How can I make sure to have “HD Voice”?

To make sure your link is in “HD Voice” mode, there are three conditions to fulfill:

• The two units involved must support the service.
• The two units must run via the same network operator, in the same country (so far the interoperability is not ensured beyond that).
• In almost every country, the two devices must access the 3G/3G+ network to get the service. Because of this, knowing the current network used, 2G or 3G, is most important to be sure the link can be wide band.

The two first conditions may not be convenient, but they are predictable and thus can be dealt with. In contrast, the last one is uncertain for field operation, because the “nomad” device may meet variable network access conditions.

First, if the network coverage is poor, the link may set up in 2G/GSM for the lack of a suitable 3G signal, and then stay in narrow band mode.

What can also happen is that during the link, because of a decrease of the 3G signal, the codec folds back on the 2G network, and so doing loses the HD Voice. That is however a good side of the HD Voice service: even if the 3G network is lost, at least the link stays active. And this handover is so efficient that the switch is hardly noticeable.
Conversely, once the link runs over the 2G access, regardless that was from the start of after folding back, there is no return to 3G, even if the conditions become favorable again. The only way to get 3G back is then to hang up and relaunch the call. Of course, that is hardly acceptable in the middle of a live transmission.

To deal at best with this rather sensitive 2G fold back issue, there are two possible policies; the best one depends on your specific requirements.

1. The standard automatic foldback mode is the safest.
   Advantage: this system provides the maximum likelihood to keep the link on even while moving and possibly running through areas with a poor coverage.
   Down side: there is some risk to switch to narrow band in the middle of a transmission, because of a momentary signal fading, and then stay this way until the line is released. One should know that the automatic foldback is rather “conservative”, i.e. it switches rather early when the 3G signal decreases. In fact, the operation in 3G can be maintained well beyond this critical level where the foldback takes place.
   Conclusion: to be preferred if your priority is to preserve the link on all circumstances. But this mode is quite suitable too if you can be sure the radio level will stay undisturbed all along the transmission.
2. To avoid an undesired foldback, you can force the device on 3G, instead of the default automatic mode. Refer to the product documentation to find the suitable instructions; these depend on the device.
   Advantage: in this way you can ensure the maximum quality, without risking a switch to narrow band during the transmission.
   Down side: if the 3G signal decreases really too much, if it is eventually lost the link is directly dropped.
   Conclusion: suitable when audio quality is the top priority.  The risk of droppping the line can be actually very low, e.g. if the reception level is stable. Even if having to move during the transmission, in many cases you can perform a “reconnaissance” in order to check that the path does not go through “dark” areas.