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gregrs



Joined: 09/01/05
Posts: 201
Loc: Herefordshire
'Line-up' procedure and phase reversal new
      #113644 - 11/04/05 08:42 PM
After my post on using a stereo bar, I've been thinking about another point mentioned in Hugh Robjohns' article on stereo mic'ing...! It refers to the 'line-up' procedure mentioned:


It is essential to calibrate the microphones and their channels at the desk before attempting to record anything in stereo. Even nominally identical microphones will have slightly differing sensitivities, and the input channels in the desk could be set up completely differently -- so it is important to run through a line-up procedure (which is far quicker to do than to read -- honest!)

What we need to achieve is identical signal levels in the left and right desk channels for a given sound pressure level in front of the microphones. The easiest and most accurate technique starts with setting the microphones' polar patterns to the desired response (if using switchable mics) and connecting them to two desk channels (or a stereo channel, if available). Turn the pan pots on paired mono channels fully left and right and use a fader clip (or some other means, such as a large bulldog clip) to mechanically fix the two faders together so they track accurately. Rig the microphones one above the other with their capsules as close together as possible, and turn them to face in the same direction while someone speaks in front of them (about two feet away and at their mid-height, if possible, to ensure minimal level differences).

In the control room, switch the loudspeaker monitoring to mono (do not use the channel pan pots, because their centre positions may not be accurate), and adjust one mic channel for the typical operating gain you expect to need, with the fader in its normal operating position. Check that there is no EQ in circuit in either channel and switch a phase reverse into the second channel. Adjust the second channel's gain until the combined output from the microphones is as quiet as possible -- there should be a very obvious null point (it will never completely cancel, because of inaccuracies in the microphones and desk channels, but it should get extremely quiet).

Next, remove the phase reversal and loudspeaker mono-ing, and with the two mics still facing forward, have your talking assistant wander in a complete circle all the way around the microphone array. If the stereo image moves away from the centre, the mics have incompatible polar patterns and will not produce accurate stereo images. Select another pair of microphones and start over.

Finally, rotate the microphones to face 45 degrees left and right (make sure the microphone connected to the panned-left channel is turned to face the left of the sound stage) and have your assistant confirm the image boundaries and left-right orientation. Having completed the line-up, do not re-plug the microphones, or adjust the channel gains, as the calibration will be destroyed and you'll have to go through the entire process all over again! In practice, this whole procedure should take about a minute and should become routine.



I'd like to try this but my desk unfortunately has no phase reverse button. So I was planning to make myself a phase reverse cable. Would turning the phantom power off, taking the phase reverse cable out and turning it back on again after 'lining up' the mics make it a pointless venture in the first place, due to possible extra resistance / characteristics of the phase reverse cable and the process of turning off and on the phantom?

After the many helpful replies to my previous question about this article, I feel empowered by my lack of knowledge to ask questions freely without feeling too stupid!

Many thanks,
Greg


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Shivanand
active member


Joined: 11/08/03
Posts: 2278
Loc: Ashgabat
Re: 'Line-up' procedure and phase reversal new [Re: gregrs]
      #113709 - 11/04/05 10:10 PM
Quote greg_malvern:


I'd like to try this but my desk unfortunately has no phase reverse button. So I was planning to make myself a phase reverse cable. Would turning the phantom power off, taking the phase reverse cable out and turning it back on again after 'lining up' the mics make it a pointless venture in the first place, due to possible extra resistance / characteristics of the phase reverse cable and the process of turning off and on the phantom?




No. A correctly wired good quality short mic cable won't make a significant difference.

--------------------
"Qui habet aures audiendi audiat"


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 22279
Loc: Worcestershire
Re: 'Line-up' procedure and phase reversal new [Re: gregrs]
      #113918 - 12/04/05 10:34 AM
Quite right -- a short cable fitted with good connectors won't make any appreciable difference.

My own solution (I often use a Mackie desk without phase reverse facilities) is to use a 'barrel' XLR connector. You can get these from the ususal suppliers (Canford, Studiospares etc) either pre-wired or empty for you to wire yourself. They have a male XLR at one end and a female XLR at the other. Some also have switches which allows you to wire up a switchable phase reverse/normal barrel.

The advantage over a cable is that it becomes much harder to mistakenly use the phase reverse adapter to wire up a system, and it's easier to quickly plug in and take out from the desk inputs.

I paint my phase-reverse barrels with a bright yellow ring to make them easy to identify quickly, and differentiate from pads and sex-changers -- which all tend to look the same at the bottom of the 'bodge box of bits'!

Hugh

--------------------
Technical Editor, Sound On Sound


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gregrs



Joined: 09/01/05
Posts: 201
Loc: Herefordshire
Re: 'Line-up' procedure and phase reversal new [Re: Hugh Robjohns]
      #114294 - 12/04/05 09:41 PM
Okay many thanks. Think I'm gonna go for a short cable with red glands on the two XLR's. Many thanks for your helpful advice once again.

Greg


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 22279
Loc: Worcestershire
Re: 'Line-up' procedure and phase reversal [Re: gregrs]
      #114436 - 13/04/05 09:12 AM
Fair enough -- the use of yellow to identify phase reverses is quite a well known and widespread practice in professional and broadcasting circles -- that's why I use that colour myself. But as long as you know which cables are reversed, all should be well.

hugh

--------------------
Technical Editor, Sound On Sound


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