JACK is an audio server is optimized for the requirements of works on creation of music. There are a few basic aspects of its activities. Let us briefly analyze them.
Settings: JACK manages your audio and MIDI settings. This allows you to select audio interface and all the necessary audio parameters such as sampling frequency (samples), buffer size and periods.
Performance: the application of the JACK allows it to achieve very low latency (delay) when working with audio and MIDI. This means that when you record a musical instrument on the computer you can control the sound through your speakers or headphones without noticeable misalignment.
Connection and collaboration: This is the strong point JACK. All inputs and outputs of your audio interface and/or JACK-compatible programs can be switched arbitrarily. JACK cope with the connections not only between programs but also within programs. JACK-compatible programs run through JACK connections of their inputs and outputs. The beauty here is that all these links available for any other JACK-compatible program. They are not limited to those that are only internal.
Qjackctl is a Very powerful and popular Manager JACK. Qjackctl allows you to get access to a large number of settings includes JACK and connection management, transmission control and even Manager JACK session. He appears in a small window, allowing you to work only for accessing settings. And you can minimize it to the tray, so it won't mess beneath your feet, when you install it.
sudo apt-get install jackd qjackctl
Suppose we want to record multi-channel composition with vocals and synthesizer as instruments. We have a MIDI keyboard, software (virtual) synthesizer on the computer and a microphone connected to the sound card input. For some reason we want to record both tracks simultaneously. Then, with the help of JACK, we can connect:
1. MIDI keyboard to a virtual synthesizer;
2. The audio output from the synth to the sound card (to hear that playing);
3.The audio output from the synthesizer to the appropriate track sound recorder;
4. The sound input of the sound card (there are connected the microphone) to the appropriate track sound recorder.
Accordingly, when we click on record, these two tracks will be recorded simultaneously.
You will have to delve into the program, but after that you won't be limited in anything.
Below is an example config that works for me.
Check out the video below. Video shot a few years ago, but to understand how Jack works and its primary setting is enough.