GNU/Linux


Apply low pass and high pass frequency filter using ffmpeg

Just a quick example that applies both a high pass and a low pass frequency filter to a video file using ffmpeg

ffmpeg -i input.mp4 -filter:a "highpass=f=200, lowpass=f=3000" output.mp4;

The frequency is set in Hz so in this example we are filtering out all frequencies that are out of [200, 3000].

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Ubuntu 18.04 and VOSK Speech Recognition API

Just some quick notes on how to install and use VOSK on Ubuntu 18.04 LTS.

  1. Install using: pip3 install vosk
  2. Get samples from https://github.com/alphacep/vosk-api/tree/master/python/example
  3. Using ffmpeg create input audio files ffmpeg -i video.mkv -c:a pcm_s16le -ac 1 output.wav
    It will create a file of the following type output.wav: RIFF (little-endian) data, WAVE audio, Microsoft PCM, 16 bit, mono 48000 Hz
  4. Export text to JSON python3 ./test_simple.py output.wav


Host or domain name not found. Name service error for name=smtp.gmail.com type=AAAA: Host not found, try again

Recently, a postfix mail server running on Ubuntu gave us the following error while trying to send an email to Bob:

Jun 16 17:02:03 gateway postfix/smtp[23522]: ED3799A0D27: to=[email protected], relay=none, delay=0.02, delays=0.01/0.01/0/0, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=smtp.gmail.com type=AAAA: Host not found, try again)

After several attempts like changing the protocol (inet_protocols) to IPv4 (since AAAA is an IPv6 A-record) in /etc/postfix/main.cf and so on we could not get it running.. We restored the configuration file to its original and then we restarted the service:

sudo service postfix restart;

It worked!…. no idea why..


youtube-dl ERROR: unable to download video data: HTTP Error 403: Forbidden

Recently, we were getting the following error in youtube-dl :

ERROR: unable to download video data: HTTP Error 403: Forbidden

We weren’t sure what that error meant (in the case of youtube-dl as the video is public) so we updated pip and youtube-dl to be sure that our problem was not that. The commands we used were the following:

python -m pip install --upgrade pip;
pip install --upgrade youtube-dl;

After the updates, we still got the same error… After some google-fu we read that it could be a caching issue! We cleared the cache as follows:

youtube-dl --rm-cache-dir;

Guess what? After that, the problem was resolved and we were able to reuse youtube-dl.


Monkey Problems

Recently, we tried to user monkeyrunner on an Ubuntu 18.04 LTS. We installed Android Studio through snap and we setup the Android SDK in ~/Andoid. monkeyrunner was installed in ~/Android/Sdk/tools/bin. To our immeasurable disappointment we found out that when we tried to execute monkeyrunner, it would give the following error:

$ ./monkeyrunner
-Djava.ext.dirs=/home/tux/Android/Sdk/tools/lib:/home/tux/Android/Sdk/tools/lib/x86_64 is not supported. Use -classpath instead.
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

At first, we had no idea what that meant, so we used file command on monkeyrunner and we found out that monkeyrunner is a bash script.

$ file monkeyrunner
monkeyrunner: POSIX shell script, ASCII text executable

After reading the code, we read the following at the last lines:

#need to use "java.ext.dirs" because "-jar" causes classpath to be ignored
#might need more memory, e.g. -Xmx128M
exec java -Xmx128M $os_opts $java_debug -Djava.ext.dirs="$frameworkdir:$swtpath" -Djava.library.path="$libdir" -Dcom.android.monkeyrunner.bindir="$progdir" -jar "$jarpath" "[email protected]"

First thing we did was to find the java version that was used, since the command that was giving the problem was that. We found out that we had version 11.

$ java -version
openjdk version "11.0.6" 2020-01-14
OpenJDK Runtime Environment (build 11.0.6+10-post-Ubuntu-1ubuntu118.04.1)
OpenJDK 64-Bit Server VM (build 11.0.6+10-post-Ubuntu-1ubuntu118.04.1, mixed mode, sharing)

After reading a bit, we found out that after Java version 8 the command line directive -Djava.ext.dirs it was deprecated and the recommendation to use -classpath was added. In the past, java.ext.dirs was used to instruct the JRE from where to load additional class and jar files. Since we had Java version 11 we had to try the recommendation to remove the -Djava.ext.dirs directive and use -classpath instead. So, we edited the file monkeyrunner and changed the last line as follows:

exec java -Xmx128M $os_opts $java_debug -classpath "$frameworkdir/*:$swtpath" -Djava.library.path="$libdir" -Dcom.android.monkeyrunner.bindir="$progdir" -jar "$jarpath" "[email protected]"

We tried executing the newly updated monkeyrunner again, only to hit another wall!

$ ./monkeyrunner
Exception in thread "main" java.lang.NoClassDefFoundError: com/android/chimpchat/ChimpChat
at com.android.monkeyrunner.MonkeyRunnerStarter.(MonkeyRunnerStarter.java:60)
at com.android.monkeyrunner.MonkeyRunnerStarter.main(MonkeyRunnerStarter.java:188)
Caused by: java.lang.ClassNotFoundException: com.android.chimpchat.ChimpChat
at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:521)
… 2 more

It turns out that when you use the -jar directive, JRE will ignore the -classpath directive and so it will again not be able to load any external class or jar files…

Solution

Instead of reinventing the wheel (We tried, we failed, it was painful. Still worth the shot!) we decided to install Java version 8 on Ubuntu 18.04 LTS side by side with Java version 11 and just used that.

First, we checked the list of installed jvm on our machine using the following command:

update-java-alternatives --list;

Where we got the following:

$ update-java-alternatives --list
java-1.11.0-openjdk-amd64 1111 /usr/lib/jvm/java-1.11.0-openjdk-amd64
java-1.8.0-openjdk-amd64 1081 /usr/lib/jvm/java-1.8.0-openjdk-amd64

It turned out we already had Java version 8 but if we didn’t we would install it as follows:

sudo apt install openjdk-8-jdk;

and then it would again appear in the list mentioned above.

Then, we switched to the Java version 8 using the following command and selecting the appropriate option:

sudo update-alternatives --config java;

$ sudo update-alternatives --config java
[sudo] password for tux:
There are 2 choices for the alternative java (providing /usr/bin/java).
Selection Path Priority Status
0 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 auto mode
1 /usr/lib/jvm/java-11-openjdk-amd64/bin/java 1111 manual mode
2 /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java 1081 manual mode
Press to keep the current choice[*], or type selection number: 2
update-alternatives: using /usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java to provide /usr/bin/java (java) in manual mode

After switching Java to Version 8 monkeyrunner was working as expected!!