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ARSCLIST  February 2013

ARSCLIST February 2013

Subject:

Re: Mono but Out-of-Phase

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Wed, 6 Feb 2013 10:32:06 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (65 lines)

Hi, Henry,

As to your question "Does anybody know how an amateur reel-to-reel 
recording could have gotten this way?" the answer is "let me count the 
ways."

First of all, just to be 100% certain, please run a calibration tape to 
make 100% certain that the playback machine is not connected in reverse 
polarity. Often this type of thing can happen when a head is swapped, 
for example. The head is basically a balanced device and could be hooked 
up in either polarity. It is uncommon to have this happen, but in some 
instances (ReVox A77, for example) the colour codes are not immediately 
obvious (I wrote the colour code on the chassis under the trim plate on 
most of my machines back in the day). This could happen to the record 
deck or the play deck as a head is swapped out.

Of course, there are other places where polarity can be swapped in a 
system, but in a consumer system with unbalanced interconnects, it is 
more difficult to invert the polarity in the wiring. If the connections 
are balanced, the polarity could be inverted at any number of places, 
including a wiring error (birth defect or maintenance error) within the 
machine.

It is possible to see an in-polarity signal at one frequency and an 
out-of-polarity signal at another frequency with badly mis-adjusted 
azimuth, but I do not think that is what you are describing. An example 
might be tuning azimuth at 16 kHz and getting it off by one cycle which 
would be 1/2-cycle at 8 kHz. That, however, when summed would show a 
loss of highs.

A funny story: I was playing a video that I had made for a family 
member's memorial service at a church I had never been to previously. I 
struggled to understand why the voice of the deceased did not play in 
the soundtrack at the church but was fine on my computer. The answer is 
described at length here:
http://richardhess.com/notes/2012/12/12/the-phone-plug%e2%80%94uses-and-misuses/

Cheers,

Richard

On 2013-02-06 9:55 AM, Henry Borchers wrote:
> Hi all,
>
> I've got a question for you that has been a real head scratcher for me and I hope there is someone here who might be able to shed some light on it.
>
> I've been digitizing a number of amateur recordings from a student radio station from the 1970s recorded on 1/4" reel tape and a number of these recordings from this collection seems to be mono and have no stereo field but the left and right channels have opposing polarity. It is easy to just digitize both channels and play them on stereo headphones but since they are 180 degrees out of phase, they cancel each other out when summed together with a mono speaker.
>
> I understand the physics of canceling waves (and I know how deal with this) but my question is this. Does anybody know how an amateur reel-to-reel recording could have gotten this way?
>
> --
> Henry Borchers
> Broadcast Media Digitization Librarian
> University of Maryland
> B0221D McKeldin Library
> College Park, MD 20742
> (301) 405-0725
>

-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada                             647 479 2800
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.

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