TCM320AC36, TCM320AC37

VOICE-BAND AUDIO PROCESSORS (VBAP

™)

SLWS003C – MAY 1992 – REVISED APRIL – 1998

16

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

encoding

The encoder internally samples the output of the transmit filter and holds each sample on an internal

sample-and-hold capacitor. The encoder performs an A/D conversion on a switched-capacitor array. Digital

data representing the sample is transmitted on the first 8 or 16 data clock cycles of the next frame.

The autozero circuit corrects for dc offset on the input signal to the encoder using the sign-bit averaging

technique. The sign bit from the encoder output is long-term averaged and subtracted from the input to the

encoder.

data word structure

The data word is eight bits long in the companded mode and all eight bits represent one audio data sample.

The sign bit is the first bit transmitted.

The data word is 16 bits long in the linear mode. The first 13 bits comprise the audio data sample, and the last

three bits form the volume control word in the receive direction (DIN) and are zero pad bits in the transmit

direction (DOUT). The sign bit is transmitted first.

receive operation

decoding

In the companded mode, the serial data word is received at DIN on the first eight clock cycles in fixed-data rate

and on the last eight clock cycles in variable-data rate. In the linear mode, the serial data word is received at

DIN on the first 13 clock cycles. D/A conversion is performed, and the corresponding analog sample is held on

an internal sample-and-hold capacitor. This sample is transferred to the receive filter.

receive filter

The receive section of the filter provides pass-band flatness and stop-band rejection that approximates both

the AT&T D3/D4 specification and CCITT recommendation G.712 when operated at the recommended

frequencies. The filter contains the required compensation for the (sin x)/x response of such decoders.

receive buffer

The receive buffer contains the volume control.

earphone amplifier

The earphone audio-output amplifier has a balanced output, as shown in Figure 6, to allow maximum flexibility

in output configuration. The output amplifier is designed to directly drive a piezo earphone in the differential

configuration without any additional external components. The output can also be used to drive a single-ended

The receive-channel output level can be adjusted between specified limits by connecting an external resistor

network to EARGS.