i wrote a long reply, but it diodn’t get posted properly 🙁
When you say FR, you mean 12.2 ? As opposed to … lower AMR codecs ?
Basically it makes sense.
12.2 FR can be used up to RXQUAL 4.5
When RxQual is better than 4.5, the FR is better than lower AMR codecs.
When RxQual is worse than 4.5, the FR is suffering from interference, so the speech frames are getting deleted once in a while. That impacts the MOS very negatively, as you can guess. At his point, it is better to switch to a “lower” AMR codec, but still within the AMR FR set.
Pushing the usage of 12.2FR means that you tend to globally decrease the RxQual. If you decide to switch to lower AMR FR codecs earlier than 4.5, then the MOS will be degraded because the speech encoding won’t be as good. However, the Rxqual *should* remain stable (rxqual is not theoretically linked to the codec used, as it is measured at the physical layer – but i’m not sure how this theory would fair in the real world.)
Now, if you are opposing FR with HR, then I can’t understand it. The HR is way more sensitive to interference than FR.