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different channel rates: 5.25, 6.0, 8.4, 10.5 and 21.0 kbits/s.
The underlying motivation is to allow the evolution of the
system to lower channel rates as voice processing technology
improves. Two of these channel rates, 5.25 and 10.5 kbitsls, do
not include coding. Potential applications of these uncoded
channels include transmitting 4.8 and 9.6 kbitds vocoded
speech in satellite systems that have limited spectrum but are
not power limited. Alternative applications could include the
use of vocoders that include their own coding, protecting the
important bits more than the others; this could eliminate the
need for coding in the channel modem.
365 U~YW
No. 75
Annex 10 - AeronauhauhcTael lecommunicntionr VoCume If1
4.5.2 C channel fmme. The frame duration at all channel
rates is 500 milliseconds. Carrier activation (burst mode) based
on speech activity is used in the to-aircraft direction. At each
activation, a preamble and unique word are transmitted to
commence the burst, and from then on further unique words
wcur every 500 milliseconds. Thus the phasing of the unique
word depends on the instant when activation commences. A
postamble is sent when there is no voice content in the
intedeaver block.
4.5.3 Voice activation. In the to-aircraft direction, carrier
activation is controlled by an electrical signal at the C channel
interface; When the C channel is used for voice, this signal is
controlled by the voice encoder. The voice encoder turns the
signal on as soon as it detects voice, but applies a hangover
time before turning the signal off to avoid excessive turnaffs
between syllables. In addition, the forward carrier is activated
by sub-band channel signalling as required.
4.5.3.1 The channel unit starts a new bunt immediately
after the "active" state is signalled. When the 'hot active" state
is signalled, the channel unit continues transmission for a
period and then drops the canier, In the case of channels with
FEC and an interleaver, the channel unit completes
transmission of the current interleaver block, plus another
complete interleaver block, which ends with a postamble to
terminate the C channel carrier burst. In the case of channels
with no FEC or interleaver, the channel unit continues
transmission for 20 milliseconds. For both cases, during this
period the required bits are taken fmm the primary channel
interface as usual.
4.53.2 The same timing rules apply to the corresponding
control signal from the sub-band signalling channel
equipment.
4.5.3.3 - In the from-aircraft direction, the canier is
transmitted continuously during the call regardless of speech
activity. The start of the uansmission is the same as for the
forward dimtion, with a preamble and unique word
transmitted at the beginning, followed by further unique words
at 500 millisecond intervals.
4.5.4 Data activation. The capability to use the C channel
for data other than vmoded speech is not required for safety
applications. However, the C channel may be used for the
transmission of data by non-safety services and some details of
its operation are included here. In the to-aircraft direction the
forward cattier is operated in h t mode-for circuit-mode data
transfer. Circuit 109 (CF) as defined in CCI'IT
Recommendation V.24 (termed DCD in EIA RS-232C), from
the GES voice band modem controls the activation interface of
the C channel. The interface is initially activated when the call
enters data mode, which is indicated by the circuit 109
transition to ON state. The interface normally remains
activated until deactivation of data mode by the AES. If the
circuit 109 changes to the OFF state indicating loss of received
carrier fern the far-end modem, the transmit circuit-mode
interface unit finishes sending the contents of the
plesiochronous buffer md then changes to "not active". The C
channel activation interface is reactivated again when (io the
circuit 109 changes to the ON state.
4.5.5 Interleaver size. In any data transmission system
where interleaving is required to randomize the errors, the
interleaver size is always a trade-off between the delay
incurred and its effectiveness at randomizing the errors. The
interleaver blocksize for the 21 kbits/s C channel is 384 bits,
which corresponds to 192 information bits. This 192 bits is the
frame size (20 milliseconds) of most 9.6 kbitds vocoders. This
direct mapping between vocoder frames and interleaver buffers
minimizes the delay caused by interleaving and its effect on
voice communications. The resulting over-all transmission
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