From 865068f158b6d02e341457706ff8da725279e278 Mon Sep 17 00:00:00 2001
From: Sam Machin <sam@sammachin.com>
Date: Fri, 1 May 2026 13:43:13 +0100
Subject: [PATCH] add new CODEC_TRANSCODE option (#225)

* add new CODEC_TRANSCODE option

allows RTP engine to add additional codecs to the outgoing offer that wern't in the invite from Freeswitch so RTP engine will transcode outbound calls.

* update README for new env var descriptions

---------

Co-authored-by: Dave Horton <daveh@beachdognet.com>
---
 README.md           | 10 ++++++++++
 lib/call-session.js | 10 ++++++++--
 lib/utils.js        |  4 ++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 07ec990..87f9c80 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,19 @@ Configuration is provided via environment variables:
 |JAMBONES_RTPENGINES| commans-separated list of ip:ng-port for rtpengines (e.g. '172.31.32.10:22222')|yes|
 |JAMBONES_TIME_SERIES_HOST| influxdb host |yes|
 |JAMBONES_RECORD_ALL_CALLS| enable auto record calls |no|
+|JAMBONES_CODEC_OFFER_WITH_ORDER| comma-separated codec list to use as the outbound offer toward the carrier; the original codecs from the feature server are stripped first (e.g. `'opus,PCMU,PCMA,telephone-event'`) |no|
+|JAMBONES_CODEC_TRANSCODE| comma-separated codec list that rtpengine should add to the outbound offer and transcode on the fly when the carrier selects them — used to support codecs the feature server does not speak (e.g. `'AMR-WB/16000'`). Requires an rtpengine build that includes the corresponding codec module. |no|
+|JAMBONES_ACCEPT_AND_TRANSCODE| comma-separated codec list to accept on the outbound leg and transcode to PCMU/PCMA toward the feature server |no|
+|JAMBONES_ACCEPT_G729| if set, accept G.729 on the outbound leg and transcode to PCMU/PCMA (shorthand for `JAMBONES_ACCEPT_AND_TRANSCODE=g729`) |no|
 |K8S| service running as kubernetes service |no|
 |K8S_RTPENGINE_SERVICE_NAME| rtpengine service name(required for K8S) |no|
 
+#### Codec transcoding notes
+
+`JAMBONES_CODEC_OFFER_WITH_ORDER` and `JAMBONES_CODEC_TRANSCODE` can be combined. When both are set, rtpengine first strips the original codecs from the SDP, then adds the codecs listed in `JAMBONES_CODEC_OFFER_WITH_ORDER` to the offer, and finally adds the codecs from `JAMBONES_CODEC_TRANSCODE` with transcoding enabled. When only `JAMBONES_CODEC_TRANSCODE` is set, the original codecs from the feature server are kept in the offer and the transcode codecs are appended.
+
+Codecs that require licensed or optional rtpengine modules (AMR, AMR-WB, G.729, etc.) only work if rtpengine was compiled with support for them. Carrier-specific fmtp parameters (for example, `octet-align=1` for AMR-WB) can be appended to the codec name as needed (e.g. `AMR-WB/16000;octet-align=1`).
+
 ### running under pm2
 Typically, this application runs under [pm2](https://pm2.io) using an [ecosystem.config.js](https://pm2.keymetrics.io/docs/usage/application-declaration/) file similar to this:
 ```js
diff --git a/lib/call-session.js b/lib/call-session.js
index e864920..7c2afbf 100644
--- a/lib/call-session.js
+++ b/lib/call-session.js
@@ -892,8 +892,14 @@ Duration=${payload.duration} `
         process.env.JAMBONES_ACCEPT_G729 ? 'g729' : '';
       offerMedia = { ...offerMedia, ...(acceptCodecs && dlg.type === 'uac' &&
         { codec: { mask: acceptCodecs, transcode: 'pcmu,pcma' } })};
-      answerMedia = { ...answerMedia, ...(process.env.JAMBONES_CODEC_OFFER_WITH_ORDER && dlg.type === 'uac' &&
-        { codec: {offer: process.env.JAMBONES_CODEC_OFFER_WITH_ORDER.split(','), strip: 'all' }})};
+      answerMedia = { ...answerMedia, ...((process.env.JAMBONES_CODEC_OFFER_WITH_ORDER ||
+        process.env.JAMBONES_CODEC_TRANSCODE) && dlg.type === 'uac' &&
+        { codec: {
+          ...(process.env.JAMBONES_CODEC_OFFER_WITH_ORDER &&
+            { offer: process.env.JAMBONES_CODEC_OFFER_WITH_ORDER.split(','), strip: 'all' }),
+          ...(process.env.JAMBONES_CODEC_TRANSCODE &&
+            { transcode: process.env.JAMBONES_CODEC_TRANSCODE.split(',') })
+        }})};
       let opts = {
         ...this.rtpEngineOpts.common,
         ...offerMedia,
diff --git a/lib/utils.js b/lib/utils.js
index 8f7947c..15996b8 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -58,6 +58,10 @@ function makeRtpEngineOpts(req, srcIsUsingSrtp, dstIsUsingSrtp, padCrypto, teams
         offer: process.env.JAMBONES_CODEC_OFFER_WITH_ORDER.split(','),
         strip: 'all'
       }),
+    ...(process.env.JAMBONES_CODEC_TRANSCODE &&
+      {
+        transcode: process.env.JAMBONES_CODEC_TRANSCODE.split(',')
+      }),
   };
 
   return {