This time, I want to have a look at what is possible using only open source software
and using only Linux.
I don't mean replacing your current OS or DAW, but rather adding to it, with things
like realtime processing, audio manipulation and extra features that are not available
in the more main stream software and plugins.
Many of you will use a DAW like PT or Logic in Mac or Windows.
Nothing wrong with that, but there are things that those apps are not designed to
do at all, or you have to use "kludges" and you have no way of changing the default
behaviour as it's all closed source.
The idea then, is to interface a modest Linux PC to your current DAW/mixer workflow.
Certain tasks can then be offloaded to this PC, but are available in near realtime
on your existing system, or can be accessed via a shared folder on a network.
Linux supports as many sound cards/interfaces as you need, and is limited only
by the resources available on your PC.
How you interface them, is entirely up to you, it can be via XLR/RCA (analog),
Firewire, USB, TCP/IP or Samba shares.
Furthermore, no hardware patch-bay is required, once multiple feeds are coming
in and out of a Linux PC, many software patch-bays are available and can be changed
in realtime.
Below, two scenarios I have created and patches can be saved and recalled at any time.
recompiled by me.
In all cases, I used the same computer, which is an entry level Celeron with a 1.8GHz
For sound cards I'm using the internal one, an extra one on PCI and two on USB-2.
Hence I have in total, 4 stereo outputs (or 8 mono) and 4 stereo inputs plus 4 monos.
mobile phone or CD player.
These are selected by means of a hardware patch panel.
rest via a passive mixer to a pair of studio monitors.
These I use as "dry" monitoring channels.
Since no amplification or EQ is required on the monitoring channels, the passive mixer
is a simple lash-up of log sliders feeding a summing matrix.
the audio before broadcast.
Connected the audio outputs of my Pioneer FM tuner to a pair of inputs and analyzed
the output of two popular stations.
Further explanation as follows:
The top trace, is the audio waveform, this then allows visual inspection in case
there is any clipping.
The bottom waveforms (green is the left audio & purple is the right).
The one just above it (brownish/orange), is the average frequency response (last 3 secs).
Note the characteristic low cut at 15 KHZ.
All FM stations do this, so as to preserve the stereo pilot tone at 19 KHz & the suppressed
carrier at 38 KHz.
Then we also have a readout of the time spent analyzing, the RMS value and the LUFS
plus overall LRA.
The bottom app, is just the patch settings.
As can be seen, to do all this in realtime, placed only a few percent CPU load and a
total latency of around 18 milliseconds.
Not bad at all for a Celeron computer with 2 GB RAM.
Let's then look at a FM talk only station:
Very Interesting results.
Let's analyze two different CD's.
A bit compressed, but expected of most of today's music.
At least it's not limited to 15 KHz like FM broadcasts or many streaming platforms.
The second one though, is just crazy, yes it's more dynamic, but just look at the clipping.
So much for standards.
Of course, the same setup could be used to monitor and analyze the audio from TV broadcasts.
You may have also noticed that on the last three tests, I used yet another app, a level histogram.
this is very useful is showing the "spread" or loudness range.
Keep in mind that is was all done in realtime.
2) Getting perceptional loudness and song EQ's:
Those that make and record music or do final mix on music, will know that each genre
of music, has some "established" EQ's or what they sound like.
Once it's time to final mix your song, you may want to make it sound more like a similar
commercial song.
No problem, simply play the commercial song you want to "emulate" through the apps above,
take some screenshots during the intro, chorus, etc and you will have a much better picture.
Some of you will also know that, humans do not perceive all sounds (pitch) at the same level
or intensity.
There has been many studies done on this, and the LUFS system also takes this into account.
There are algorithms available so we can easily make an app to see what a specific mix
will sound like.
Below is the response of some music, with relation to this.
It's a 3 second average of how it will be perceived, it's not a frequency response in the
traditional sense, but intensity of frequencies as they will be perceived by the average person.
I have also added a LUFS meter, plus another very handy app, an instant recorder.
By clicking on the big green icon, it will record the audio and time stamp it for later reference.
all files that are placed in a special watch folder.
Yet another use, is to create effects for TV or radio shows.
One effect that is quite difficult to achieve, is the realistic simulation of a cellphone or
other digital call that is breaking up.
Normally it involves reducing the sample rate and some EQ.
However, this is not that realistic, to get that mangled sound, one should ideally feed
the audio through a real digital channel and create the break up or decimation there.
Again, open source to the rescue, there are uLAW, GSM and other codecs available
that can be manipulated to adjust the "break-down".
To that, just add some EQ and some compression to replicate those conversations.
The settings I used for GSM "mangling":
I have full control of level, phase and simulated bit error rate.
On top of this, some compression and more "envelope mangling":
The results?
Before and after: