Extract Center Channel From Stereo

[Bran Bast]

Eitherselect audio in the Center, Left, Right, or Surroundchannel, or select Custom and specify the precise phase degree,pan percentage, and delay time for audio you want to extract orremove. (The Surround option extracts audio that is perfectly outof phase between the left and right channels.)

In general, higher numbers work better for extracting the centerchannel, while lower values work better for removing the centerchannel. Lower values allow more bleed through and may not effectivelyseparate vocals from a mix, but they may be more effective at capturingall the center material. In general, a range from 2 to 7 works well.

The Stereo Imagery effect positions and expands the stereo image. Because the Stereo Expander is VST-based, however, you can combine it with other effects in the Mastering Rack and Effects Rack. In Multitrack View, you can also vary the effect over time by using automation lanes.

The Center Extract module preserves (using Keep Center) or removes (using Keep Sides) the centerchannel of a stereo file. Extracting the center will retain the center of a stereo field and attenuateeverything on the sides, such as signals panned to the left or right. See the Use Cases sectionbelow for more contextual examples and additional information about Center Extract processing.

I have some 3 part harmony acapella music in stereo files. I want to isolate the three voices (not perfectly, just enough to hear clearly the different lines for the purpose of learning the songs or transcribing the different parts).

Using centre channel extract (CCE from here on) I'm able to get useable separation, it appears the main voice was mixed to centre, with the two others panned left and right respectively. So far so good. I can have three tracks, with different pan settings in the CCE on each, and solo the three different voices while playing back the multitrack.

In all cases, the effect is being applied, as you can hear that it isn't the original mix, and you can hear artefacts of the CCE effect. When I go back to the effect and check the settings (or in the case of clip effect, undo the apply button and check the reinstated effect), I find that the pan setting which I had adjusted to the left or right to get the left or right channels to the fore, has reverted to the centre.

It seems that all of the above procedures result in the pan being reset to the centre. Which means the effect is useless unless you're using it to extract the centre. Obviously, it is called the CENTRE Channel Extractor for a reason, but surely there wouldn't be a pan control if its setting wasn't intended to be reflected in the output.

No, it has nothing to do with the output at all - the pan control controls where the extracted signal comes from in the original. I've just tried using the CCE in multitrack, and mixed down the file, and got exactly what I expected - the stereo spread remains the same, but the extracted signal is where it was in the mix previously, but louder.

In your scenario, with the mix as you describe, you'd probably be better off doing this in Waveform view, and yes you'll have to do the extraction three times, just to reduce the level of the centre voice for the side extractions. To play safe, do this on a copy of the file!. So you do one pass with the CCE pan control set to left, one with it set to centre, and one with it set to right. Each time you do this, you do either a 'Save As' or a 'Copy to New File' and save the result there. With the original file still open, you should be able to undo the extraction you just made to get the original back, and do the next one. When you've finished, you'll have the three files you need.

See screenshot. Pan control is set to left approx 50%. When I play back, i hear what I wanted to hear ( as you said, the voice is still where it was in the mix, but louder relatively to what it was. See audio meter. Left is consistently louder than right). At this point I am hearing the desired effect.

When I then hit Apply in the effects rack (or export or pre-render in multitrack), immediately I find that the left is no longer consistently louder than the right and indeed I hear the centre channel more prominently than left or right. See meter in below screenshot.

In short, it seems like my pan setting in CCE is reset to centre before the file is processed, regardless of whether I apply it as a clip effect, or place it in a track effect and pre-render or export.

In short, it seems like my pan setting in CCE is reset to centre before the file is processed, regardless of whether I apply it as a clip effect, or place it in a track effect and pre-render or export.

I don't get that - I get the effect applied correctly, but I've noted that it disappears from the rack if you apply it from there, and certainly when you undo it, it comes back into the rack with the pan control centered. You only get it back correctly if you save it as a rack preset, and select it from the list. Now I'd say that it was arguable as to whether this is 'correct' or not - I really don't know. What I do know though is that I'm going to refer it to the devs for comment...

If I select my rack effect preset (which also contains the CCE effect preset), I get my CCE in the rack slot as I expected, but when I open it, the pan is centred, and the preset dropdown has reverted to "Custom".

On examination of the Effect Presets XML file, I can pick out these and see that they are indeed saved, with all params identical except for one, the second float tag. I'm extrapolating that the one that differs is the pan property, though I have no way to confirm.

I made a rack preset called "CCE Left Extract" which had an instance of "Extract Left" in the first slot. The paramlist for "Extract Left" is reproduced verbatim in that preset under the tag "Effect Settings". So its baked right into the rack preset.

I want to make a good stereo sound from the front three channels (Front, Center, Right) without the back channels - I'm going to play those through another amp. My thoughts are copying some/all of the center channel onto the left/right channels of the stereo output?

Semi-recent versions of VirtualDub have an audio filter known as "center cut." This filter attempts to isolate the central components of the incoming signal and separate them from the side signals. The result is a stereo output with the ambience, and a mono output with the foreground sounds and vocals. To test this filter in VirtualDub, enable advanced audio filtering, then add input/center cut/output/discard filters to the filter graph, in that order (or swap discard and output).

Disclaimer: I'm not an audio researcher and am not familiar with the audio literature, so excuse me if I use the wrong terms or fail to acknowledge past work, as I am not familiar with existing advanced algorithms for vocal removal. I came up with this algorithm one day after a discussion about vector projection in lower-division math class, so I didn't do any research before devising the algorithm.

"Center cut" is a separation algorithm that works in frequency domain; it analyzes the phase of audio components of the same frequency on the left and right channels and attempts to determine the approximate center channel. The center channel is then subtracted from the original input to produce the side channels. Thus, one immediate limitation that should be apparent is that center cut requires stereo input. However, unlike the traditional method of vocal separation, taking the difference of left and right channels, the center cut algorithm is able to both produce stereo ambience output and extract the center channel.

Obviously, it isn't necessary to use a Fast Hartley Transform to do this; a Fast Fourier Transform (FFT) would do equally well as the results of a FHT and a real FFT can be easily exchanged. Also, the vector normalizations and the quadratic solve must be guarded against degenerate cases. As one or both source vectors shrink, the problem becomes increasingly ill-conditioned and the derived phase of the center vector becomes erratic; fortunately in this case the magnitude of the center vector also shrinks and the phase stability matters less. Finally, the components at DC and Nyquist rate only have a real component, so they are simply set to zero for the center channel.

The center cut algorithm whacks the phase of the left and right channels, so it has a tendency to move them apart in time and cause echoing effects in the side channels. This phenomenon becomes worse as the FHT window is increased, which is unfortunate as increasing the window size improves the quality of separation.

Overall imbalances in volume between the incoming left and right channels result in center leakage into the louder channel. It may be possible to add an adaptive normalizer into the algorithm to fix this.

This blog was originally open for comments when this entry was first posted, but was later closed and then removed due to spam and after a migration away from the original blog software. Unfortunately, it would have been a lot of work to reformat the comments to republish them. The author thanks everyone who posted comments and added to the discussion.

I have currently my Apple TV connected to an AV receiver with outputs to the Family room speakers. This works fine. I would like to get sound from the Apple TV to another room which has Sonos speakers through a audio matrix switch.

To feed the audio to Sonos bridge, I need to extract audio from the HDMI port on Apple TV.. I have seen some products available on amazon that do this. However, I am also told that by doing that, I will likely

As for using an extractor to get stereo (or digi) audio off of an HDMI, there's plenty of devices out there that do it - but many have limited support for 4K. They'll sey they are, but only at 30Hz, or limiting int hat they won't to yyc 444, only yyc 422 or 420, don't do HDR, or HDR only at 30Hz, or lower number of color bits etc etc.

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