and a thermal resistance junction to ambient, as follows: LM832N b 90§ c/w, LM832M-115§ c/w. Note 2: To force the DNR system into maximum bandwidth, connect a 2k resistor from pin 9 to GND. AC ground pin 9 or pin 6 to select minimum bandwidth. To change minimum and maximum bandwidth, see Application Hints. Note 3: The maximum noise reduction CCIR/ARM weighted is about 14 dB. This is accomplished by changing the bandwidth from maximum to minimum. In actual operation, minimum bandwidth is not selected, a nominal minimum bandwidth of about 2 kHz gives 10 dB of noise reduction. See Application Hints. 2 External Component Guide (See Figure 1 ) P/N Recommended Value Effect Purpose Remarks Smaller Larger C1 10 mF Power supply decoupling Poor supply rejection Better supply rejection Do not use less than 10 mF C2,C11 1 mF Input coupling capacitor Increases frequency of lowfrequency roll-off Reduces frequency of lowfrequency roll-off DC voltage at pin 2 and pin 13 is 0.35V fe C3,C10 22 nF for Stereo, 15 nF for mono C4,C8 1 mF C5 C6 0.1 mF 820 pF 1 2qC2RIN Establishment of Min and Max Bandwidth Bandwidth becomes wider Bandwidth becomes narrower See Note 4 Output coupling capacitor Increases frequency of lowfrequency roll-off Reduces frequency of lowfrequency roll-off DC voltage at pin 4 and pin 11 is 0.35V Works with R1 and R2 to set one of the lowfrequency corners in control path Works with input resistance of pin 6 to set one of the low-frequency corners in the control path C7 39 nF Works with input resistance of pin 9 to form part of control path frequency weighing C9 1 mF Sets attack time R1,R2 R1 a R2 e 1 kX R3 # 2 kX This voltage divider sets control path sensitivity Sets gain amp load when DNR is OFF Some high frequency program material may be attenuated Same as above Bandwidth may increase due to low-frequency inputs, causing ‘‘Breathing’’ Same as above fe 1 2qC4RLOAD fe 1 e 1.6 kHz 2qC5(R1 a R2) See Note 4 fe 1 e 4.8 kHz 2qC6RPIN6 See Note 4 Same as above Same as above Reduces attack and decay time Increases attack and decay time fe 1 e 4.8 kHz 2qC7RPIN7 See Note 4 Ð Loads gain amp output, may cause distortion Ð See Note 4 Sensitivity should be set for maximum noise reduction and minimum audible frequency program effect on high Max bandwidth will be reduced Note 4: The values of the control path filter components (C5, C6, C7, C9, R1, R2) and the integrating capacitors (C3, C10) should not be changed from the recommended values unless the characteristics of the noise or program material differ substantially from that of FM or tape sources. Failure to use the correct values may result in degraded performance, and therefore the application may not be approved for DNR trademark usage. Please contact National Semiconductor for more information and technical assistance. 3 Typical Performance Characteristics TL/H/5176–2 FIGURE 2. Supply current vs supply voltage TL/H/5176–5 FIGURE 5. Output level change vs supply voltage FIGURE 4. Power supply rejection ratio vs frequency TL/H/5176 – 7 TL/H/5176 – 6 FIGURE 6. Output level vs frequency TL/H/5176 – 9 TL/H/5176–8 FIGURE 8. Output vs frequency and control path signal TL/H/5176 – 4 TL/H/5176 – 3 FIGURE 3. Channel separation vs frequency FIGURE 9. Frequency response for various input levels TL/H/5176 – 11 FIGURE 11. Change in main signal path maximum bandwidth vs temperature 4 FIGURE 7. THD vs frequency TL/H/5176 – 10 FIGURE 10. Gain of control path vs frequency Circuit Operation acts as an integrator and is unable to detect it. Because of this, signals of sufficient energy to mask noise open the bandwidth to 90% of the maximum value in less than 1 ms. Reducing the bandwidth to within 10% of its minimum value is done in about 60 ms: long enough to allow the ambience of the music to pass through, but not so long as to allow the noise floor to become audible. 3. Reducing the audio bandwidth reduces the audibility of noise. Audibility of noise is dependent on noise spectrum, or how the noise energy is distributed with frequency. Depending on the tape and the recorder equalization, tape noise spectrum may be slightly rolled off with frequency on a per octave basis. The ear sensitivity on the other hand greatly increases between 2 kHz and 10 kHz. Noise in this region is extremely audible. The DNR system low pass filters this noise. Low frequency music will not appreciably open the DNR bandwidth, thus 2 kHz to 20 kHz noise is not heard. The LM832 has two signal paths, a main signal path and a bandwidth control path. The main path is an audio low pass filter comprised of a gm block with a variable current, and a unity gain buffer. As seen in Figure 1 , DC feedback constrains the low frequency gain to Av e b1. Above the cutoff frequency of the filter, the output decreases at b6 dB/oct due to the action of the 0.022 mF capacitor. The purpose of the control path is to generate a bandwidth control signal which replicates the ear’s sensitivity to noise in the presence of a tone. A single control path is used for both channels to keep the stereo image from wandering. This is done by adding the right and left channels together in the summing amplifier of Figure 1 . The R1, R2 resistor divider adjusts the incoming noise level to slightly open the bandwidth of the low pass filter. Control path gain is about 60dB and is set by the gain amplifier and peak detector gain. This large gain is needed to ensure the low pass filter bandwidth can be opened by very low noise floors. The capacitors between the summing amplifier output and the peak detector input determine the frequency weighting as shown in the typical performance curves. The 1 mF capacitor at pin 10, in conjunction with internal resistors, sets the attack and decay times. The voltage is converted into a proportional current which is fed into the gm blocks. The bandwidth sensitivity to gm current is 70 Hz/mA. In FM stereo applications a 19 kHz pilot filter is inserted between pin 8 and pin 9 as shown in Figure 16 . Normal methods of evaluating the frequency response of the LM 832 can be misleading if the input signal is also applied to the control path. Since the control path includes a frequency weighting network, a constant amplitude but varying frequency input signal will change the audio signal path bandwidth in a non-linear fashion. Measurements of the audio signal path frequency response will therefore be in error since the bandwidth will be changing during the measurement. See Figure 9 for an example of the misleading results that can be obtained from this measurement approach. Although the frequency response is always flat below a single high-frequency pole, the lower curves do not resemble single pole responses at all. A more accurate evaluation of the frequency response can be seen in Figure 8 . In this case the main signal path is frequency swept while, the control path has a constant frequency applied. It can be seen that different control path frequencies each give a distinctive gain roll-off. Application Hints The DNR system should always be placed before tone and volume controls as shown in Figure 1 . This is because any adjustment of these controls would alter the noise floor seen by the DNR control path. The sensitivity resistors R1 and R2 may need to be switched with the input selector, depending on the noise floors of different sources, i.e., tape, FM, phono. To determine the value of R1 and R2 in a tape system for instance; apply tape noise (no program material) and adjust the ratio of R1 and R2 to slightly open the bandwidth of the main signal path. This can easily be done by viewing the capacitor voltage of pin 10 with an oscilloscope, or by using the circuit of Figure 12 . This circuit gives an LED display of the voltage on the peak detector capacitor. Adjust the values of R1 and R2 (their sum is always 1 kX) to light the LEDs of pin 1 and pin 18. The LED bar graph does not indicate signal level, but rather instantaneous bandwidth of the two filters; it should not be used as a signal-level indicator. For greater flexibility in setting the bandwidth sensitivity, R1 and R2 could be replaced by a 1 kX potentiometer. To change the minimum and maximum value of bandwidth, the integrating capacitors, C3 and C10, can be scaled up or down. Since the bandwidth is inversely proportional to the capacitance, changing this 0.022 mF capacitor to 0.015 mF will change the typical bandwidth from 1 kHz – 30 kHz to 1.5 kHz – 44 kHz. With C3 and C10 set at 0.022 mF, the maximum bandwidth is typically 30 kHz. A double pole double throw switch can be used to completely bypass DNR. The capacitor on pin 10 in conjunction with internal resistors sets the attack and decay times. The attack time can be altered by changing the size of C9. Decay times can be decreased by paralleling a resistor with C9, and increased by increasing the value of C9. When measuring the amount of noise reduction of DNR in a cassette tape system, the frequency response of the cassette should be flat to 10 kHz. The CCIR weighting network has substantial gain to 8 kHz and any additional roll-off in the cassette player will reduce the benefits of DNR noise reduction. A typical signal-to-noise measurement circuit is shown in Figure 13 . The DNR system should be switched from maximum bandwidth to nominal bandwidth with tape noise as a signal source. The reduction in measured noise is the signal-to-noise ratio improvement. PSYCHOACOUSTIC BASICS The dynamic noise reduction system is a low pass filter that has a variable bandwidth of 1 kHz to 30 kHz, dependent on music spectrum. The DNR system operates on three principles of psychoacoustics. 1. Music and speech can mask noise. In the absence of source material, background noise can be very audible. However, when music or speech is present, the human ear is less able to distinguish the noiseÐthe source material is said to mask the noise. The degree of masking is dependent on the amplitude and spectral content (frequencies) of the source material, but in general multiple tones around 1 kHz are capable of providing excellent masking of noise over a very wide frequency range. 2. The ear cannot detect distortion for less than 1 ms. On a transient basis, if distortion occurs in less than 1 ms, the ear 5 Application Hints (Continued) TL/H/5176 – 12 FIGURE 12. Bar Graph Display of Peak Detector Voltage TL/H/5176 – 13 FIGURE 13. Technique for Measuring S/N Improvement of the DNR System CASCADE CONNECTION Additional noise reduction can be obtained by cascading the DNR filters. With two filters cascaded the rolloff is 12 dB per octave. For proper operating bandwidth the capacitors on pin 3 and 12 are changed to 15 nF. The resulting noise reduction is about 17 dB. Figure 15 shows the monaural cascade connection. Note that pin 14 is grounded so only the pin 2 input is fed to the summing amp and therefore the control path. Figure 14 shows the stereo cascade connection. Note that pin 14 is open circuit as in normal stereo operation. *R1 a R2 e 1 kX (refer to application hints) FIGURE 14. Stereo Cascade Connection 6 TL/H/5176 – 14 Application Hints (Continued) *R1 a R2 e 1 kX (refer to application hints) TL/H/5176 – 15 FIGURE 15. Monaural Cascade Connection FM STEREO When using the DNR system with FM stereo as the audio source, it is important to eliminate the ultrasonic frequencies that accompany the audio. If the radio has a multiplex filter to remove the ultrasonics there will be no problem. This filtering can be done at the output of the demodulator, before the DNR system, or in the DNR system control path. Standard audio multiplex filters are available for use at the output of the demodulator from several filter companies. Figure 16 shows the additional components L1, C15 and C16 that are added to the control path for FM stereo applications. The coil must be tuned to 19 kHz, the FM pilot frequency. *R1 a R2 e 1 KX (refer to application hints) TL/H/5176 – 16 FIGURE 16. FM Stereo Application FOR FURTHER READING Noise Masking 1. ‘‘Masking and Discrimination’’, Bos and De Boer, JAES, Volume 39, Ý4, 1966. 2. ‘‘The Masking of Pure Tones and Speech by White Noise’’, Hawkins and Stevens, JAES, Volume 22, Ý1, 1950. Tape Noise Levels 1. ‘‘A Wide Range Dynamic Noise Reduction System’’ Blackmer, ‘dB’ Magazine, August-September 1972, Volume 6, Ý8. 2. ‘‘Dolby B-Type Noise Reduction System’’, Berkowitz and Gundry, Sert Journal, May-June 1974, Volume 8. 3. ‘‘Cassette vs Elcaset vs Open Reel’’, Toole, Audioscene Canada, April 1978. 4. ‘‘CCIR/ARM: A Practical Noise Measurement Method’’, Dolby, Robinson, Gundry, JAES, 1978. 3. ‘‘Sound System Engineering’’, Davis, Howard W. Sams and Co. 4. ‘‘High Quality Sound Reproduction’’, Moir, Chapman Hall, 1960. 5. ‘‘Speech and Hearing in Communication’’, Fletcher, Van Nostrand, 1953. 7 TL/H/5176 – 17 LM832 Simple Circuit Schematic 8 9 LM832 Dynamic Noise Reduction System DNR Physical Dimensions inches (millimeters) Order LM832M NS Package Number M14A Order LM832N NS Package Number N14A LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Europe Fax: (a49) 0-180-530 85 86 Email: cnjwge @ tevm2.nsc.com Deutsch Tel: (a49) 0-180-530 85 85 English Tel: (a49) 0-180-532 78 32 Fran3ais Tel: (a49) 0-180-532 93 58 Italiano Tel: (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-043-299-2309 Fax: 81-043-299-2408 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. This datasheet has been download from: www.datasheetcatalog.com Datasheets for electronics components.