TrueSpeech® Co-Processor
PRELIMINARY/CONFIDENTIAL
Version: 1.18
9.7.8
G.723.1 Frame Type Encoding
The two least significant bits of the first word of the TrueSpeech 6.3 and 5.3 frames are defined by the ITU-T
G.723.1 v5.1 (final formal release) specification to indicate the speech data rate as shown:
Bit 1-0
Frame Type
Data Rate
Number of
Significant Data
Words
00
01
10
0
1
2
TrueSpeech 6.3 data frame (12 words)
TrueSpeech 5.3 data frame (10 words)
Silence/Comfort Noise Generation frame (only first two words
contain useful data). All other data words are set to zero
12
10
2
11
3
Repeat last CNG frame (only two least significant bits of first data
word are used). All other data words are set to zero
2 Bits
Compatibility Note:
The Bit 1-0 encoding of the CT8022 is different from that used in older firmware versions of the
CT8020 which implemented pre-release versions of G.723.1, (firmware revisions 0109 and 0112 part
numbers CT8020A11AQC and CT8020A11BQC). The full production release, CT8020D11AQC
(firmware revision 0114) implements the final v5.1 G.723.1 release, as do all versions of the CT8022.
The Host can examine these two bits to distinguish between 6.3 and 5.3 frames (only) to determine
the data rate and the number of valid data words in the frame. This bit encoding applies only to the
6.3 and 5.3 TrueSpeech frames. There is no bit encoding provided for the 8.5, 4.8 and 4.1 rates.
When implementing a real-time digital compressed speech link, the Host and data transport service should be able to
transfer all four sub-types of speech frame end-to-end. The Host may take advantage of the smaller effective frame
size of the silence frames by using the unused portion of the frame for transporting non-speech data. However,
before writing the silence frames back to the decoder, the Host should restore the unused portions of the silence
frames to zero.
The CT8022 implements additional non-standard frame encoding as follows:
A speech frame that consists of all zeros will be interpreted as an instruction to the playback channel to output pure
silence for one frame period. This applies to all TrueSpeech rates, including 8.5, 6.3, 5.3, 4.8 and 4.1.
A speech frame that has the first two data words set to FFFFH, with the remaining words set to all zeros, will be
interpreted as an instruction to activate the G.723.1 frame erasure mechanism. This should be used where the Host
needs to instruct the CT8022 playback channel to replace a dropped or corrupt speech frame by extrapolating speech
data from previous speech frames. This feature is available only with the TrueSpeech 6.3, 5.3, 4.8 and 4.1 data rates.
When reading or writing speech frames to or from the CT8022, the number of physical words transferred for each
individual frame is fixed during each record/playback session. For example, if the record/playback session starts in
6.3 mode, then all frames will contain 12 words.
Frames that use less than this number of words for actual data will have the unused portion of the frame set to zero.
This approach simplifies the coding of low-level driver software such that it does not need to deal with variable-sized
data frames. This is particularly significant when dealing with DMA-type transfer.
66
DSP GROUP, INC., 3120 SCOTT BOULEVARD CT8022A11AQC FW Revision 0118
SANTA CLARA, CA 95054 PH: 408 986 – 4300 FAX: 408 986 – 4490
All specifications are subject to change without prior notice.
ETC [ ETC ]
