diff --git a/app/app.c b/app/app.c
index 0f4de2d..c4a97fb 100644
--- a/app/app.c
+++ b/app/app.c
@@ -1783,9 +1783,10 @@ void APP_time_slice_10ms(void)
 						RADIO_enableTX(false);
 						BK4819_TransmitTone(true, 500);
 						SYSTEM_DelayMs(2);
+						
 						GPIO_SetBit(&GPIOC->DATA, GPIOC_PIN_SPEAKER);
+						g_speaker_enabled = true;
 
-						g_speaker_enabled     = true;
 						g_alarm_tone_counter_10ms = 0;
 					}
 				}
diff --git a/app/main.c b/app/main.c
index 7bbc2af..104f540 100644
--- a/app/main.c
+++ b/app/main.c
@@ -29,7 +29,9 @@
 #endif
 #include "audio.h"
 #include "board.h"
+#include "bsp/dp32g030/gpio.h"
 #include "driver/bk4819.h"
+#include "driver/gpio.h"
 #include "driver/uart.h"
 #include "dtmf.h"
 #include "frequencies.h"
@@ -959,6 +961,10 @@ void MAIN_Key_UP_DOWN(bool key_pressed, bool key_held, scan_state_dir_t Directio
 				return;
 			}
 
+			// suppress audio click
+			GPIO_ClearBit(&GPIOC->DATA, GPIOC_PIN_SPEAKER);
+			g_speaker_enabled = false;
+
 			Next = RADIO_FindNextChannel(Channel + Direction, Direction, false, 0);
 			if (Next == 0xFF)
 				return;
@@ -992,11 +998,19 @@ void MAIN_Key_UP_DOWN(bool key_pressed, bool key_held, scan_state_dir_t Directio
 		return;
 	}
 
+	GPIO_ClearBit(&GPIOC->DATA, GPIOC_PIN_SPEAKER);
+	g_speaker_enabled = false;
+
 	// jump to the next channel
 	APP_channel_next(false, Direction);
 
-	g_scan_pause_10ms = 0;   // go NOW
-
+	// go NOW
+	g_scan_pause_10ms      = 0;   
+	g_scan_pause_time_mode = false;
+	g_squelch_open         = false;
+	g_rx_reception_mode    = RX_MODE_NONE;
+	FUNCTION_Select(FUNCTION_FOREGROUND);
+	
 	g_ptt_was_released = true;
 }
 
diff --git a/driver/bk4819.c b/driver/bk4819.c
index 10d9b34..fbd46b7 100644
--- a/driver/bk4819.c
+++ b/driver/bk4819.c
@@ -1931,7 +1931,7 @@ void BK4819_start_fsk_rx(const unsigned int packet_size)
 void BK4819_PlayRogerMDC1200(void)
 {
 	uint16_t fsk_reg59;
-	uint8_t  packet[40];
+	uint8_t  packet[42];
 
 	const uint8_t  op  = MDC1200_OP_CODE_POST_ID;
 	const uint8_t  arg = 0x80;
diff --git a/firmware.bin b/firmware.bin
new file mode 100644
index 0000000..fb5fe82
Binary files /dev/null and b/firmware.bin differ
diff --git a/firmware.packed.bin b/firmware.packed.bin
new file mode 100644
index 0000000..0284bcd
Binary files /dev/null and b/firmware.packed.bin differ
diff --git a/mdc1200.c b/mdc1200.c
index 700572d..8264670 100644
--- a/mdc1200.c
+++ b/mdc1200.c
@@ -2,19 +2,16 @@
 #include <string.h>
 
 #include "bsp/dp32g030/crc.h"
+#include "driver/uart.h"
 #include "mdc1200.h"
 #include "misc.h"
 
-// MDC1200 sync bit reversals and packet magic
+// MDC1200 sync bit reversals and packet sync
 //
 // 24-bit pre-amble
 // 40-bit sync
 //
-//static const uint8_t header[] = {0x00, 0x00, 0x05, 0x55, 0x55, 0x55, 0x55, 0x07, 0x09, 0x2a, 0x44, 0x6f};
-//static const uint8_t header[] = {0x00, 0x00, 0x0A, 0xAA, 0xAA, 0xAA, 0xAA, 0x07, 0x09, 0x2a, 0x44, 0x6f};
-//
-//static const uint8_t header[] = {0x00, 0x00, 0x0A, 0xAA, 0xAA, 0xAA, 0xA0, 0xb6, 0x8e, 0x03, 0xbb, 0x14};
-static   const uint8_t header[] = {0x00, 0x00, 0x05, 0x55, 0x55, 0x55, 0x50, 0x29, 0x71, 0xfc, 0x44, 0xeb};
+static const uint8_t header[] = {0x55, 0x55, 0x55, 0x00, 0x00, 0x00, 0x00, 0x0A, 0x07, 0x09, 0x2a, 0x44, 0x6f};
 
 uint8_t bit_reverse_8(uint8_t n)
 {
@@ -263,7 +260,18 @@ uint8_t * encode_data(uint8_t *data)
 			data[FEC_K + i] = bo;
 		}
 	}
-
+/*
+	#if defined(ENABLE_UART) && defined(ENABLE_UART_DEBUG)
+	{
+		const unsigned int size = FEC_K * 2;
+		unsigned int i;
+		UART_printf("mdc1200 tx1 %u ", size);
+		for (i = 0; i < size; i++)
+			UART_printf(" %02X", data[i]);
+		UART_SendText("\r\n");
+	}
+	#endif
+*/
 	{	// interleave the bits
 
 		unsigned int i;
@@ -285,7 +293,7 @@ uint8_t * encode_data(uint8_t *data)
 			}
 		}
 
-		// copy the interleaved bits back to the input/output buffer
+		// copy the interleaved bits back to the data buffer
 		for (i = 0, k = 0; i < (FEC_K * 2); i++)
 		{
 			int bit_num;
@@ -301,22 +309,22 @@ uint8_t * encode_data(uint8_t *data)
 }
 
 void delta_modulation(uint8_t *data, const unsigned int size)
-{	// xor succesive bits in the entire packet, including the bit reversing pre-amble
-	uint8_t b1;
+{	// exclusive-or succesive bits
 	unsigned int i;
-	for (i = 0, b1 = 1u; i < size; i++)
+	uint8_t prev_bit = 0;
+	for (i = 0; i < size; i++)
 	{
 		int bit_num;
 		uint8_t in  = data[i];
 		uint8_t out = 0;
 		for (bit_num = 7; bit_num >= 0; bit_num--)
 		{
-			const uint8_t b2 = (in >> bit_num) & 1u;
-			if (b1 != b2)
-				out |= 1u << bit_num;        // previous bit and new bit are different
-			b1 = b2;
+			const uint8_t new_bit = (in >> bit_num) & 1u;
+			if (new_bit != prev_bit)
+				out |= 1u << bit_num;        // previous bit and new bit are different - send a '1'
+			prev_bit = new_bit;
 		}
-		data[i] = out;
+		data[i] = out ^ 0xff;
 	}
 }
 
@@ -340,18 +348,18 @@ unsigned int MDC1200_encode_single_packet(uint8_t *data, const uint8_t op, const
 
 	p = encode_data(p);
 
-#if 0
-	{	// test packet
-		//
-		// op 0x01, arg 0x80, id 0xB183
-		//
-		const uint8_t test_packet[] = {0x07, 0x25, 0xDD, 0xD5, 0x9F, 0xC5, 0x3D, 0x89, 0x2D, 0xBD, 0x57, 0x35, 0xE7, 0x44};
-		memcpy(data + sizeof(header), test_packet, sizeof(test_packet));
-	}
-#endif
-
 	size = (unsigned int)(p - data);
-
+/*
+	#if defined(ENABLE_UART) && defined(ENABLE_UART_DEBUG)
+	{
+		unsigned int i;
+		UART_printf("mdc1200 tx2 %u ", size);
+		for (i = 0; i < size; i++)
+			UART_printf(" %02X", data[i]);
+		UART_SendText("\r\n");
+	}
+	#endif
+*/
 	delta_modulation(data, size);
 
 	return size;