Send dead carrier before and after

It would be better to send silence, but in this situation
we don't have control of the signal amplitude, just its
frequency deviation.

Also added a function to dump the frame header, and
made a minor formatting change in the output of an
access token.

apps/opv-mod.cpp

@@ -250,15 +250,11 @@ void output_frame(std::array<uint8_t, 2> sync_word, const bitstream_t& frame)
 }
 
 
-// create and ouput a preamble frame to cout in the desired format
-void send_preamble()
+// create and output a frame with a constant byte value (preamble or dead carrier)
+void send_constant_frame(const uint8_t value)
 {
-    // Preamble is simple... bytes -> symbols -> baseband.
-
-    if (config->verbose) std::cerr << "Sending preamble: " << stream_type4_size + 16 << " bits." << std::endl;
-
     std::array<uint8_t, (stream_type4_size + 16)/8> preamble_bytes;
-    preamble_bytes.fill(0x77);  // +3, -3, +3, -3 == 01 11 01 11 == 0x77
+    preamble_bytes.fill(value);
     if (config->bitstream)
     {
         for (auto c : preamble_bytes) std::cout << c;
@@ -269,8 +265,30 @@ void send_preamble()
         auto preamble_baseband = symbols_to_baseband(preamble_symbols);
         for (auto b : preamble_baseband) std::cout << uint8_t(b & 0xFF) << uint8_t(b >> 8);
     }
+
 }
 
+
+// create and output a preamble frame to cout
+void send_preamble()
+{
+    if (config->verbose) std::cerr << "Sending preamble: " << stream_type4_size + 16 << " bits." << std::endl;
+
+    send_constant_frame(0x77);  // +3, -3, +3, -3 == 01 11 01 11 == 0x77
+}
+
+
+// create and output a frame of dead carrier to cout
+// (We'd like to send silence instead, but can't do that when we're outputting
+// frequency modulation values and not magnitudes.)
+void send_dead_carrier()
+{
+    if (config->verbose) std::cerr << "Sending dead carrier: " << stream_type4_size + 16 << " bits." << std::endl;
+
+    send_constant_frame(0);     // +1, +1, +1, +1 = 00 00 00 00 == 0x00
+}
+
+
 constexpr std::array<uint8_t, 2> STREAM_SYNC_WORD = {0xFF, 0x5D};
 constexpr std::array<uint8_t, 2> EOT_SYNC = { 0x55, 0x5D };
 
@@ -585,7 +603,7 @@ int main(int argc, char* argv[])
     invert = config->invert;
 
     OPVFrameHeader::token_t access_token;
-    std::cerr << "Access token: " << std::hex << std::setw(6) << config->token << std::dec << std::endl;
+    std::cerr << "Access token: 0x" << std::hex << std::setw(6) << config->token << std::dec << std::endl;
     access_token[0] = (config->token & 0xff0000) >> 16;
     access_token[1] = (config->token & 0x00ff00) >> 8;
     access_token[2] = (config->token & 0x0000ff);
@@ -593,8 +611,25 @@ int main(int argc, char* argv[])
     auto fh = fill_fheader(config->source_address, access_token, config->bert != 0);
     auto encoded_fh = encode_fheader(fh);
 
+    //!!! debug
+    dump_fheader(fh);
+    std::cerr << "Encoded: "
+            << std::hex     // output numbers in hex
+            << std::setfill('0');   // fill with 0s
+
+    for (auto hbyte: encoded_fh)
+    {
+        std::cerr << std::setw(2) << int(hbyte) << " ";
+    }
+
+    std::cerr << std::endl << std::dec;
+
+
+    
     signal(SIGINT, &signal_handler);
 
+    send_dead_carrier();    // in simulation, this coincides with the "initialization" period of the demod
+    send_dead_carrier();    // in simulation, this provides some space before the preamble starts
     send_preamble();
 
     if (!config->bert) {
@@ -652,6 +687,8 @@ int main(int argc, char* argv[])
         }
 
         std::cerr << "Output " << frame_count << " frames of BERT data." << std::endl;
+
+        send_dead_carrier();    // simulate loss of signal
     }
 
     return EXIT_SUCCESS;