Added instructions for general audio broadcasting (issue markondej#144)

README.md

@@ -22,7 +22,7 @@ make
 ```
 After a successful build you can start transmitting by executing the "fm_transmitter" program:
 ```
-sudo ./fm_transmitter -f 102.0 acoustic_guitar_duet.wav
+sudo ./fm_transmitter -f 100.6 acoustic_guitar_duet.wav
 ```
 Where:
 * -f frequency - Specifies the frequency in MHz, 100.0 by default if not passed
@@ -33,7 +33,7 @@ Other options:
 * -b bandwidth - Specifies the bandwidth in kHz, 100 by default
 * -r - Loops the playback
 
-After transmission has begun, simply tune an FM receiver to chosen frequency, You should hear the playback.
+After transmission has begun, simply tune an FM receiver to chosen frequency, you should hear the playback.
 ### Raspberry Pi 4
 On Raspberry Pi 4 other built-in hardware probably interfers somehow with this software making transmitting not possible on all standard FM broadcasting frequencies. In this case it is recommended to:
 1. Compile executable with option to use GPIO21 instead of GPIO4 (PIN 40 on GPIO header):
@@ -45,6 +45,22 @@ make GPIO21=1
 echo "performance"| sudo tee /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
 ```
 3. Using lower FM broadcasting frequencies (below 93 MHz) when transmitting.
+### Use as general audio output device
+[hydranix](https://github.com/markondej/fm_transmitter/issues/144) has came up with simple method of using transmitter as an general audio output device. In order to achieve this you should load "snd-aloop" module and stream output from loopback device to transmitter application:
+```
+sudo modprobe snd-aloop
+arecord -D hw:1,1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 - &
+```
+Please keep in mind loopback device should be set default ALSA device (see [this article](https://www.alsa-project.org/wiki/Setting_the_default_device)). Also parameter "-D hw:X,1,0" should be pointing this device (use card number instead of "X").
+### Microphone support
+In order to use a microphone live input use the `arecord` command, eg.:
+```
+arecord -D hw:1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -
+```
+In cases of a performance drop down use ```plughw:1,0``` instead of ```hw:1,0``` like this:
+```
+arecord -D plughw:1,0 -c 1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -
+```
 ### Supported audio formats
 You can transmitt uncompressed WAV (.wav) files directly or read audio data from stdin, eg. using MP3 file:
 ```
@@ -62,15 +78,6 @@ Or you could also use FFMPEG:
 ffmpeg -i example.webm -f wav -bitexact -acodec pcm_s16le -ar 22050 -ac 1 converted-example.wav
 sudo ./fm_transmitter -f 100.6 converted-example.wav
 ```
-### Microphone support
-In order to use a microphone live input use the `arecord` command, eg.:
-```
-arecord -D hw:1,0 -c1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -
-```
-In cases of a performance drop down use ```plughw:1,0``` instead of ```hw:1,0``` like this:
-```
-arecord -D plughw:1,0 -c1 -d 0 -r 22050 -f S16_LE | sudo ./fm_transmitter -f 100.6 -
-```
 ## Legal note
 Please keep in mind that transmitting on certain frequencies without special permissions may be illegal in your country.
 ## New features

fm_transmitter.cpp

@@ -109,9 +109,11 @@ int main(int argc, char** argv)
         std::cout << "Error: " << catched.what() << std::endl;
         result = EXIT_FAILURE;
     }
-    auto temp = transmitter;
-    transmitter = nullptr;
-    delete temp;
+    if (transmitter != nullptr) {
+        auto temp = transmitter;
+        transmitter = nullptr;
+        delete temp;
+    }
 
     return result;
 }

makefile

@@ -1,20 +1,17 @@
 EXECUTABLE = fm_transmitter
-VERSION = 0.9.5
+VERSION = 0.9.6
 FLAGS = -Wall -O3 -std=c++11
 TRANSMITTER = -fno-strict-aliasing -I/opt/vc/include
 ifeq ($(GPIO21), 1)
 	TRANSMITTER += -DGPIO21
 endif
 
-all: fm_transmitter.o mailbox.o sample.o wave_reader.o transmitter.o
-	g++ -L/opt/vc/lib -o $(EXECUTABLE) fm_transmitter.o mailbox.o sample.o wave_reader.o transmitter.o -lm -lpthread -lbcm_host
+all: fm_transmitter.o mailbox.o wave_reader.o transmitter.o
+	g++ -L/opt/vc/lib -o $(EXECUTABLE) fm_transmitter.o mailbox.o wave_reader.o transmitter.o -lm -lpthread -lbcm_host
 
 mailbox.o: mailbox.c mailbox.h
 	g++ $(FLAGS) -c mailbox.c
 
-sample.o: sample.cpp sample.hpp
-	g++ $(FLAGS) -c sample.cpp
-
 wave_reader.o: wave_reader.cpp wave_reader.hpp
 	g++ $(FLAGS) -c wave_reader.cpp