TCM320AC36, TCM320AC37

VOICE-BAND AUDIO PROCESSORS (VBAP

™)

SLWS003C – MAY 1992 – REVISED APRIL – 1998

15

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

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

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.

TCM320AC36/37 VBAP

2 k

Ω

17

VMID

18

MICIN

19

MICGS

20

MICBIAS

To Transmit Filters

Microphone Amplifier

VMID Buffer

10 k

Ω

+

–

VMID Reference

For Amplifiers

1

µF

VMID

Generator

Electret

Microphone

–

+

3.3

µF

MICMUTE

6

10 k

Ω

–

+

VDD

470 pF

+

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.