Rabbit & Pork

Sounds easy - well, almost.

I was looking for a set-up that provides a Skype recording of three independent tracks:

• Track 1: My voice (as Skype host) only
• Track 2: Voices of Skype participants (minus host) only
• Track 3: Auxiliary sound effects & pre-recorded material

Independent tracks give fine post-processing control e.g. volume level balancing, application of different audio effects. Also, track 1 & 3 can be recorded directly at source, rather than via Skype, thus improving clarity.

Basically, I wanted Mac software to support the following (as inspired by my colleague’s MS Windows set-up):

In particular, I wanted to feed auxiliary sound effects & pre-recorded material into the Skype conference allowing all participants to hear them “live”.

The solution is built using the neat Mac software packages Audio Hijack Pro and Soundflower (a virtual audio device), but the secret sauce is in “how to configure them”.

Here goes the configuration…

In Audio Hijack Pro:

1. Create a session named ‘Skype Host’
1. Source Type: Audio Source
2. Input Device: Built-in Mic (or mic of your choice)
3. Output Device: Soundflower 2ch
4. Recording: AAC
2. Create a session named ‘Skype Participants (minus host)’
1. Source Type: Application
2. Application: Skype
3. Recording: AAC
4. Effects: Channel Tweaker: Kill Left, Channel Tweaker: Duplicate Right
3. Create a session named ‘Auxiliary’
1. Source Type: Application
2. Application: GarageBand
3. Recording: AAC
4. Effects: Auxiliary Device Output: Soundflower 2ch

The trick appears in 2.4 - When hijacking an Application, both Input & Output are recorded. However, Input and Output are sent to different channels, so we can use a chain of effects to filter out the left channel (Skype input) and duplicate the right channel (Skype output).

AAC recording format is chosen in all sessions to minimise CPU usage (i.e. no compression is required) during the Skype conference.

The virtual Soundflower audio device is used as the sync for the hijacked Mic and Garageband inputs. This allows Audio Hijack Pro to record them, but also pass them thru to Skype. Thus, in Skype, the following preferences are required:

1. Audio Output: Built-in Output (or output of your choice)
2. Audio Input: Soundflower 2ch

With the setup complete, here goes the recording…

1. Start Skype Conference
2. Start GarageBand
3. In Audio Hijack Pro, “Hijack” and start “Record” on the three configured sessions
4. Chat In Skype, play audio & sound effects in GarageBand
5. In Audio Hijack Pro, stop “Record” on the three configured sessions

At this point you’ll have three AAC files which may be imported into Garageband for post-processing and exported to a single audio file for Podcast publishing.