RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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8.2 VOICE DECODER AND AUDIO DECODER
Decoder error correction coding must be enabled for voice decoder playback of messages compressed with error correction
and disabled for messages compressed without error correction (see control bit HDLC in Table 3-1).
The decoder is enabled by setting control bit DCDEN (1A:5). The decoder input data is transferred from the host to the
decoder one byte at a time. Each byte is written to DBUFF (see Table 3-1) in response to a modem generated interrupt (see
status bit B2A in Table 3-1). The maximum time interval between interrupts is three sample periods for the voice decoder
and one sample period for the audio decoder. The voice codec time interval is programmable having a default value of three
sample periods (see parameter 66 in Section 4.2). Since this parameter is used for both the encoder and decoder, the host
must rewrite the desired interrupt time interval between encoder and decoder operations if this value is different from the
default value of three sample periods.
The host writes blocks of decoder input data to the decoder. The variable rate voice decoder has variable length data blocks
from 4 to 38 bytes. The fixed rate voice decoder’s input is a repeated sequence of fixed length data blocks. Without error
correction coding this sequence is 36, 36, 36, 35 bytes ,and with error correction coding this sequence is 38, 38, 37, 38, 38,
37, 38, 37 bytes. The audio decoder’s input is a fixed length data block of 120 bytes at 32 kbps or 90 bytes at 24 kbps.
A data underrun condition will occur when the decoder has completed decoding and playback of its last data block, and the
host has not completed transfer of the next data block. When an underrun occurs the decoder sets status bit VOVUN (17:0)
and suspends message playback. During this suspension the decoder output samples are forced to zero. To intentionally
pause the decoder, the host may choose to halt data transfer to force an underrun condition. Message playback continues
after data transfer resumes and the decoder resets VOVUN.
The voice decoder provides the host with four optional pitch synchronous playback speeds. The playback speed may be
arbitrarily changed at any time during message playback (see control bits FAST33, FAST50, NORM, and SLOW in Table 3-
1). At 33% faster playback speed, the message is played back in 67% of the original recorded message time. At 50% faster
playback speed, the message is played back in 50% of the original recorded message time. At normal playback speed, the
message is played back in the same period of time as the original recorded message. At 50% slower playback speed, the
message is played back in 150% of the original recorded message time.
Decoder output volume control provides 2 dB gain when control bit VOLUP (0E:6) is set and 2 dB loss when control bit
VOLDWN (0E:5) is set. The decoder resets bits VOLUP and VOLDWN after making the appropriate volume adjustment. The
volume control range is 0 dB to -40 dB. The lower limit is programmable (see parameter 67 in section 4-2).
For decoder implementation refer to flowchart shown in Figure 8-6.
Summary of Contents for RFX144V24-S23
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