TCM320AC36 Datasheet, PDF (16/23 Pages) Texas Instruments – VOICE-BAND AUDIO PROCESSORS VBAPE

TCM320AC36, TCM320AC37
VOICE-BAND AUDIO PROCESSORS (VBAP™)
SLWS003C – MAY 1992 – REVISED APRIL – 1998
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
load with the output signal voltage centered around V_CC /2.
The receive-channel output level can be adjusted between specified limits by connecting an external resistor
network to EARGS.
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