Four Ways to Vary Image Width

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Four Ways to Vary Image Width

Post by mediatechnology » Thu Jul 24, 2014 7:45 am

I get lots of questions about using the Precision MS board to vary Width.
There are many ways to get Width adjustment of the stereo image.
Here's four.
Some use Mid Side relative level adjustment and can be performed using the Precision MS Matrix others use Left, Right and Side.

1) In the normal configuration of the Precision MS Board all you have to do to change image width is vary the gain in the Side or Mid channel relative to the other. This allows width to be trimmed but does not provide an easy panoramic adjustment from full Side to Mono. Its best used as a Width trim function. So if you have a compressor in Mid, all you have to do is trim its makeup or input gain. Less gain in Mid is wider...

2) In the width adjustment configuration I show linked below, the decoder is made encoder and vice-versa. A dual pot is ahead of the 5532/LME49860 stage to vary the relative side to mid levels. The output of the encoder (actually the decoder) is looped into the "decoder." Those loops are the Mid/Side process inserts. So yes, you can do both width and process with one board. In the photo here the insert points are the twisted pair looping around the board: ... 459&p=6015

3) The circuit to make the MS work as an elliptic equalizer provides a very cool panoramic width control that doesn't have the gain build-up when panned to full mono method 2 has. Another added advantage is that it only requires a single gang pot (if done single-ended not balanced). The post and circuit is here: ... l&start=36 By making the EQ block a variable gain polarity flipper (-1, 0 +1) there is full panoramic control from Side to Mid. I persoanlly think this is the best way to do Width and is similar to a circuit Ric Chenn posted some time ago. The signal never leaves the Left Right domain and has varying degrees of side injected into them with a varying sign. The polarity flipper is found here: ... p?f=6&t=71 The variable gain polarity flipper is only one of the 1240 blocks shown in the drawing.

4) Here is another Width Control which doesn't use the MS board. It's based on an earlier Ric Chenn design but uses THAT1240s instead of 5532s and precision resistors. It requires a dual pot. Similar to method "3" Left and Right never enter the MS domain. That circuit is posted here: ... p?f=6&t=71

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Re: Four Ways to Vary Image Width

Post by ricardo » Fri Jul 25, 2014 6:57 pm

I was hired by Calrec to do mikes but probably did more Electronic stuff than mikes cos Broadcast Mixing Desks was the big cash earner.

The 'Mike Dept.' got all the odd ball design work, like EQs & Compressor/Limiters, for which listening tests were required .. including the Width module for M-series.

I ended up with a 'Sum & Difference' to get M/S and another to get back to L/R with a simple 0 to x2 amp on the Difference chain. This gave the best subjective results on the mainly classical stuff the BBC were interested in. Abba bought the 1st Broadcast desk by a 'music' studio but I went back to speakers soon after that so dunno how it works on pop stuff.

There was one twiddle preset to ensure the -zillion dB crosstalk spec was obtained somewhere within the slop of the '0' detent of the Width pot. :D

i still think this, very straightforward approach, gives the best user interface ... though I've played with loadsa SupaStereo stuff which involved processing M & S for better stereo on speakers & phones including LF blend ..

I rather like viewtopic.php?f=6&t=71 which uses one less device .. but requires a 2 gang pot which might be less easy to trim for da -zillion dB crosstalk.

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