How to detect signal level without creating a "fakesink" audio-pipeline?

Osmo Antero osmoma at
Sat Aug 13 04:05:56 PDT 2011


I would like improve my audio-recorder software. It uses GStreamer for 
the recording task.

The recorder has a "timer" that can start recording if it detects sound 
input on the line. The timer is simply a textual command that is 
interpreted by the program.

These are examples of the "timer" actions:

start if voice 5s
# pause if silence 8 seconds
# start at 14:50 pm
# stop if silence

To my question:
Let's take the first command (start if voice 5s) as an example.

The timer process notices the "start if voice" command and creates a 
audio-pipeline that listens to the selected device. It will start 
recording if detects a signal/audio/voice that lasts at least 5 seconds 

The timer calls the listener module that creates this pipeline:

$ gst-launch-0.10 pulsesrc 
device="alsa_output.pci-0000_06_01.0.analog-stereo.monitor" name=source0 
! level name=level ! queue ! audioconvert ! audio/x-raw-int,channels=1 ! 
fakesink name=output-sink

Notice that this records to a "fakesink" sink and its output is 
discarded. The listener does only care about the signal level.

This pipeline sets up a callback function on "message::element" signal 
(in src/gst-listener.c module)

g_signal_connect(bus, "message::element", 
G_CALLBACK(listener_message_handler), NULL);

That calls (in src/gst-listener.c module)
static gboolean listener_message_handler(GstBus * bus, GstMessage * 
message, gpointer data) {


This is how the timer/listener can start the REAL RECORDING PROCESS if 
it detects some signal/audio/voice on the line.

Please see the src/gst-listener.c (and src/timer.c) modules here:

The annoying thing is that this fakesink recorder (pipeline) is also 
VISIBLE in the GNOME's "Sound Preferences" dialog.

Take a look at this picture:   (the fakesink recorder/listener is 
annoyingly visible in the settings)

Here is another picture:
This picture shows both the "fakesink" listener pipeline that tries to 
detect "silence". The second is the REAL GStreamer-recorder that outputs 
the a disc file.

The question:

Is there any better way to detect the signal level (audio level) in 
GStreamer? I want a method that avoids this visible "fakesink" recorder. 
Can I monitor signal level on any device (eg. audio card, webcam, 
microphone) without pipeline?

I hope you understand my question ;-)

Here are couple of pictures of the recorder

Source code on the Launchpad

Most kindly
  Moma Antero
  Oslo, Norway

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