transcriberbin: expose mix-matrix property on audio sink pads

As the application expects to have the bin buffer the audio stream
internally and output it again unchanged, and transcribers might
expect a set number of channels, we need to expose a property to
let the user control how to downmix the audio stream teed through
the transcriber.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1969>

docs/plugins/gst_plugins_cache.json

@@ -8799,6 +8799,17 @@
                         "type": "GstElement",
                         "writable": true
                     },
+                    "transcription-mix-matrix": {
+                        "blurb": "Initial transformation matrix for the transcriber audioconvert",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "mutable": "ready",
+                        "readable": true,
+                        "type": "GstValueArray",
+                        "writable": true
+                    },
                     "translation-languages": {
                         "blurb": "A map of language codes to caption channels, e.g. translation-languages=\"languages, transcript={CC1, 708_1}, fr={708_2, CC3}\" will map the French translation to CC1/service 1 and the original transcript to CC3/service 2",
                         "conditionally-available": false,

video/closedcaption/src/transcriberbin/imp.rs

@@ -2509,6 +2509,27 @@ impl ObjectImpl for TranscriberSinkPad {
                     .blurb("A map of language codes to bin descriptions, e.g. synthesis-languages=\"languages, fr=awspolly\" will use the awspolly element to synthesize speech from French translations")
                     .mutable_playing()
                     .build(),
+                gst::ParamSpecArray::builder("transcription-mix-matrix")
+                    .nick("Transcription mix matrix")
+                    .blurb("Initial transformation matrix for the transcriber audioconvert")
+                    .element_spec(
+                        &gst::ParamSpecArray::builder("rows")
+                            .nick("Rows")
+                            .blurb("A row in the matrix")
+                            .element_spec(
+                                &glib::ParamSpecFloat::builder("columns")
+                                .nick("Columns")
+                                .blurb("A column in the matrix")
+                                .minimum(-1.)
+                                .maximum(1.)
+                                .default_value(0.)
+                                .build()
+                            )
+                            .build(),
+                    )
+                    .mutable_ready()
+                    .build()
+
         ]
         });
 
@@ -2664,6 +2685,15 @@ impl ObjectImpl for TranscriberSinkPad {
                     }
                 }
             }
+            "transcription-mix-matrix" => {
+                let mut ps = self.state.lock().unwrap();
+                let Ok(pad_state) = ps.as_mut() else {
+                    return;
+                };
+                pad_state
+                    .transcriber_aconv
+                    .set_property("mix-matrix", value);
+            }
             _ => unimplemented!(),
         }
     }
@@ -2697,6 +2727,16 @@ impl ObjectImpl for TranscriberSinkPad {
                     Err(_) => None::<gst::Element>.to_value(),
                 }
             }
+            "transcription-mix-matrix" => {
+                let ps = self.state.lock().unwrap();
+                match ps.as_ref() {
+                    Ok(ps) => ps.transcriber_aconv.property_value("mix-matrix"),
+                    Err(_) => {
+                        let v: Vec<gst::Array> = vec![];
+                        gst::Array::new(&v).to_value()
+                    }
+                }
+            }
             _ => unimplemented!(),
         }
     }