8.2.4 Periodic muting and insertion of resynchronization sequence
26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS
In order to guarantee that the CTM signal can be reliably transmitted via any kind of speech codec, a periodic muting of the transmitted signal is applied that prevents that the modem signal is classified as non-speech by the speech encoder’s VAD. The functional goal of the process described here is to insert muting intervals of 80 ms with a periodicity of 960 ms. Furthermore, the transmitted bit-stream provides a training sequence that allows a resynchronization after an interrupted transmission, e.g. a cell hand-over.
The periodic muting and the insertion of the resynchronization information is applied to the bit‑stream between the FEC Error Protection and the interleaver. Both functions are implemented by means of two switches, as it is indicated in Figure 4. The control of the two switches depends on the value of the index k, which is the time index of the bit-stream 
. This bit-stream contains the information bits coming from the FEC error protection as well as the bits that are inserted and marked as to be muted.
Figure 4 – Periodic muting and insertion of resynchronisation sequence
The default position of the first switch is such that the bits coming from the FEC error protection are forwarded to the next block. Only for certain indices 
, which are stored in a look up table, the first switch is set to its lower position in order to insert a "mute bit". In this case, the bit that is actually available from the FEC error protection is maintained until the switch is in its default position again. Therefore, the switch acts as an insertion device so that no bit coming from the FEC error protection is discarded.
The indices 
, which indicate at what time instants a "mute bit" has to be inserted, are stored in a look up table. The indices can be calculated as follows:
This results in the following indices:
07, 22, 37, 52, 67, 82, 97, 112,
15, 30, 45, 60, 75, 90, 105, 120,
23, 38, 53, 68, 83, 98, 113, 128,
31, 46, 61, 76, 91, 106, 121, 136.
As can bee seen in Figure 6, these bits are consecutive in the bit‑stream after the interleaver and therefore generate a silent period of 80 ms.
The second switch is also controlled by the actual value of k. In case that k is equal to 
, all 32 bits of the resynchronization sequence 
are inserted. Similar to the first switch, also this switch acts like an insertion device, so that all bits coming from the previous block are maintained. After the insertion of the resynchronization sequence, the index k is reset to zero in order to obtain a periodic muting and resynchronization with a periodicity of 352+32=384 bits. The organization of the resulting bit‑stream that is sent to the interleaver is depicted in Figure 5.
The resynchronization sequence 
consists of 32 elements:
sRESYNC = { 0, 1, 0, 1, 0, 1, 1, 0,
0, 1, 1, 0, 1, 1, 1, 0,
1, 1, 0, 1, 0, 0, 1, 0,
0, 1, 1, 1, 0, 0, 0, 1 .}
Figure 5 – Organization of the bit‑stream bOUT to the interleaver