Encoder and Decoder Control
Analogue Input and Output Switching
The Control Register, Byte 0 – bits 0 to 5, are used, in conjunction with the codec Powersave Bit (Byte 1 – bit 3) to control codec
input/output conditions and sample rates. Figure 3 shows the codec functional situation.
AUDIO IN
MOD
DEMOD
AUDIO OUT
CVSD Codec
INPUT
BIAS
200k
Ω
(nom)
V
BIAS
AUDIO
BYPASS
V
BIAS
500k
Ω
(nom)
OUTPUT
BIAS
Fig.3 Analogue Control – with reference to Fig.1
Control Register
Codec
Decoder
Powersave
Control
Bit
“5”
“4”
0
0
0
0
0
0
0
0
1
0
0
1
–
–
–
0
1
1
1
0
0
1
0
0
1
0
1
1
0
1
–
–
–
1
1
1
Encoder
Control
“2”
“1”
0
0
0
0
0
0
0
0
1
0
0
1
–
–
–
0
1
1
1
0
0
–
–
–
1
1
1
Circuit Switches
Audio
By-Pass
OFF
ON
OFF
OFF
–
OFF
OFF
ON
OFF
OFF
–
OFF
“3”
0
1
0
1
–
1
0
1
0
1
–
1
Audio
Out
ON
OFF
OFF
ON
–
ON
OFF
OFF
OFF
OFF
–
OFF
Output
Bias
OFF
OFF
OFF
OFF
–
OFF
ON
OFF
ON
ON
–
ON
OFF
ON
=
=
Switch Open
Switch Closed
Note
Decoder ‘idling’ fed with “1010101
....” pattern at 32kb/s.
Decoder running at the selected
sampling rate.
1
1
Decoder circuits Powersaved
“0”
0
1
0
1
–
1
0
–
1
Input
Bias
ON
OFF
OFF
OFF
–
OFF
ON
2
Encoder running at 32kb/s but
Encoder Data Output forced to ‘idle’
pattern “01010 ...”
Encoder running at selected
Sampling Rate
–
ON
Encoder circuits Powersaved
Table 5 Analogue Control – with reference to Fig.3
Notes
1.
If the Delta Codec is in the Direct Access mode, these
sampling rates will be as provided by the externally
applied clock.
2.
The Input Bias switch is operated by the Control Register
Codec Powersave’ and ‘Encoder Control’ bits to provide a
relatively low impedance path for V
BIAS
to charge the input
coupling capacitor whenever the codec is powersaved, or the
Encoder control bits are set to “0,” so that input bias can be
established quickly prior to operation.
Time Compression of Speech
The 25kb/s and 50kb/s sampling rate options are provided
for time compression (and subsequent expansion) of speech
signals.
For example, 1.0 second of speech stored at 50kb/s may
be transmitted in 0.8 seconds if played out at 64kb/s, and
finally restored to its original speed at the receiver by storing
9
at 64kb/s and playing out at 50kb/s. A similar result (with a
degraded SINAD) may be achieved by using 25kb/s and
32kb/s sampling rates.
However, the speech frequencies are raised by time
compression, and since the signal transmitted to air must be
band limited to 3400Hz, the effective end-to-end bandwidth
is 0.8 x 3400Hz, which is approximately 2700Hz.
CMLMICRO [ CML MICROCIRCUITS ]
