diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..397b4a7
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+*.log
diff --git a/README.md b/README.md
index 7798477..9178494 100644
--- a/README.md
+++ b/README.md
@@ -129,13 +129,15 @@ dvledtx uses a JSON config file with three sections:
 | **interfaces** | `name` | PCI BDF address of the NIC (e.g. `0000:06:00.0`) |
 | | `sip` | Source IP address |
 | | `dip` | Destination multicast IP address |
-| **video** | `width` | Frame width in pixels |
-| | `height` | Frame height in pixels |
+| **video** | `width` | Source frame width in pixels |
+| | `height` | Source frame height in pixels |
+| | `tx_url` | Path to the source video file |
+| **tx_video** | `scale_width` | (Optional) Output width after scaling |
+| | `scale_height` | (Optional) Output height after scaling |
 | | `fps` | Frames per second (25, 30, 50, 60) |
 | | `fmt` | Pixel format (see [Supported Formats](#supported-formats)) |
-| | `tx_url` | Path to the source video file |
 | **tx_sessions[]** | `udp_port` | UDP port for the session |
-| | `payload_type` | RTP payload type (typically 96) |
+| | `payload_type` | (Optional) RTP payload type — defaults to `96` if not present |
 | | `crop` | Region to transmit: `x`, `y`, `w`, `h` in pixels |
 
 Example (`config/tx_fullhd_single_session.json`):
@@ -146,12 +148,14 @@ Example (`config/tx_fullhd_single_session.json`):
     { "name": "0000:06:00.0", "sip": "192.168.50.29", "dip": "239.168.85.20" }
   ],
   "video": {
-    "width": 1920, "height": 1080, "fps": 30,
-    "fmt": "yuv422p10le",
+    "width": 1920, "height": 1080,
     "tx_url": "bbb_sunflower_1080p_30fps_normal.mp4"
   },
+  "tx_video": {
+    "fps": 30, "fmt": "yuv422p10le"
+  },
   "tx_sessions": [
-    { "udp_port": 20000, "payload_type": 96, "crop": { "x": 0, "y": 0, "w": 1920, "h": 1080 } }
+    { "udp_port": 20000, "crop": { "x": 0, "y": 0, "w": 1920, "h": 1080 } }
   ]
 }
 ```
@@ -236,6 +240,43 @@ When `log_file` is set, log output is written to that file in addition to the co
 
 > **Note:** Maximum supported resolution is 3840x2160. Width must be even for YUV format alignment.
 
+### Video Scaling
+
+dvledtx supports upscaling and downscaling via optional `scale_width` and `scale_height` fields in the video configuration block. When set, the decoded source video is scaled to the specified dimensions before crop regions are applied.
+
+| Feature | Description |
+|---------|-------------|
+| **Upscale** | Scale smaller source (e.g. 1080p) to larger output (e.g. 4K) |
+| **Downscale** | Scale larger source (e.g. 4K) to smaller output (e.g. 1080p) |
+| **Single session** | Full scaled frame transmitted as one stream |
+| **Multi-session** | Crop regions applied to scaled frame for tiled LED walls |
+| **Max output** | 3840x2160 (4K UHD) |
+
+Example — upscale 1080p source to 4K output:
+```json
+{
+  "video": {
+    "width": 1920, "height": 1080,
+    "tx_url": "source_1080p.mp4"
+  },
+  "tx_video": {
+    "scale_width": 3840, "scale_height": 2160,
+    "fps": 30, "fmt": "yuv422p10le"
+  },
+  "tx_sessions": [
+    { "udp_port": 20000, "crop": { "x": 0, "y": 0, "w": 3840, "h": 2160 } }
+  ]
+}
+```
+
+Log output when scaling is active:
+```
+[INFO ] Video: 1920x1080 -> scale 3840x2160 30fps yuv422p10le  tx_url=source_1080p.mp4
+[INFO ]   Session 0: udp_port=20000 pt=96 crop=[0,0 3840x2160]
+```
+
+> **Note:** When `scale_width`/`scale_height` are set, crop bounds are validated against the scaled dimensions, not the source dimensions. Both fields must be specified together, and the scaled dimensions must satisfy the pixel format's chroma-alignment constraints (e.g. `yuv420` requires even width and height; `yuv422*` requires even width).
+
 ## Performance Considerations
 
 ### Optimization Features
diff --git a/config/tx_2k_multi_session.json b/config/tx_2k_multi_session.json
index 2b4c9f3..ebaea16 100644
--- a/config/tx_2k_multi_session.json
+++ b/config/tx_2k_multi_session.json
@@ -2,7 +2,7 @@
   "log_file": "dvledtx.log",
   "interfaces": [
     {
-      "name": "0000:03:00.1",
+      "name": "0000:06:00.0",
       "sip":  "192.168.50.29",
       "dip":  "239.168.85.20"
     }
@@ -10,24 +10,25 @@
   "video": {
     "width":  2560,
     "height": 1440,
+    "tx_url": "bbb_sunflower_1080p_30fps_normal.mp4"
+  },
+  "tx_video": {
+    "scale_width":  2560,
+    "scale_height": 1440,
     "fps":    30,
-    "fmt":    "yuv422p10le",
-    "tx_url": "ball_2k_yuv420p_30fps_5min.mp4"
+    "fmt":    "yuv422p10le"
   },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
       "crop": { "x": 0,    "y": 0, "w": 854, "h": 1440 }
     },
     {
       "udp_port":     20002,
-      "payload_type": 96,
       "crop": { "x": 854,  "y": 0, "w": 854, "h": 1440 }
     },
     {
       "udp_port":     20004,
-      "payload_type": 96,
       "crop": { "x": 1708, "y": 0, "w": 852, "h": 1440 }
     }
   ]
diff --git a/config/tx_2k_single_session.json b/config/tx_2k_single_session.json
index a2a5c86..0be6239 100644
--- a/config/tx_2k_single_session.json
+++ b/config/tx_2k_single_session.json
@@ -10,14 +10,17 @@
   "video": {
     "width":  2560,
     "height": 1440,
-    "fps":    30,
-    "fmt":    "yuv422p10le",
     "tx_url": "ball_2k_yuv420p_30fps_5min.mp4"
   },
+  "tx_video": {
+    "scale_width":  2560,
+    "scale_height": 1440,
+    "fps":    30,
+    "fmt":    "yuv422p10le"
+  },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
       "crop": { "x": 0, "y": 0, "w": 2560, "h": 1440 }
     }
   ]
diff --git a/config/tx_4k_multi_session.json b/config/tx_4k_multi_session.json
index bb83a02..63800f2 100644
--- a/config/tx_4k_multi_session.json
+++ b/config/tx_4k_multi_session.json
@@ -10,25 +10,26 @@
   "video": {
     "width":  3840,
     "height": 2160,
+    "tx_url": "ball_4k_gbrp10le_60fps_5min.mp4"
+  },
+  "tx_video": {
+    "scale_width":  1920,
+    "scale_height": 1080,
     "fps":    30,
-    "fmt":    "yuv422p10le",
-    "tx_url": "ball_4k_yuv420p_30fps_5min.mp4"
+    "fmt":    "yuv422p10le"
   },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
-      "crop": { "x": 0,    "y": 0, "w": 1280, "h": 2160 }
+      "crop": { "x": 0,    "y": 0, "w": 640, "h": 1080 }
     },
     {
       "udp_port":     20002,
-      "payload_type": 96,
-      "crop": { "x": 1280, "y": 0, "w": 1280, "h": 2160 }
+      "crop": { "x": 640, "y": 0, "w": 640, "h": 1080 }
     },
     {
       "udp_port":     20004,
-      "payload_type": 96,
-      "crop": { "x": 2560, "y": 0, "w": 1280, "h": 2160 }
+      "crop": { "x": 1280, "y": 0, "w": 640, "h": 1080 }
     }
   ]
 }
diff --git a/config/tx_4k_single_session.json b/config/tx_4k_single_session.json
index df69353..5fa2bbc 100644
--- a/config/tx_4k_single_session.json
+++ b/config/tx_4k_single_session.json
@@ -10,15 +10,18 @@
   "video": {
     "width":  3840,
     "height": 2160,
+    "tx_url": "ball_4k_gbrp10le_60fps_5min.mp4"
+  },
+  "tx_video": {
+    "scale_width":  900,
+    "scale_height": 600,
     "fps":    30,
-    "fmt":    "yuv422p10le",
-    "tx_url": "ball_4k_yuv420p_30fps_5min.mp4"
+    "fmt":    "yuv422p10le"
   },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
-      "crop": { "x": 0, "y": 0, "w": 3840, "h": 2160 }
+      "crop": { "x": 0, "y": 0, "w": 900, "h": 600 }
     }
   ]
 }
diff --git a/config/tx_fullhd_multi_session.json b/config/tx_fullhd_multi_session.json
index 7c195c4..3e98b9e 100644
--- a/config/tx_fullhd_multi_session.json
+++ b/config/tx_fullhd_multi_session.json
@@ -10,24 +10,25 @@
   "video": {
     "width":  1920,
     "height": 1080,
-    "fps":    30,
-    "fmt":    "yuv422p10le",
     "tx_url": "bbb_sunflower_1080p_30fps_normal.mp4"
   },
+  "tx_video": {
+    "scale_width":  1920,
+    "scale_height": 1080,
+    "fps":    30,
+    "fmt":    "yuv422p10le"
+  },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
       "crop": { "x": 0,    "y": 0, "w": 640, "h": 1080 }
     },
     {
       "udp_port":     20002,
-      "payload_type": 96,
       "crop": { "x": 640,  "y": 0, "w": 640, "h": 1080 }
     },
     {
       "udp_port":     20004,
-      "payload_type": 96,
       "crop": { "x": 1280, "y": 0, "w": 640, "h": 1080 }
     }
   ]
diff --git a/config/tx_fullhd_single_session.json b/config/tx_fullhd_single_session.json
index 82bcdff..2939030 100644
--- a/config/tx_fullhd_single_session.json
+++ b/config/tx_fullhd_single_session.json
@@ -2,7 +2,7 @@
   "log_file": "dvledtx.log",
   "interfaces": [
     {
-      "name": "0000:03:00.1",
+      "name": "0000:06:00.0",
       "sip":  "192.168.50.29",
       "dip":  "239.168.85.20"
     }
@@ -10,15 +10,18 @@
   "video": {
     "width":  3840,
     "height": 2160,
-    "fps":    60,
-    "fmt":    "gbrp10le",
-    "tx_url": "ball_4k_gbrp10le_30fps_5min.mp4"
+    "tx_url": "bbb_sunflower_1080p_30fps_normal.mp4"
+  },
+  "tx_video": {
+    "scale_width":  3840,
+    "scale_height": 2160,
+    "fps":    30,
+    "fmt":    "yuv422p10le"
   },
   "tx_sessions": [
     {
       "udp_port":     20000,
-      "payload_type": 96,
-      "crop": { "x": 0, "y": 0, "w": 3840, "h": 2160 }
+      "crop": { "x": 0, "y": 0, "w": 1920, "h": 1080 }
     }
   ]
 }
diff --git a/include/app_context.h b/include/app_context.h
index 784e27c..f07def7 100644
--- a/include/app_context.h
+++ b/include/app_context.h
@@ -37,6 +37,8 @@ struct dvledtx_context {
   /* Video parameters */
   uint32_t width;
   uint32_t height;
+  uint32_t scale_width;       /* output width after scaling (0 = no scaling) */
+  uint32_t scale_height;      /* output height after scaling (0 = no scaling) */
   int fps;                    /* frames per second: 25, 30, 50, 60 */
   enum AVPixelFormat fmt;     /* e.g. AV_PIX_FMT_YUV422P10LE */
 
diff --git a/include/util/config_reader.h b/include/util/config_reader.h
index 227717a..5209fb3 100644
--- a/include/util/config_reader.h
+++ b/include/util/config_reader.h
@@ -30,6 +30,8 @@ struct dvledtx_config {
   /* video block */
   uint32_t width;
   uint32_t height;
+  uint32_t scale_width;   /* 0 = no scaling (use source width) */
+  uint32_t scale_height;  /* 0 = no scaling (use source height) */
   int      fps;
   char     fmt[32];         /* e.g. "yuv422p10le" */
   char     tx_url[256];
diff --git a/src/core/session_manager.c b/src/core/session_manager.c
index 0c90f86..6246e82 100644
--- a/src/core/session_manager.c
+++ b/src/core/session_manager.c
@@ -51,8 +51,10 @@ static void* st20p_tx_thread_shared(void* arg) {
   struct shared_decode_ctx* dec = ctx->shared_dec;
   int crop_x = ctx->crop_x_offset;
   int crop_y = ctx->crop_y_offset;
-  int crop_w = ctx->crop_width  > 0 ? ctx->crop_width  : (int)ctx->app->width;
-  int crop_h = ctx->crop_height > 0 ? ctx->crop_height : (int)ctx->app->height;
+  int eff_w  = (int)(ctx->app->scale_width  > 0 ? ctx->app->scale_width  : ctx->app->width);
+  int eff_h  = (int)(ctx->app->scale_height > 0 ? ctx->app->scale_height : ctx->app->height);
+  int crop_w = ctx->crop_width  > 0 ? ctx->crop_width  : eff_w;
+  int crop_h = ctx->crop_height > 0 ? ctx->crop_height : eff_h;
 
   LOG_INFO("ST20P TX(%d): shared thread started (crop x=%d y=%d w=%d h=%d)",
            ctx->idx, crop_x, crop_y, crop_w, crop_h);
@@ -169,8 +171,10 @@ static void* st20p_tx_thread(void* arg) {
    * Path B yuv_frame is already crop-sized (strip) → no offset needed. */
   int crop_x = (ctx->use_ffmpeg == true) ? ctx->crop_x_offset : 0;
   int crop_y = (ctx->use_ffmpeg == true) ? ctx->crop_y_offset : 0;
-  int crop_w = ctx->crop_width  > 0 ? ctx->crop_width  : (int)ctx->app->width;
-  int crop_h = ctx->crop_height > 0 ? ctx->crop_height : (int)ctx->app->height;
+  int eff_w  = (int)(ctx->app->scale_width  > 0 ? ctx->app->scale_width  : ctx->app->width);
+  int eff_h  = (int)(ctx->app->scale_height > 0 ? ctx->app->scale_height : ctx->app->height);
+  int crop_w = ctx->crop_width  > 0 ? ctx->crop_width  : eff_w;
+  int crop_h = ctx->crop_height > 0 ? ctx->crop_height : eff_h;
 
   while (ctx->app->exit == false && session_manager_should_exit() == false) {
     const AVFrame* frame = tx_fetch_next_frame(ctx);
@@ -217,13 +221,15 @@ int create_st20p_tx_session(session_manager_t* manager, struct dvledtx_context*
   /* Fallback: divide the frame width evenly across all sessions */
   if (ctx->crop_width == 0 || ctx->crop_height == 0) {
     int total   = app->st20p_sessions > 0 ? app->st20p_sessions : 1;
-    int strip_w = (int)app->width / total;
+    int eff_w   = (int)(app->scale_width  > 0 ? app->scale_width  : app->width);
+    int eff_h   = (int)(app->scale_height > 0 ? app->scale_height : app->height);
+    int strip_w = eff_w / total;
     ctx->crop_x_offset = session_idx * strip_w;
     ctx->crop_y_offset = 0;
     ctx->crop_width  = (session_idx == total - 1)
-                       ? ((int)app->width - ctx->crop_x_offset)
+                       ? (eff_w - ctx->crop_x_offset)
                        : strip_w;
-    ctx->crop_height = (int)app->height;
+    ctx->crop_height = eff_h;
   }
   LOG_INFO("ST20P TX session %d: crop rect x=%d y=%d w=%d h=%d",
            session_idx, ctx->crop_x_offset, ctx->crop_y_offset,
diff --git a/src/ffmpeg/ffmpeg_decoder.c b/src/ffmpeg/ffmpeg_decoder.c
index 7cee9dc..5274a5c 100644
--- a/src/ffmpeg/ffmpeg_decoder.c
+++ b/src/ffmpeg/ffmpeg_decoder.c
@@ -321,9 +321,11 @@ static void close_ffmpeg_decoder(
  * ========================================================================= */
 int open_shared_ffmpeg(struct shared_decode_ctx* dec, const char* filename) {
   const struct dvledtx_context* app = dec->app;
+  int target_w = (int)(app->scale_width  > 0 ? app->scale_width  : app->width);
+  int target_h = (int)(app->scale_height > 0 ? app->scale_height : app->height);
   return open_ffmpeg_decoder(
     filename, "Shared decode",
-    app->fmt, (int)app->width, (int)app->height,
+    app->fmt, target_w, target_h,
     &dec->fmt_ctx, &dec->codec_ctx, &dec->sws_ctx,
     &dec->av_frame, &dec->yuv_frame, &dec->av_packet,
     &dec->video_stream_idx);
@@ -341,9 +343,11 @@ void close_shared_ffmpeg(struct shared_decode_ctx* dec) {
 static int open_ffmpeg_source(struct st20p_tx_ctx* ctx, const char* filename) {
   char log_prefix[64];
   snprintf(log_prefix, sizeof(log_prefix), "ST20P TX(%d)", ctx->idx);
+  int target_w = (int)(ctx->app->scale_width  > 0 ? ctx->app->scale_width  : ctx->app->width);
+  int target_h = (int)(ctx->app->scale_height > 0 ? ctx->app->scale_height : ctx->app->height);
   int ret = open_ffmpeg_decoder(
     filename, log_prefix,
-    ctx->app->fmt, (int)ctx->app->width, (int)ctx->app->height,
+    ctx->app->fmt, target_w, target_h,
     &ctx->fmt_ctx, &ctx->codec_ctx, &ctx->sws_ctx,
     &ctx->av_frame, &ctx->yuv_frame, &ctx->av_packet,
     &ctx->video_stream_idx);
diff --git a/src/util/config_reader.c b/src/util/config_reader.c
index c475e07..e9d7b48 100644
--- a/src/util/config_reader.c
+++ b/src/util/config_reader.c
@@ -199,7 +199,8 @@ int peek_config_log_file(const char* config_file, char* out_buf, size_t out_size
  * Expected JSON structure:
  *   {
  *     "interfaces": [ { "name": "...", "sip": "...", "dip": "..." } ],
- *     "video": { "width": N, "height": N, "fps": N, "fmt": "...", "tx_url": "..." },
+ *     "video": { "width": N, "height": N, "tx_url": "..." },
+ *     "tx_video": { "scale_width": N, "scale_height": N, "fps": N, "fmt": "..." },
  *     "log_file": "/path/to/dvledtx.log",  (optional — omit for console-only logging)
  *     "tx_sessions": [
  *       { "udp_port": N, "payload_type": N, "crop": { "x":N, "y":N, "w":N, "h":N } },
@@ -261,11 +262,21 @@ int parse_tx_config(const char* config_file, struct dvledtx_config* config) {
         int v;
         v = extract_json_int(video_obj, video_end, "width");  if (v > 0) config->width  = v;
         v = extract_json_int(video_obj, video_end, "height"); if (v > 0) config->height = v;
-        v = extract_json_int(video_obj, video_end, "fps");    if (v > 0) config->fps    = v;
-        extract_json_string(video_obj, video_end, "fmt",    config->fmt,    sizeof(config->fmt));
         extract_json_string(video_obj, video_end, "tx_url", config->tx_url, sizeof(config->tx_url));
     }
 
+    /* --- tx_video block (transmission parameters) --- */
+    const char* tx_video_obj = find_object(json, buf_end, "tx_video");
+    if (tx_video_obj != NULL) {
+        const char* tx_video_end = find_object_end(tx_video_obj, buf_end);
+        if (tx_video_end == NULL) tx_video_end = buf_end;
+        int v;
+        v = extract_json_int(tx_video_obj, tx_video_end, "scale_width");  if (v > 0) config->scale_width  = v;
+        v = extract_json_int(tx_video_obj, tx_video_end, "scale_height"); if (v > 0) config->scale_height = v;
+        v = extract_json_int(tx_video_obj, tx_video_end, "fps");    if (v > 0) config->fps    = v;
+        extract_json_string(tx_video_obj, tx_video_end, "fmt",    config->fmt,    sizeof(config->fmt));
+    }
+
     /* --- optional top-level log_file --- */
     extract_json_string(json, buf_end, "log_file", config->log_file, sizeof(config->log_file));
 
@@ -309,10 +320,7 @@ int parse_tx_config(const char* config_file, struct dvledtx_config* config) {
         if (v > 0 && v <= 255) {
             s->payload_type = (uint8_t)v;
         } else {
-            LOG_ERROR("session %d: payload_type not set or invalid (%d)",
-                   config->session_count, v);
-            free(json);
-            return -1;
+            s->payload_type = 96; /* default to 96 (first dynamic RTP payload type) */
         }
 
         /* crop sub-object */
@@ -427,6 +435,44 @@ int validate_tx_config(const struct dvledtx_config* config) {
         return -1;
     }
 
+    /* Scale dimensions validation (optional — 0 means no scaling) */
+    if (config->scale_width != 0 || config->scale_height != 0) {
+        if (config->scale_width == 0 || config->scale_height == 0) {
+            LOG_ERROR("scale_width and scale_height must both be set or both omitted");
+            return -1;
+        }
+        if (config->scale_width > 3840 || config->scale_height > 2160) {
+            LOG_ERROR("scale resolution %ux%u exceeds maximum 3840x2160",
+                   (unsigned)config->scale_width, (unsigned)config->scale_height);
+            return -1;
+        }
+        /* Validate scaled dimensions against pixel format chroma alignment */
+        {
+            const char* fmt_lookup = config->fmt[0] ? config->fmt : "yuv422p10le";
+            if (strcmp(fmt_lookup, "yuv420") == 0) fmt_lookup = "yuv420p";
+            enum AVPixelFormat pix_fmt = av_get_pix_fmt(fmt_lookup);
+            const AVPixFmtDescriptor* desc =
+                (pix_fmt != AV_PIX_FMT_NONE) ? av_pix_fmt_desc_get(pix_fmt) : NULL;
+            int x_align = desc ? (1 << desc->log2_chroma_w) : 2;
+            int y_align = desc ? (1 << desc->log2_chroma_h) : 1;
+
+            if (x_align > 1 && config->scale_width % x_align != 0) {
+                LOG_ERROR("scale_width %u must be a multiple of %d for pixel format '%s'",
+                       (unsigned)config->scale_width, x_align, config->fmt);
+                return -1;
+            }
+            if (y_align > 1 && config->scale_height % y_align != 0) {
+                LOG_ERROR("scale_height %u must be a multiple of %d for pixel format '%s'",
+                       (unsigned)config->scale_height, y_align, config->fmt);
+                return -1;
+            }
+        }
+    }
+
+    /* Effective output dimensions (after scaling) for crop validation */
+    uint32_t eff_width  = config->scale_width  > 0 ? config->scale_width  : config->width;
+    uint32_t eff_height = config->scale_height > 0 ? config->scale_height : config->height;
+
     /* FPS validation */
     if (config->fps != 25 && config->fps != 30 &&
         config->fps != 50 && config->fps != 60) {
@@ -494,16 +540,16 @@ int validate_tx_config(const struct dvledtx_config* config) {
                    s->crop_w, s->crop_h);
             return -1;
         }
-        if ((uint32_t)s->crop_x + (uint32_t)s->crop_w > config->width) {
-            LOG_ERROR("session %d: crop x=%d + w=%d = %u exceeds video width %u",
+        if ((uint32_t)s->crop_x + (uint32_t)s->crop_w > eff_width) {
+            LOG_ERROR("session %d: crop x=%d + w=%d = %u exceeds effective width %u",
                    i, s->crop_x, s->crop_w,
-                   (uint32_t)s->crop_x + (uint32_t)s->crop_w, config->width);
+                   (uint32_t)s->crop_x + (uint32_t)s->crop_w, eff_width);
             return -1;
         }
-        if ((uint32_t)s->crop_y + (uint32_t)s->crop_h > config->height) {
-            LOG_ERROR("session %d: crop y=%d + h=%d = %u exceeds video height %u",
+        if ((uint32_t)s->crop_y + (uint32_t)s->crop_h > eff_height) {
+            LOG_ERROR("session %d: crop y=%d + h=%d = %u exceeds effective height %u",
                    i, s->crop_y, s->crop_h,
-                   (uint32_t)s->crop_y + (uint32_t)s->crop_h, config->height);
+                   (uint32_t)s->crop_y + (uint32_t)s->crop_h, eff_height);
             return -1;
         }
         if (s->crop_w % 2 != 0) {
@@ -516,8 +562,9 @@ int validate_tx_config(const struct dvledtx_config* config) {
          * Use the pixel format descriptor (if available) to determine
          * the required alignment from log2_chroma_w/h. */
         {
-            enum AVPixelFormat pix_fmt = av_get_pix_fmt(
-                config->fmt[0] ? config->fmt : "yuv422p10le");
+            const char* fmt_lookup = config->fmt[0] ? config->fmt : "yuv422p10le";
+            if (strcmp(fmt_lookup, "yuv420") == 0) fmt_lookup = "yuv420p";
+            enum AVPixelFormat pix_fmt = av_get_pix_fmt(fmt_lookup);
             const AVPixFmtDescriptor* desc =
                 (pix_fmt != AV_PIX_FMT_NONE) ? av_pix_fmt_desc_get(pix_fmt) : NULL;
             int x_align = desc ? (1 << desc->log2_chroma_w) : 2;
@@ -589,6 +636,8 @@ int load_and_apply_config(struct dvledtx_context* app, const char* config_file)
     /* Video */
     app->width  = config.width;
     app->height = config.height;
+    app->scale_width  = config.scale_width;
+    app->scale_height = config.scale_height;
     app->fps    = config.fps;
 
     if (strcmp(config.fmt, "yuv422p10le") == 0)       app->fmt = AV_PIX_FMT_YUV422P10LE;
@@ -636,9 +685,16 @@ int load_and_apply_config(struct dvledtx_context* app, const char* config_file)
            config_file, config.interface_name,
            config.interface_sip[0] ? config.interface_sip : "dhcp",
            config.interface_dip);
-    LOG_INFO("Video: %dx%d %dfps %s  tx_url=%s",
-           config.width, config.height, config.fps, config.fmt,
-           config.tx_url[0] ? config.tx_url : "<none>");
+    if (config.scale_width > 0 && config.scale_height > 0)
+        LOG_INFO("Video: %ux%u -> scale %ux%u %dfps %s  tx_url=%s",
+               config.width, config.height,
+               config.scale_width, config.scale_height,
+               config.fps, config.fmt,
+               config.tx_url[0] ? config.tx_url : "<none>");
+    else
+        LOG_INFO("Video: %ux%u %dfps %s  tx_url=%s",
+               config.width, config.height, config.fps, config.fmt,
+               config.tx_url[0] ? config.tx_url : "<none>");
     for (int i = 0; i < config.session_count; i++)
         LOG_INFO("  Session %d: udp_port=%u pt=%u crop=[%d,%d %dx%d]", i,
                config.sessions[i].udp_port, config.sessions[i].payload_type,
diff --git a/tests/test_config_reader.c b/tests/test_config_reader.c
index 241536b..ca9648a 100644
--- a/tests/test_config_reader.c
+++ b/tests/test_config_reader.c
@@ -560,7 +560,7 @@ static void test_parse_session_udp_port_exceeds_65535_fails(void **state)
     assert_int_equal(ret, -1);
 }
 
-static void test_parse_session_missing_payload_type_fails(void **state)
+static void test_parse_session_missing_payload_type_defaults_to_96(void **state)
 {
     (void)state;
     char *path = write_tmpfile(
@@ -574,7 +574,8 @@ static void test_parse_session_missing_payload_type_fails(void **state)
     struct dvledtx_config cfg;
     int ret = parse_tx_config(path, &cfg);
     unlink(path); free(path);
-    assert_int_equal(ret, -1);
+    assert_int_equal(ret, 0);
+    assert_int_equal(cfg.sessions[0].payload_type, 96);
 }
 
 static void test_parse_session_no_crop_object_fails(void **state)
@@ -640,6 +641,130 @@ static void test_validate_4k_height_exceeds_max_fails(void **state)
     assert_int_equal(validate_tx_config(&cfg), -1);
 }
 
+/* ==========================================================================
+ * validate_tx_config — scaling tests
+ * ========================================================================== */
+
+static void test_validate_scale_upscale_1080p_to_4k_passes(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.width  = 1920;
+    cfg.height = 1080;
+    cfg.scale_width  = 3840;
+    cfg.scale_height = 2160;
+    cfg.sessions[0].crop_w = 3840;
+    cfg.sessions[0].crop_h = 2160;
+    assert_int_equal(validate_tx_config(&cfg), 0);
+}
+
+static void test_validate_scale_downscale_4k_to_1080p_passes(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.width  = 3840;
+    cfg.height = 2160;
+    cfg.scale_width  = 1920;
+    cfg.scale_height = 1080;
+    cfg.sessions[0].crop_w = 1920;
+    cfg.sessions[0].crop_h = 1080;
+    assert_int_equal(validate_tx_config(&cfg), 0);
+}
+
+static void test_validate_scale_width_only_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.scale_width  = 3840;
+    cfg.scale_height = 0;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_height_only_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.scale_width  = 0;
+    cfg.scale_height = 2160;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_exceeds_max_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.scale_width  = 4096;
+    cfg.scale_height = 2160;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_odd_width_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.scale_width  = 1921;
+    cfg.scale_height = 1080;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_odd_height_yuv420_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    strncpy(cfg.fmt, "yuv420", sizeof(cfg.fmt) - 1);
+    cfg.scale_width  = 1920;
+    cfg.scale_height = 1081; /* yuv420 requires even height */
+    cfg.sessions[0].crop_w = 1920;
+    cfg.sessions[0].crop_h = 1080;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_crop_exceeds_scaled_dims_fails(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.width  = 3840;
+    cfg.height = 2160;
+    cfg.scale_width  = 1920;
+    cfg.scale_height = 1080;
+    /* crop is 3840x2160 but effective dims are 1920x1080 */
+    cfg.sessions[0].crop_w = 3840;
+    cfg.sessions[0].crop_h = 2160;
+    assert_int_equal(validate_tx_config(&cfg), -1);
+}
+
+static void test_validate_scale_crop_within_scaled_dims_passes(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.width  = 3840;
+    cfg.height = 2160;
+    cfg.scale_width  = 2560;
+    cfg.scale_height = 1440;
+    cfg.sessions[0].crop_w = 2560;
+    cfg.sessions[0].crop_h = 1440;
+    assert_int_equal(validate_tx_config(&cfg), 0);
+}
+
+static void test_validate_no_scale_passes(void **state)
+{
+    (void)state;
+    struct dvledtx_config cfg;
+    fill_valid_config(&cfg);
+    cfg.scale_width  = 0;
+    cfg.scale_height = 0;
+    assert_int_equal(validate_tx_config(&cfg), 0);
+}
+
 static void test_validate_duplicate_udp_ports_fails(void **state)
 {
     (void)state;
@@ -1027,7 +1152,8 @@ static void test_load_and_apply_config_nonexistent_file_returns_minus1(void **st
     "{" \
     "  \"interfaces\": [{\"name\":\"0000:06:00.0\"," \
     "    \"sip\":\"192.168.50.29\",\"dip\":\"239.168.85.20\"}]," \
-    "  \"video\": {\"width\":1920,\"height\":1080,\"fps\":30,\"fmt\":\"" fmt_str "\"}," \
+    "  \"video\": {\"width\":1920,\"height\":1080}," \
+    "  \"tx_video\": {\"fps\":30,\"fmt\":\"" fmt_str "\"}," \
     "  \"tx_sessions\": [{\"udp_port\":20000,\"payload_type\":96," \
     "    \"crop\":{\"x\":0,\"y\":0,\"w\":1920,\"h\":1080}}]" \
     "}"
@@ -1110,7 +1236,8 @@ static void test_load_and_apply_config_copies_log_file(void **state)
         "{"
         "  \"log_file\": \"myapp.log\","
         "  \"interfaces\": [{\"name\":\"0000:06:00.0\",\"sip\":\"192.168.50.29\",\"dip\":\"239.168.85.20\"}],"
-        "  \"video\": {\"width\":1920,\"height\":1080,\"fps\":25,\"fmt\":\"yuv422p10le\"},"
+        "  \"video\": {\"width\":1920,\"height\":1080},"
+        "  \"tx_video\": {\"fps\":25,\"fmt\":\"yuv422p10le\"},"
         "  \"tx_sessions\": [{\"udp_port\":20000,\"payload_type\":96,"
         "    \"crop\":{\"x\":0,\"y\":0,\"w\":1920,\"h\":1080}}]"
         "}");
@@ -1146,7 +1273,7 @@ int main(void)
         cmocka_unit_test(test_parse_returns_zero_fields_when_video_missing),
         cmocka_unit_test(test_parse_session_missing_udp_port_fails),
         cmocka_unit_test(test_parse_session_udp_port_exceeds_65535_fails),
-        cmocka_unit_test(test_parse_session_missing_payload_type_fails),
+        cmocka_unit_test(test_parse_session_missing_payload_type_defaults_to_96),
         cmocka_unit_test(test_parse_session_no_crop_object_fails),
         cmocka_unit_test(test_parse_session_negative_crop_x_fails),
         cmocka_unit_test(test_parse_session_zero_crop_w_fails),
@@ -1178,6 +1305,16 @@ int main(void)
         cmocka_unit_test(test_validate_2k_resolution_passes),
         cmocka_unit_test(test_validate_4k_resolution_passes),
         cmocka_unit_test(test_validate_4k_height_exceeds_max_fails),
+        cmocka_unit_test(test_validate_scale_upscale_1080p_to_4k_passes),
+        cmocka_unit_test(test_validate_scale_downscale_4k_to_1080p_passes),
+        cmocka_unit_test(test_validate_scale_width_only_fails),
+        cmocka_unit_test(test_validate_scale_height_only_fails),
+        cmocka_unit_test(test_validate_scale_exceeds_max_fails),
+        cmocka_unit_test(test_validate_scale_odd_width_fails),
+        cmocka_unit_test(test_validate_scale_odd_height_yuv420_fails),
+        cmocka_unit_test(test_validate_scale_crop_exceeds_scaled_dims_fails),
+        cmocka_unit_test(test_validate_scale_crop_within_scaled_dims_passes),
+        cmocka_unit_test(test_validate_no_scale_passes),
         cmocka_unit_test(test_validate_duplicate_udp_ports_fails),
         cmocka_unit_test(test_validate_crop_x_misaligned_for_yuv422_fails),
         cmocka_unit_test(test_validate_tx_url_nonexistent_file_fails),