TCM320AC36 Datasheet, PDF (15/23 Pages) Texas Instruments – VOICE-BAND AUDIO PROCESSORS VBAPE

TCM320AC36, TCM320AC37
VOICE-BAND AUDIO PROCESSORS (VBAP™)
PRINCIPLES OF OPERATION
SLWS003C – MAY 1992 – REVISED APRIL – 1998
conversion laws
The TCM320AC36 provides µ-law companding operation that approximates the CCITT G.711
recommendation. The TCM320AC37 provides A-law companding operation that approximates the CCITT
G.711 recommendation. The linear mode of operation uses a 13-bit two’s-complement format and is the same
for both the TCM320AC36 and the TCM320AC37.
transmit operation
microphone input
The microphone input amplifier is designed specifically to interface to electret-type microphone elements, as
shown in Figure 5. The VMID buffer circuit provides a voltage (MICBIAS) equal to 1/2 V_CC as a reference for
the microphone amplifier and a bias voltage to the electret microphone. The microphone amplifier output
(MICGS) is used in conjunction with a feedback network and applied to the amplifier inverting input (MICIN) to
set the amplifier gain. In the companded mode, when the MICIN signal level decreases to a level near the noise
floor, the VBAP mutes the signal and outputs zero bits while continuing to monitor the signal level. When the
input level once again exceeds the noise threshold, the mute is released and normal operation resumes. Input
hysteresis is provided to ensure noiseless transitions into and out of the muted condition. VMID appears at a
terminal to provide a place to filter the VMID voltage.
1 µF
470 pF
VMID
17
MICBIAS
20
VMID Buffer
+
–
VMID Reference
For Amplifiers ^VDD
VMID
Generator
+
–
2 kΩ
3.3 µF
+
10 kΩ
Electret
Microphone
10 kΩ
MICGS
19
MICIN
18
MICMUTE
6
Microphone Amplifier
–
+
To Transmit Filters
TCM320AC36/37 VBAP
NOTE A: Terminal numbers shown are for the DW and N packages.
Figure 5. Typical Microphone Interface
microphone mute function
The MICMUTE input causes the digital circuitry to transmit all zero code on DOUT.
transmit filter
A low-pass antialiasing section is included on the device and achieves a 35-dB attenuation at the sampling
frequency. No external components are required to provide the necessary antialiasing function for the
switched-capacitor section of the transmit filter.
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