Under the Adaptive Multi Rate (AMR) speech codec standard, the codec mode applied to the voice data exchanged between a mobile communication device and a communication network is dynamically selected by the communication network based on changing radio channel conditions and capacity requirements. The codec modes that may be applied during an active voice call are chosen from an Active Codec Set (ACS), which include codec modes defined by the communication network as part of the initial voice call setup. When the communication network determines based on one or more channel quality indicators that radio channel conditions are poor, the communication network may select a lower bit rate codec mode from the ACS in order to accommodate an increase in redundant data added as a result of channel coding. But if the radio channel conditions improve, the communication network may select a higher bit rate codec mode from the ACS, which enhances the perceived voice quality.
In general, a conventional mobile communication device will apply the codec mode commanded by the communication network. As such, a codec mode command (CMC) from the communication network may trigger an immediate change (e.g., upgrade or downgrade) in the current uplink (UL) codec mode applied by the mobile communication device. In this respect, codec mode adaptation may in fact compromise the performance of the conventional mobile communication device, particularly in the case of a dual-subscriber identity module (SIM), dual active (DSDA) mobile communication device.
In a DSDA mobile communication device, a timeslot collision may occur when both subscriptions simultaneously attempt to perform an activity. To resolve a timeslot collision when both subscriptions attempt to perform an activity (e.g., an UL activity), the transmission of voice data (e.g., speech frames) on one subscription may be inhibited if the voice data transmission has a lower priority transmission than the transmission (e.g., signaling) on the other subscription. Also, the transmission of voice data on one subscription may be inhibited (i.e., blanked) in favor of higher priority receptions on the other subscription. As a result of frequently blanking the transmission of voice data, UL voice data from the DSDA mobile communication device tends to exhibit numerous interruptions. The communication network typically misconstrues these interruptions in the UL voice data from the DSDA mobile communication device to imply fluctuating radio channel conditions. As a result, CMCs from the communication network require the DSDA mobile communication device to change its codec mode at an excessive rate and to unnecessarily downgrade to codec modes with low bit rates, which consequently lowers the Mean Opinion Score (MOS) of the voice call.