Functional Description
TRANSMIT SECTION
(Continued)
(due to encoding delay) which totals 290
ms
Any offset
voltage due to the filters or comparator is cancelled by sign
bit integration
RECEIVE SECTION
The receive section consists of an expanding DAC which
drives a fifth order switched-capacitor low pass filter
clocked at 256 kHz The decoder is A-law (TP3057) or
m-law
(TP3054) and the 5th order low pass filter corrects for
the sin x x attenuation due to the 8 kHz sample hold The
filter is then followed by a 2nd order RC active post-filter
power amplifer capable of driving a 600X load to a level of
7 2 dBm The receive section is unity-gain Upon the occur-
rence of FS
R
the data at the D
R
input is clocked in on the
falling edge of the next eight BCLK
R
(BCLK
X
) periods At
the end of the decoder time slot the decoding cycle begins
and 10
ms
later the decoder DAC output is updated The
total decoder delay is
E
10
ms
(decoder update) plus
110
ms
(filter delay) plus 62 5
ms
( frame) which gives
approximately 180
ms
The transmit section input is an operational amplifier with
provision for gain adjustment using two external resistors
see
Figure 4
The low noise and wide bandwidth allow gains
in excess of 20 dB across the audio passband to be real-
ized The op amp drives a unity-gain filter consisting of RC
active pre-filter followed by an eighth order switched-ca-
pacitor bandpass filter clocked at 256 kHz The output of
this filter directly drives the encoder sample-and-hold circuit
The A D is of companding type according to
m-law
(TP3054) or A-law (TP3057) coding conventions A preci-
sion voltage reference is trimmed in manufacturing to pro-
vide an input overload (t
MAX
) of nominally 2 5V peak (see
table of Transmission Characteristics) The FS
X
frame sync
pulse controls the sampling of the filter output and then the
successive-approximation encoding cycle begins The 8-bit
code is then loaded into a buffer and shifted out through D
X
at the next FS
X
pulse The total encoding delay will be ap-
proximately 165
ms
(due to the transmit filter) plus 125
ms
4
NSC [ NATIONAL SEMICONDUCTOR ]
