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EiInEtiEf, DOLBY.B NOISE PROCESSOR545 LINEAR INTEGRAIED CIRCUITS
DESCRIPTION
The NE545 is the monolithic implementation of the Dolby" B noise reduction system. This circuit is used to reduce the level of background noise introduced during recording and playback of audio signals on magnetic tape, and to improve the noise level in FM broadcast reception. This circuit is available only to licencees of Dolby Labs. Licensing and application information may be obtained from Dolby Labs. London or New York. ABSOLUTE MAXI MUM RATINGS Supply Voltage 24 Volts Operation Temperature Range 9"6 1e +70"C Storage Temperature Range 65"C to +i50"C Lead Temperature (Soldering, 60 sec.) +300"C BLOCK DIAGRAM DOLBY A TYPE INTEGRATED CIRCUII I SIGNETICS NE545A 15 J PIN CONFIGURATION (Top View) B PACKAGE Input Input Output Reference lnput Output O utput By Pass By Pass By Pass Reference Input Output By Pass G rou nd Order Part No. NE545B
SIGNETICS DOLBY-B NOISE PROCESSOR T 545 ELECTR ICAL CHARACTER ISTICS Vg6 = 12 volts, f = 2OHz to 20kHz. All levels referenced to 580mV RMS (0dB) at Pin 3, TA = +25"C (Unless Otherwise Noted) PARAMETER TEST CONDITIONS LIMITS UNIT MIN TYP MAX Supply Voltage Range Supply Current Voltage Gain (Pins 5-3) Voltage Gain (Pins 3-7) Distortion Signal Handling Signal-To-Noise Ratio Record Mode Frequency Response (at Pin 7) Back-to- Back F requency Response lnput Resistance Output Resistance f - lkHz (Pins 6 & 2 Connected) f = 1kHz,OdB at Pin 3, Noise Reduction Out f = 'l kHz,OdB at Pin 7 I = 1kHz, +10d8 at Pin 7 f - 'l kHz Distortion <.O.,Yo Record (Pins6 & 2Connected) Record (lnput on Pin 2) Playback (Pins6 & 2 Connected) Playback (lnput on Pin 2) f = 1.4kHz Input at Pin 5 - OdB = -20d8 = 30dB f = 5kHz Input at Pin 5 = OdB = 20dB - 30dB = 40dB f = 1OkHz Input at Pin 5 = OdB = -20d8 - 30dB Using Typical Record Mode Response Pin 5 Pin 2 Pin 6 Pin 3 Pin 7 I /.O - 24.5 a 18.8 - 23.8 I 19.4 25.5 +10 oz oz 3.9 1.9 zo n nnE o.2 +12 to to tc-o 22.5 16.8 21.8 29.7 -11.4 23.5 100 c.o 0.1 0.5 tt - tJ.o -20.5 AZ 14.8 - 19.8 -27.1 1tr A -21.5 az 6.7 3.1 "t20 120 dB k ohms k ohms k ohms onms ohms mA dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB
SIGNETICS DOLBY-B NOISE PROCESSOR I545 TYPICAL PER FORMANCE CURVES FIGURE 13 BACK-TO-BACK FREOUENCY R ESPONSE VERSUS TEMPERATURE ENcoDE AT +25"c AND -3odB FIGURE 14 BACK-TO.BACK FREOUENCY RESPONSE VERSUS TEMPERATURE ENcoDE AT +25"9 AND -4odB +50 E
01 I 10
FR€OUENCY lkH/) 0l I t0 too TUFOUfNCY l[H/] FIGURE 16 MAXIMUM OUTPUT SWING VEBSUS FREQUENCY REF. = 1%THD FIGURE 15 LOW LEVEL FREOUENCY RESPONSE l =44 ! roo FIGURE 17 ENCODE CHARACTER ISTICS FIGURE 18 LEVEL MISMATCH RESPONSE 2dB LOSS BETWEEN ENCODE AND DECODE a '/\\) 2dB
SIGNETICS DOLBY-B NOISE PROCESSOR r 545 SCHEMATIC DIAGRAM FIGURE 23 NE545B TEST CIRCUIT FIGURE 24 DOt BY B TYPE INTEGRATED CIRCUII SIGNETICS NE5458 c20 1 c208 200pF I 10 15 c303
4 IOO1F
1,, t00k c309 t0/15 R307 10/15 120 : c206 I0|5 c301 560{lr)l R306 180 c304 I**' c307 0.33"''_l 0027= l'ti,_L I NOI L: Al. t RFSISTORS STANDARD AND ARI MFASURED lN OH|S J,C202 T 250/rs NOTE: A unique phenomenon may be observed in the rapid testing of the NE545. lt is possible for C3O9 to become charged in a reverse polarity when the power to a device is switched on and off rapidly as might be encountered in a testing situation. This results in an increase in turn-on delay of the circuit. This condition can be prevented by placing a 15OkQ resistor in parallel with C309. This resistor is only requirecl in the test circuit and not in the actual application in which the NE545 is used. lf desired, the'l50kf) resistor can be used in the actual application without any degrada- tion of the performance of the circuit.
SIGNETICS DOLBY-B NOISE PROCESSOR T 545 NE545B TEST PROCEDURE EOUIPMENT 1. Low distortion audio generator (H.P. 204D) 2. RMS Voltmeter (H.P. 400E) Connect audio generator to the input of the test jig (Fis. 2al . Adjust the input level to obtain OdB (580mV) at pin 3 of the lC. Perform spot frequency measurement at the output (pin 7) of the lC. All readings should fall within 12dB with respect to the standard recording data as shown in the Processor Characteristics Analysis Form. Optionally, rather than checking at spot frequencies, the entire frequency spectrum can be checked using a sweep generator and recorder. INTRODUCTION The Dolby B-type noise reduction system is designed for the reduction of hiss in consumer products. Employing techniques similar to those used in the wideband Dolby Laboratories A-type professional noise reduction equ ip- ment, the B-type system retains the advantage of inaudible action, good matching, and low distortion. The B-type noise reduction system operates by boosting low-level high-frequency signals in the encode mode and attenuating the same signals in a complementary manner in the decode mode. During the latter process, noise in the treated frequency range is also attenuated. In applying noise reduction to consumer tape products and FM receivers, it is necessary to appreciate that the full subjective improvement offered by a 10dB reduction in hiss can be obtained only if the other performance para' meters of the system are of b comparable standard. The hum level, in particular, must be reduced to a lower value than is acceotable with normal hiss levels. The Signetics NE545B Dolby-B Noise Processor can be used in three basic noise reduction circuits - encoders, decoders, and switchable processors which will operate in either mode. CIRCUIT OPERATION The operation of the noise reduction system will be described with reference to Figure 25, which is a block diagram of the encode and decode processor. Referring to Figure 25, signals enter the encoder via amplif ier A and pass to a low-pass f ilter J which attenuates all unwanted frequencies. such as tape recorder bias or FM multiplex signals, to a level of less than 45d8. lf such spurious signals are above the threshold of the compressor, the full 10dB of low level pre-emphasis will not be obtained. The signal at the output of amplifier B is split into two paths, one, known as the main path, provides an unaltered signal component d irectly to the output via adder E, while the other, known as the side chain, controls the dynamic characteristics of the processor. The side chain contains a high-pass dynamic f ilter comprising capacitors C301, C302, C303 and resistors R30i, R302 and the controlled resistance F. At low signal levels F has a high value and the first part of the filter (i.e. C301, C302, and R301) then controls the response of the side chain. The output of the filter is amplified by amplifierC and the resulting noise reduction signal is added to the main signal in adder E. The low-level gain of the side-chain is such that the overall output is increased by i0dB at 5kHz when the noise reduction signal is added. FIGURE 25 ENCODE-DECODE BLOCK DIAGRAM R302 NOTE: ALL RESISTORS STANDARD AND ARE IVIEASURED lN OHMS c 301
SIGNETICS DOLBY-B NOISE PROCESSOR I545 FIGURE 27 IMPEDANCE CONVERTER BEFERFNCL VOLTAGF NOTE: ALL RESISTORS STANDARD AND ARE fulEASURED IN OHMS CIRCUIT DESCRIPTION (con't.) Amplifier'A'(Figure 27) is a unity gain impedance converter. providing high input and a given output impe- dance. A Darlington conf iguration is used for minimum bias current, keeping the output offset with respect to the reference voltage to a minimum. R21 gives the appropriate source impedance for the external low pass filter required to eliminate supersonic signals originating from FM tuners or from the bias and erase oscillators of tape recorders. One of the most challenging aspects of the lC design was the development of a variable resistor (block F) with a wide dynamic range and a resistance versus control voltage law similar to that of the field effect transistor employed in the discrete component circuit. The drain source conductance Gp5 of a field effect transistor operating as a variable resistor is given by
2 Inss Vns
Gos = .iix [(vcs----::)- vnl Vo' 2 Y- where IDSS is drain saturation current, Vp is gate pinch- off voltage, V65 is gate-source bias, Vp5 is drain-source voltage. Provided the signal level applied to the device is small, Gpg is therefore a linear function of the gate-source voltage. For a forward biased junction, the slope resistance RJ is given by KT R l=- ol where k is Boltzmann's constant, T is absolute temperature, q is electronic charge, and I is forward current through ju nction. Hence the slope conductance of the junction G1 is a linear function of the forward current, and it is possible to match the FET. The chosen variable resistor circuit is shown in its basic form in Figure 28. BV varying the current through the differential pair 01, 02 with the current sources 1 and 2, the input resistance at point 3 changes and its value is approximately
26 X 10'3
2 X_ohms
at room temoerature.
SIGNETICS DOLBY-B NOISE PROCESSOR I545 CIRCUIT DESCRIPTION (Con't.t In Figure 29 this principle has been transformed into a circuit as used in the lC. Additional diodes are inserted in the emitters of the differential pair in order to have a more manageable current range for the current source. The matching of the FET curve with the new circuit is shown in Figure 30. Since the variable resistance is a feedback amplifier, very little resistance modulation takes place; this is shown in a linearity graph (Figure 31) where THD is plotted against the input signal. For minimum loading of the variable resistance circuit, a Darlington buffer amplifier is used to drive amolif ier "C". One important feature of the Dolby noise reduction system is that it is possible to substantially eliminate the over- shoots which are normally associated with a compressor, and which give rise to transient distortion in the recording or transmission medium with consequent incorrect expan- sion (1). In order to generate sufficient signal to operate the over- shoot suppressor ll, the noise reduction signal level is raised in amplifier C. The suppressor itself consists of a simple diode clipper. The control amplifier "D" has a frequency dependent response determined by an external resistor and capacitor, and drives the rectifier section G via an AC coupling. The rectifier of the discrete component circuit employs a germanium diode, and the lC had to reproduce closely its threshold characteristics. Prolonged development work showed that good matching could be obtained by the use of a Schottky barrier diode. The smoothing of the control signal is performed by a two stage integration circuit with a level dependent charging rate, providing rapid attack under conditions when the overshoot suppressor might operate. and more gradual gain variation under less stringent conditions. External components provide the necessary trme constants. Bias and reference voltages for the individual sections are generated by the internal regulated power supply. Each section has its own bias supply to insure good isolation. CONCLUSIONS The device provides the desirable interfacing conditions for most economical incorporation in consumer equipment, so that, for example, the only additional electronics needed to complete one channel of a cassette recorder are a bias oscillator, a recording amplifier (one transistor), and a microphone and head amplifier (two transistors). The use of the lC permits the elimination of about 50 components/ channel, with a saving of approximately 100 soldered loints. In large quantity production the lC providesworth- while reductions in overall component cost and in assembly and testing times. Integrated circu it processors meet the originally stated requirements comfortably, and the standard Dolby B-type characteristics (originally those of the discrete component circuit) are met typically within a tolerance of 1dB. RE FERENCES 1. R.M. Dolby, "An audio noise reduction system", Journal of the Audio Engineering Society, Vol. 15.4 (1967). 2. R.M. Dolby, "A noise reduction system for consumer tape recording". Presented at Audio Eng. Soc. 1970 Convention in New York. 3. R. Berkovitz and K.J. Gundry, "Dolby B-type noise reduction system". Audio, Sept./Oct. '1973. 4. D.P. Robinson, "Production of Dolby B-type cassettes",. Journal of the Audio Engineering Society, Vol. 20.i0 (19721. D.P. Robinson, "Dolby B-type noise reduction for FM broadcasts", Journal of the Audio Engineering Society, Yo1.21.5 (1973). R.M. Dolby, "Optimum use of noise reduction in FM broadcasting", Journal of the Audio Engineering Society, Yol.21.5 (1973). FIGUBE 30 RE LATIVE CONTROL VO LTAGE VEBSUS RESISTANCE o FIGURE 31 HARMONIC DISTORTION VERSUS SIGNAL LEVEL to I z { or 100nrv 1000D,V stGNAr LEVEL VtN MiLLiVOLTS itlitlttlttl ,/ r" r,iou _uor, ,,.1?, VrN orVrru_ Sl(INAL FHtOUtNCY lkHz RD DrOOa AISTSTANCI KT ,rlF
FIGURE 39. CLASS I PROCESSOR SHOWING CIRCUIT VARIATIONS
SIGNETICS DOLBY.B NOISE PROCESSOR r 545 PARTS LIST FOR DOLBY.B CLASS I PROCESSOR PART VALUE WATT MAX. TOL. TYPE-PART NO. R301 302 303 304 305 306 307 308 309 310 311 312 3'14 c201 202 203 204 205 206 208 c301 302 303 304 305 306 307 308 309 L201 202 D301 3.3ks2 47kfl i50ko 27okSL 680ko 18oO r zoS) 1 00k 1kO 100ks) l MesS/
1 Mes f)
25OpF]sV 3900pF 3300pF 22OOpF 10pF 15V 200pF 5600pF .O271tF 4700pF .O471tF 10pF 15V .1 ltF .33trtF 1OpF 15V 10pF 15V 20 60MH 15 40MH Germanium OA91 112 114 114 114 114 1t4 "t/4 1t4 R q F Adjustable Ad ju stab le 3321F RN6OD Carbon Comp R1048 CTS Trimmer Carbon Comp E lectrolytic Myf ar Elpac Z 1R 392J ,' z 1R 332J " z 1R 222J E lectro lytic Ceram ic Mylar Elpac Z 1R 562F " z'tR 273F " z 1R 472F " z lR 4t3J E lectrolytic Mylar Elpac Z 1R 1O4K " z 1R 334K E lectrolytic J.W. M iller 9063 J.W. Miller 9062 IN34A, IN644
SIGNETICS DOLBY.B NOISE PROCESSOR T 545 AVENUE I SUNNYVALE, CALIFOFINIA I 940E!6 I T\\A/X: (91O) 339-92E|3suilltiEs El'I1 EAST AFIGUES TEL: (4OB) 739-77OO