Finding Gold in Random Places

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When coming into a new position at any location where you are the lead tech I strongly recommend looking at EVERY system in every venue of your location. This last week, I was introduced to a room that I have never been in that had an audio system installed in it. I for one didn’t know the room existed nor even had an audio system installed.

Installed in the audio rack that held the mixer was a digital CD/DVD audio recorder, the Tascam DV-RA1000 which is a high resolution audio mixdown recorder that can record 192kHz/24-Bit audio to a DVD or normal resolution to a CD.

The unit was not connected to anything and wasn’t even plugged in! This CD recorder was much better quality than the Tascam CD-RW750 which we have been using to record our sermons for years. The DV-RA1000 includes internal DSP for limiting/compression and EQ not to mention a lot of different inputs both digital and analog.

The largest benefit was the time it takes to finalize the CD. With the CD-RW750 it took about 1.5 minutes to complete a CD after the recording has been stopped. The DV-RA1000 takes less than 30 seconds. Also the time it takes the unit to stop recording and be ready to record a 2nd track is much less time.

All in all it was a great decision to switch to this unit, but even better is that I didn’t have to spend any money as we already had this. I just had to find it! So the moral of the story is that you need to look in all of your rooms/systems/venues and document every piece of gear you have.

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Polarity issues in CD recording from bad wiring

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Balanced audio signals are a great thing for the audio industry. If you don’t know much about balanced vs. unbalanced signals, I highly suggest doing some reading on Wikipedia.

Balanced signals have a differential amplifier on the output and the input side. They basically carry a mono signal down 3 conductors. Two wires carry the signal and then the last wire carries the ground. The beauty of the differential amplifier is that two wires that carry the signal. One wire carries the normal signal, then the other inverts the polarity. So, the two signals are out of polarity of each other. At the receiving end of the balanced signal, there is another differential amplifier which inverts one of the wires and sums them together to get the signal back to normal.

If there is any noise injected into the wire on its run, it is injected into both wires in polarity. When the signal reaches the receiving end of the wire, that differential amplifier inverts the polarity of one of the wires which cancels out the noise. This is because the noise is now out of polarity with each other and, in the summing process, cancels out itself.

Illustration from http://www.ians-net.co.uk/images/articles/balanced/balanced.gif showing how noise is rejected in a balanced signal.

At North Ridge Community Church, NRCC, we have a Tascam CD recorder which we use to record the spoken word of our services. I noticed one day that I had to turn up the CD really loud in my car to be able to hear it. This surprised me because earlier in the day I was almost clipping the meters with the CD recording levels. I decided to rip the CD into Cubase 5 to see what was going on. Here is what I found:

Out of Polarity signals which were recorded to the CD like this.

As you can see, the left and right channels are recorded out of polarity to each other. Basically, they are cancelling each other out from the inversion.

After seeing this on the screen, I went back to the church to find that the CD recording feed is coming from a balanced mono send off of a matrix on the Allen & Heath ML4000. The cable then goes into a 1/4 inch stereo to R and L RCA adapter. The way this adapter works is for a headphone cable which takes the left from the tip and sleeve and the right from the ring and sleeve. With a balanced signal, the tip and ring are inverted in polarity. So this is sending the + into the left while sending the – into the right.

I recorded a video of myself explaining what is going on when you have a signal that is out of polarity. In the video, I accidentally mention that this could be called 180 degs out of phase which is incorrect. The only term to call this is out of polarity.

After making the correct type of adapter with the balanced to RCA, the recordings will now have full volume on the CD recording with a full spectrum sound. This is another reason not to trust any installed wires without checking them.

Drew Brashler