David,

Most likely your mod_opus is configured to respect the remote's fmtp and not the maxaveragebitrate and maxplaybackrate that you configure. You have bitrate-negotiation set to true.
It looks it's going in WB mode if you are sure it starts sending FEC at over 16 kbit/s . 
Value 14400 was chosen as config default only because is one step above 14000 which is one of the LBRR thresholds (Opus's bitrate works in steps of 400 bits/s) and it also provides a good compromis loss/audio quality for NB. 
https://github.com/xiph/opus/blob/master/silk/define.h#L52

/* LBRR thresholds */
#define LBRR_NB_MIN_RATE_BPS 12000
#define LBRR_MB_MIN_RATE_BPS 14000
#define LBRR_WB_MIN_RATE_BPS 16000


FEC threshold in FS is this:
#define SWITCH_OPUS_MIN_FEC_BITRATE 12400 - one step above codec's LBRR threshold for NB. LBRR means Low Bit Rate Redundancy which means FEC. 

Regards,
Dragos




On Sat, Dec 18, 2021 at 3:06 AM David Villasmil <david.villasmil.work@gmail.com> wrote:
Hello all,

i've been testing FEC and how low we can go with maxavgbiterate

<param name="use-vbr" value="1"/>
<param name="use-dtx" value="1"/>
<param name="complexity" value="10"/>
<param name="packet-loss-percent" value="15"/>
<param name="asymmetric-sample-rates" value="1"/>
<param name="bitrate-negotiation" value="1"/>
<param name="keep-fec-enabled" value="1"/>
<param name="use-jb-lookahead" value="true"/>
<param name="maxaveragebitrate" value="14400"/>
<param name="maxplaybackrate" value="8000"/>
<param name="sprop-maxcapturerate" value="0"/>
<param name="adjust-bitrate" value="1"/>

we only see FEC if we set maxavgbiterate to 18000, we see no FEC with 14400

anyone knows what i'm missing?

Regards,

David Villasmil
phone: +34669448337
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