Recently I’ve been tasked with mixing a few podcasts – and me being me (in other words reluctant to bother using a mouse if I don’t have to… and only possessing 50usd raspberry pi computers) I decided to try using command line utils. Really a raspberry pi 3 is easily up to the task – haven’t tried it on my zero, I’m not that much of a masochist!
Happily, a combination of ffmpeg and ecasound has seemed to do a pretty good job. It was fiddly to set up but as tends to be the case with CLI setups, it holds the potential for mixing future episodes to be very fast indeed to mix.
Recording and File Prep
The two ladies recording the podcast are situated on opposite sides of the Atlantic. Therefore each has a mic and recording device and also a phone. That way the call is conducted over the phone and two pristine recordings are made locally so we get the highest possible quality.
Then I need to mix the two recordings at a later date. One of the recordings actually gets saved as m4a (odn’t ask why:) and at 48khz while the other is a wav at 44.1khz. Seeing as I like to mix at 44.1 and ecasound can only really seek effectively with wavs, I used ffmpeg to convert the m4a to a wav at the correct frequency.
ffmpeg -i input.m4a -ar 44100 output.wav
Synch and Volume
Then I need to synch the two files from the two podcast participants. I’ve used ecasound’s simple cli syntax for this:
ecasound -c \ -a:1 -chcopy:1,2 -ea:70 -i:playat,$1,input_A.wav \ -a:2 -chcopy:1,2 -ea:100 -i:input_B.wav \ -a:1,2 -o alsahw,1,0
Let’s break that down.
- ‘-c’ means to go into interactive mode – more on that in a moment…
- The ‘-a:x’ bits mean an effect chain (ie input followed by a load of cables, followed by output). In this case we’re specifying multiple chains going to the same output so we have to put them in one line (ie the last one). I couldn’t really tell you why, but as long as you only mention any input or output once in the file then it seems happy connecting them to as few or many chains as you like!
- The ‘-chcopy:1,2’ bits are simply there because the input files are mono and I have headphones and two ears! Therefore I want to be able to hear the mono channel through both sides of my headphones.
- ‘-ea:’ is an amplifier. The value is a percentage of the natural amplitude of the file.
- The ‘-i’ part means specifying an input. ‘playat’ means we’ve got an offset for one of the files to get them in synch. It also happens that I’ve made the offset a parameter for the bash script. In this way I can do ‘bash ./synch_podcast.sh 4’ to start input_A.wav after 4 seconds.
- The ‘-o alsahw,1,0’ is to play through alsa to my soundcard.
But wait, what if my guess of 4 seconds turns out to be wrong??!:-)
Well if we run our command we’ll have ecasound’s interactive mode which serves as a control surface and more. Unfortunately, pretty much the only thing it can’t edit easily is that little parameter in ‘playat’ that tells it at what point to play the sample.
Therefore we press ‘t’ to start ecasound playing – then probably a whole lot of ‘fw 30’, ‘rw 10’ etc to send the transport back and forward to understand if our guess of 4 seconds to synchronise was close or not. If so then cool… If not then we simply press ‘q’ and start our script with 5.5 seconds etc…
The other thing you’ll want to do is balance the volumes between the files. This is easy interactive mode.
You should see something like this:
ecasound ('h' for help)> cop-status ### Chain operator status (chainsetup 'untitled-chainsetup') ### Chain "1": 1. Channel copy:  from-channel 1.000,  to-channel 2.000 2. Amplify:  amp-% 70.000 Chain "2": 1. Channel copy:  from-channel 1.000,  to-channel 2.000 2. Amplify:  amp-% 100.000
Volumes are a percentage – so to change that volume from 70 to 90:
c-iselect 1 cop-set 2,1,90
In other words, select chain 1, then set chain-operator 2, parameter 1 to 90.
The beauty of this is you can press the up arrow and change the 90 to 95 etc then press return and execute the command again with a different value.
In this instance you’ll have to note down the two volumes you used as ecasound doesn’t edit the ‘playat’ command interactively unfortunately. When they’re noted down just press ‘q’, edit the two vols in the original script and start up ecasound again (by pressing up arrow a couple of times and return given were in bash…)
Then when you’re done just note down the number you used to synchronise and edit the last line of your script so that the output is ‘output.wav’ instead of alsa.
Everyone’s human and I’ve been asked to cut bits out…
Here I tend to fire up jack so I can have multiple outputs. (something like this: jackstart -d alsa -p 128 -n 2 -r 44100) and set up a screen or tmux session with one terminal showing:
ecasound -c -i podcast.wav -o jack,system,notransport
This way I can whizz back and forth in playing the file and find the places I want. The ‘notransport’ bit means it doesn’t impact the whizzing back and forth I’ll be doing on the editted copy…
Then I have another tab with an sh script open which will look something like this:
ecasound -c \ -a:1 -i:playat,0,select,0,30,podcast.wav -a:2 -i:playat,30,select,35,60,podcast.wav ...etc -a:1,2,<and more...> -o jack,system,notransport
Above I just removed a 5 second gap.
Again, when I’m done I just replace the output for ‘podcast_edited.wav’.
Add the Intro and Outro
The last thing you’ll need to do is stick an intro and outro on it:
ecasound -c \ -a:1 -ea:60 -i intro.wav -a:2 -ea:130 -i playat,30,podcast_edited.wav -a:3 -ea:190 -i playat,5030,outro.wav -a:1,2,3 -o final.wav
And we’re done!