libspeexenc: Add an option for enabling DTX

Discontinuous transmission is an addition to VAD/VBR operation, that
allows to greatly reduce bitrate for silent chunks or stationary
noises.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/libspeexenc.c

@@ -85,6 +85,7 @@ typedef struct {
     int cbr_quality;            ///< CBR quality 0 to 10
     int abr;                    ///< flag to enable ABR
     int vad;                    ///< flag to enable VAD
+    int dtx;                    ///< flag to enable DTX
     int pkt_frame_count;        ///< frame count for the current packet
     AudioFrameQueue afq;        ///< frame queue
 } LibSpeexEncContext;
@@ -120,6 +121,7 @@ static av_cold void print_enc_params(AVCodecContext *avctx,
     av_log(avctx, AV_LOG_DEBUG, "packet size: %d\n",
            avctx->frame_size * s->frames_per_packet);
     av_log(avctx, AV_LOG_DEBUG, "voice activity detection: %d\n", s->vad);
+    av_log(avctx, AV_LOG_DEBUG, "discontinuous transmission: %d\n", s->dtx);
 }
 
 static av_cold int encode_init(AVCodecContext *avctx)
@@ -196,6 +198,13 @@ static av_cold int encode_init(AVCodecContext *avctx)
     if (s->vad)
         speex_encoder_ctl(s->enc_state, SPEEX_SET_VAD, &s->vad);
 
+    /* Activiting Discontinuous Transmission */
+    if (s->dtx) {
+        speex_encoder_ctl(s->enc_state, SPEEX_SET_DTX, &s->dtx);
+        if (!(s->abr || s->vad || s->header.vbr))
+            av_log(avctx, AV_LOG_WARNING, "DTX is not much of use without ABR, VAD or VBR\n");
+    }
+
     /* set encoding complexity */
     if (avctx->compression_level > FF_COMPRESSION_DEFAULT) {
         complexity = av_clip(avctx->compression_level, 0, 10);
@@ -318,6 +327,7 @@ static const AVOption options[] = {
     { "cbr_quality",       "Set quality value (0 to 10) for CBR",       OFFSET(cbr_quality),       AV_OPT_TYPE_INT, { .i64 = 8 }, 0,  10, AE },
     { "frames_per_packet", "Number of frames to encode in each packet", OFFSET(frames_per_packet), AV_OPT_TYPE_INT, { .i64 = 1 }, 1,   8, AE },
     { "vad",               "Voice Activity Detection",                  OFFSET(vad),               AV_OPT_TYPE_INT, { .i64 = 0 }, 0,   1, AE },
+    { "dtx",               "Discontinuous Transmission",                OFFSET(dtx),               AV_OPT_TYPE_INT, { .i64 = 0 }, 0,   1, AE },
     { NULL },
 };