Under The Microscope

Change The Channel

Not long ago, a user named Jonathan emailed us asking how to record audio from the voice chat application Skype, using Audio Hijack Pro. For those elite few who’ve read the manual (found under the Help menu) we detail how to record from Skype and other voice chat programs (such as iChat), enabling users to record any conversations they have. This is often used for things such as interviews or calls to relatives.

This intrepid user, however, was looking to do something a bit more complex. Specifically, he wanted to record each half of the conversation onto a separate channel. If you’re not aware, a stereo audio file contains two channels, a left channel and a right channel. In music, these channels are usually blended or mirrored, but they occasionally contain unique information.

Here, Jonathan was hoping to have the audio from Alan (the local user) on the right channel and audio from Bob (his remote conversation partner) on the left channel. When this recording is listened to with headphones, all of Alan’s audio would be heard in the right ear and Bob’s in the left.But how to make such a recording? It’s a little tricky, but we worked it out and the folowing screenshots show how to do it.

For this setup, the source in the Input tab is set to the Audio Device that Skype is using for input. This must match what’s found for the Audio Input in Skype’s Audio preferences. The output is set to Silence, to avoid echoes. The real magic occurs in the Effects area.

The top row of plugins shown below handles audio from the local microphone, Alan’s audio. In order, these plugins are PassThru, a second PassThru, Monomizer, and finally Channel Tweaker set to “Kill Left”. The PassThru plugins simply prevent this audio stream from being mixed with the second stream prematurely. Monomizer ensures that the audio stream from the input device is in Mono. The Channel Tweaker plugin removes all audio from the left side, keeping it all on the right.

The next step is to get the remote half of the conversation from Skype, Bob’s half, onto the left side of the recording. That’s what the second row of plugins does for us. Here, we have Gain set to -infinity (or, zero, to non-audio geeks) creating an empty audio stream, followed by Application Mixer pulling audio from Skype. Because of the way Skype outputs audio this will just be Bob’s half of the conversation, but you’ll need to be sure to turn on the Monitor checkbox, so you can hear the output. Then we use Monomizer, again to ensure that the audio stream is in Mono and finally Channel Tweaker set to Kill Right.

Click to hear a sample recording

So, our final recording consists of two distinct streams, Alan and Bob. Each stream is reduced to just one channel, and then they’re blended together into a single stream for the final recording. The end result is a file with Alan on the right and Bob on the left, just as desired.

This technique can be extended to almost any sort of recording. To record from two different applications, one on each channel, set up the Silence Input in the Input tab, then use two instances of the Application Mixer, one in each row. To record microphone commentary over a movie, set your movie player as the Application Mixer source. There are lots of weird and great uses for this – try it out and have fun.

Update (January 21st, 2006): A few users have mentioned that the resulting recording has a slight bit of the left channel audio on the right track, and vice versa. This is occurring because Audio Hijack Pro records MP3 in joint stereo. To get pure recordings, record to AIFF. In the future, we may add true Stereo to our recording options.

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