Stereo Mastering

M/S Compressor
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M/S CompressorU786 as M/S Compressor

The U786 bass compressor can be used as M/S compressor as well. The M/S mode switch inserts M/S matrix stages pre and post the compressor. The input matrix is post split filter, which makes possible to combine both functions and use M/S mode also when the module is used as bass compressor.

Two special controls allow variation of the behavior in M/S mode. The WEIGHT control operates quite similar to a balance control. It determines how the to M/S channels M (mid) and S (side) affect the compression. At the center position, both channels are treated identically. With settings all to the left or all to the right only the particular channel causes regulation. This allows to change the characteristics of the compression to be more or less 'center accentuated' or 'side accentuated'. However, regulation of the audio signals in both channels is precisely identical all the time.

The BALANCE control can be used for special effects, like dynamic control of the stereo base width. The default position of this control is the center position. When set all to right, only the S signal is affected by the regulation, while a setting all to the left avoids any regulation of the S channel and causes regulation in the M channel only. Any other setting than 0 results in a modulation of the stereo base width that changes with the gain reduction. Using theses section can produce some nice effects; however, moving a guitar more to left with each kick of the bass drum is not something that can be used in general. Effects like that can are possible as soon as the regulation causes any change of the level relation between M and S.

M/S Compressor Specials
Convential dynamics treatment of stereo signals maintain the channel balance by linking the two channels precisely. Both channels need to have the same gain reduction to avoid any strange effects. The control signal is generated from the stereo signal with the higher momentary values. This control signal is applied to both stereo channels. There is no real difference with M/S compression; however, the two signals that are processed are now M and S instead of left and right. The M and S signals are converted from the left/right signals in a matrix. The M = mid signal is actually the mono signal from the addition of both stereo channels. The S = side signal contains all spatial components of the stereo signal. Another matrix can reconvert the M/S format back to left and right. Any processing of the signal in the M/S format will of course changes of the reconverted left/right signal as well. The advantage of the M/S format is that some regulations that are very hard to achieve are very simple in this format (and vice versa).

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If the levels and the phase of both channels in the M/S format are not treated absolutely identically, the base width of the stereo signals changes. It is of crucial importance to consider this issue, or you end up with a mix that shifts the positions of the instruments on the stereo base depending on the high level signals that cause regulation. If only the M channel is compressed and the kick drum leads the regulation due to the high level with each kick, the gain reduction alters the level of the M channel and therefore also the level relation between the M and S channel in time with kick drum. Since the higher level of the kick reduces the level of the M channel, the base width is enhanced during the kick. An instrument that is located half to the left will be moved more to the left by this effect. This is a nice special effect but in general the result sounds weird and will be confusing and disturbing. When using M/S mode with a setting of the balance control different from 0, it is absolutely necessary to check the compressed result carefully for effects like that.

The real advantage of M/S compressor is simply the better weighting of the mono components of a mix. The reason is quite simple to understand. The level of the M channel that represents the mono signal, is almost always a lot higher than the level of the S channel that only contains the spatial information. Compression is well controlled by the basic rhythm instruments that are usually located near the center of the stereo panorama. This was actually the reason for us to include the M/S feature into the bass compressor; it can be of advantage in the bass range a lot more than in the mid and high range.

External M/S Processing
The signal in M/S format can be switched to the insert section of the U786 M/S compressor by the INS MS switch. External EQ processing of the M/S signal is possible using this feature. The special W785MS M/S equalizer is designed for this purpose and can be used together with U786. This switch override any other internal routing of the insert and can cause problems when using this features in a multiband array. In this case, the linked compressor would get the M/S signal at the compressor input instead of the frequency band that is not internally treated. To avoid confusion if this switch is pressed by chance, a jumper can block the activation of M/S to Insert if SPLIT MODE is active.

Weight and Balance in Stereo Mode
The controls WEIGHT and BALANCE are also active in stereo mode. While weight changes the influence of the left and right channel but does not change identical regulation of both stereo channels, the balance control determines the regulation of both stereo channels independently. This is in stereo mode as dangerous as it is in M/S mode, even though when the effects are a little different. If only the left channel is regulated, the right channel remains unchanged and vice versa - you can image how the results will sound.

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