SL34118

SL34118 Voice Switched Speakerphone Circuit The SL34118 Voice Switched Speakerphone Circuit incorporates the necessary amplifiers, attenuators, level detectors, and control algorithm to form the heart of a high quality hands-free speakerphone system. Included are a microphone amplifier with adjustable gain and MUTE control, Transmit and Receive attenuators which operate in a complementary manner, level detectors at both input and output of both attenuators, and background noise monitors for both the transmit and receive channels. A Dial Tone Detector prevents the dial tone from being attenuated by the Receive background noise monitor circuit. Also included are two line driver amplifiers which can be used to form a hybrid network in conjunction with an external coupling transformer. A high pass filter can be used to filter out 60 Hz noise in the reseive channel, or for other filtering functions. A Chip Disable pin permits powering down the entire circuit to conserve power on long loops where loop current is at a minimum. The SL34118 may be operated from a power supply, or it can be powered from the telephone line, requiring typically 5.0 mA. The SL34118 can be interfaced directly to Tip and Ring (through a coupling transformer) for stand-alone operation, or it can be used in conjunction with a handset speech network and or other features of a featurephone. • • • • • • • • Improved Attenuator Gain Range: 52 dB Between Transmit and Receive Low Voltage Operation for Line-Powered Applications (3.0-6.5 V) 4 Point Signal Sensing for Improved Sensitivity Background Noise Monitors for Both Transmit and Receive Paths Microphone Amplifier Gain Set by External Resistors - Mute Function Included Chip Disable for Active Standby Operation On Board Filter Pinned-Out for User Deined Function Dial Tone Detector to Inhibit Receive Idle Mode During Dial Tone Presence ORDERING INFORMATION SL34118N Plastic SL34118D SOIC TA = -25° to 70° C for all packages PIN ASSIGNMENT SLS System Logic Semiconductor SL34118 SIMPLIFIED BLOCK DIAGRAM FUNCTIONAL DESCRIPTION INTRODUCTION The fundamental difference between the operation of a speakerphone and a handset is that of halfduplex versus full-duplex. The handset is full duplex since conversation can occur in both directions (transmit and receive) simultaneousiy. A speakerphone has higher gain levels in both paths, and attempting to converse full duplex results in oscillatory problems due to the loop that exists within the system. The loop is formed by the receive and transmit paths, the hybrid, and the acoustic coupling (speaker to microphone). The only practical and economical solution used to data is to design the speakerphone to function in a half duplex mode - i.e., only one person speaks at a time, while the other listens. To achieve this requires a circuit which can detect who is talking, switch on the appropriate path (transmit or receive), and switch off (attenuate) the other path. In this way, the loop gain is maintained less than unity. When the talkers exchange function, the circuit must quickly detect this, and switch the circuit appropriately.By providing speech level detectors, the circuit operates in a “hand-free” mode, eliminating the need for a “push-to-talk” switch. The handset, by the way, has the same loop as the speakerphone. But since the gains are considerably lower, and since the acoustic coupling from the earpiece to the mouthpiece is almost nonexistent (the receiver is normally held against a person’s ear), oscillations don’t occur. The SL34118 provides the necessary level detectors, attenuators, and switching control for a properly operating speakerphone. The detection sensitivity and timing are externally controllable. Additionally, the SL34118 provides background noise monitors which make the circuit insensitive to room and line noise, hybrid amplifiers for interfacing to Tip and Ring, the microphone amplifier, and other associated functions. ATTENUATORS The transmit and receive attenuators are complementary in function, i.e., when one is at maximum gain (+6.0 dB), the other is at maximum attenuation (-46 dB), and vice versa. They are never both fully on or both fully off. The sum of their gains remains constant (within a nominal error band of ±0.1 dB) at a typical value of -40 dB. Their purpose is to control the transmit and receive paths to provide the half-duplex operation required in a speakerphone. The attenuators are non-inverting, and have a 3.0 dB (from max gain) frequency of ≈100 KHz. The input impedance of each attenuator (TXI and RXI) is nominally 10 kΩ (see Figure 1), and the input signal should be limited to 350 mVrms (990 mVp-p) to prevent distortion. That maximum SLS System Logic Semiconductor SL34118 recommended input signal is independent of the volume control setting. The diode clamp on the inputs limits the input swing, and therefore the maximum negative output swing. This is the reason for V RXOL a nd V TXOL specification being defined as they are in the Electrical Characteristics. The output impedance is