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Abridged data for the NE572 programmable analog compandor Elsewhere in this issue we have published the complete circuit of a CD Compressor, based on the Signetics NE572 IC. This dual channel gain control circuit can be used as a compressor or expandor and in noise reduction systems. ~ - - -- ----------<-(5,11) (8.10) In use, the two channels may be used independently for dynamic range compression or expansion. As shown in the block diagram of Fig.1, each channel has a full wave rectifier to detect the average value of the input signal, a variable gain cell and a signal buffer. The buffer stage permits independent control of dynamic attack and recovery time with a minimum of external components. This leads to improved distortion compared with previous compandors. (1.15) r---+---+-+- Main features (18) (8) (4,12) (:Z,14) Fig.1: each channel of the NE572 has a full wave rectifier to detect the average value of the input signal, a variable gain cell and a signal buffer. v+ + ,, 140µA The gain cell IQ 02 ., 6 .8K IG I Vin Fig. 2: the gain cell circuitry. Qt, Q2, Q3 & Q4 form two differential pairs with bases connected to the inputs and output of op amp Al. 42 The main features of the NE572 compandor chip are as follows: • Independent control of attack and recovery time. • Improved low frequency gain control ripple. • Complementary gain compression and expansion with external op amp; suitable for noise reduction systems. • Wide dynamic range () 1 lOdB). • Temperature-compensated gain control. • Low distortion gain cell. • Low noise - 6µ V typical. • Wide supply voltage range 6-22V. • System level adjustable with external components. SILICON CHIP Fig.2 shows the circuit of the gain cell. Ql, QZ, Q3 and Q4 are two differential pairs with the bases connected to the inputs and output of op amp Al. The negative feedback through Ql holds the VEE of Q3 and Q4 equal. There is an offset voltage at the output due to mismatching of the transistors and this leads to even harmonic distortion. The offset voltage can be trimmed out by R,4 R3 17.3t< (5 , 11~ ~G CIN1 YIN o----j ,_____.. 2.2µF VREF (4.12) CIN3 2.2µF JJ R2 3.3K CA ....i-- (3.13) CR 1µF 10µF (18) (8) -=- +vcc Fig.3: the basic expandor circuit. The NE572 acts as a variable resistance in the input signal path to op amp A2. At acts as an input buffer. feeding a current source within ± 25µ,A into the THD (total harmonic distortion) trim pin. Effectively, the gain cell works by varying the current through the second differential pair, Q3 and Q4. The gain current is controlled by VREF· The output current 10 must feed the virtual ground input of an operational amplifier in noninverting mode. The non-inverting input of the op amp has to be biased at VREF if the output current 10 is to be DC coupled. The buffer amplifier In audio systems it is desirable to have fast attack time and slow 9,11( recovery time for a tone burst input. The fast attack time reduces transient channel overload but also causes low frequency ripple distortion. The low frequency ripple distortion is cured by a slow recovery period. The buffer amplifier makes it easy to have independent attack and recovery times. Basic expandor The basic expandor circuit shown in Fig.3 can be viewed as an inverting amplifier circuit based around op amp A2. The NE572 acts as a variable input resistance that changes according to the average input signal level detected by the ½coc Basic compressor 10µF C2 , 1µF c .. , ... e>------j 1--,vv,,......+--2,2,uF ,:;K 2 .2,uF C..3 2.2µF l1 * CR 10µF 3 ,31( R2 CA 1µF t---+------vv\r(3,13) vcc (1e1 rectifier. The rectifier alters the output current of the gain cell which in turn controls the overall gain of the amplifier. The amplifier stage is preceded by op amp A1 which acts as an input buffer. The resulting output signal range is greater than that present at the input. The expansion is centred about input levels of lO0mV so there is no change to a signal of this magnitude. Signals above lO0mV can be boosted by up to 14dB and those below this level can be attenuated down by as much as 40dB. In Fig.3 resistors Rl and R2 are tied to internal summing nodes. Rl is an internal 6.8kfl resistor. The maximum input current into the gain cell can be as large as 140µ,A. This corresponds to a voltage level of 952mV peak (6.8kn x 140µ,A). The input peak current into the rectifier is limited to 300µ,A. Note that the addition of external resistances in series with Rl will accommodate a higher input level. R2 and R3 are external resistors and the ratio of R3/R2 is set to obtain desirable system voltage and current levels. A small R2 results in higher gain control current and smaller static and dynamic tracking error. In high performance applications, A2 has to be low noise, high speed and wide band so that the high performance output of the gain cell will not be degraded. The capacitor CA connected to the NE572 sets the attack time and CR sets the recovery time constant. Fig.4: the compressor circuit. This uses an NE572 in the feedback loop of inverting op amp stage At. The resistance of the gain cell varies according to the signal level on the output of At. The arrangement of the NE572 in a compressor circuit is shown in Fig.4. The NE572 is placed in the feedback loop of op amp Al. This circuit has the opposite effect of an expandor on input signals. The operation is essentially the same as that of our CD Compressor. Compandor systems involve the combined use of both a compressor and an expandor to process a signal. These systems are usually applied to tape/disc noise reduction, digital audio, and bucket brigade delay lines. More elaborate compandor systems have features such as band limiting, band splitting, pre-emphasis, de-emphasis and equalisation incorporated. ~ MARCH 1989 43