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As a result of how FR/HR/EFR DTX interacts with block diagonal interleaving, at the beginning and end of each DTX pause a correctly functioning TCH receiver will always pick up an artifact consisting of 4 received bursts (2 for TCH/HS) and same number of omitted bursts. Standard channel decoding of this Rx artifact will produce a "half-block" in which half of the bits prior to convolutional decoding will come from a SID repetition whose Tx was notionally suppressed, while the other half will be garbage. As a result of convolutional decoding, the result will often appear as valid SID per classification rules - but passing it as such to the Rx DTX handler is wrong. Classic E1 BTS and GSM MS implementations include a kind of SID filter at this point, setting BFI on these received half-blocks, so that the Rx DTX handler will see an invalid SID condition. Invalid SID means that comfort noise generation is to be continued, but no updated CN parameters are available - which is the truth in half-block Rx situations. Implement the same filter. Additional background info can be found here: https://osmocom.org/projects/retro-gsm/wiki/DTXu_half-blocks Change-Id: I46c62312316b04567bcadf6050597673f071247d