Processing 101

     A discussion and definition of commonly used term

       By Mike Raide, W2ZE

  After reading a post on the AM Forum by Paul, WA3VJB, prompted me to do some thinking. After reviewing the East Coast sound page, I noticed that there wasn’t a topic describing anything about processing, or how to setup processing at one’s station. I thought that maybe a tutorial on the subject might be in order, so I volunteered myself to the Amfone.net “gang”, and was greeted with an enthusiastic "Go Ahead!”

  The purpose of this tutorial is not to recommend or condemn anyone particular unit(s), or to say it has to be done this way or it’s wrong.  I have found over the years, that opinions are like elbows, everyone has one, and certainly, everyone in the AM community has different opinions about processing. The purpose of this discussion is to define general terms and examine how they can help everyone achieve the “signature sound” that they want to achieve from their station.

Well, lets start with the unit for measuring power and voltage ratios in audio and RF. The Bel is named after Alexander Graham Bell, who not only invented the telephonium, but did a lot of work with sound and how humans perceive sound. The decibel is 1/10 of a Bel, and is the most commonly used unit. The decibel is calculated using the following formula:

                                                 10Log(Pout/Pin)

So, we have an amplifier that takes a 1w signal and makes it a 20 watt signal out, so we calculate it by using the above formula:

                                          10Log x (20/1) = 13.01dB

The amp has a power gain of 13 dB.

When power is doubled, the gain is 3 dB, but when voltage is doubled the gain is 6 dB. When power is reduced in half, the loss is 3dB, but when voltage is reduced in half, the loss is 6dB. Voltage gain is found using the following formula: 20Log(Vout/Vin). What starts to get a little confusing, it when you open a manual and look at the spec.’s of a particular piece of gear and it measures it’s input level or output level as say dBm or dBv or even dBu.

What that tells us is what reference the manufacturer used to find its output ratings. Many pieces of professional gear use some sort of metering to display levels, and are measured with VU (volume unit) meters, and are marked with decibels on the meter (1). Volume units are referenced to 1mW of power produced by a 1KHz tone across a terminated 600 ohm load (2). Using Ohm’s law, this gives us .775V of audio, and references 0 dB on the meters, and the dBm is used to reference all power levels above and below this level. DBu is a decibel measurement made unterminated or measured in an open circuit, and dBv is decibel-volts measured using an open circuit as well, but using 1V as its reference. Since these circuits are unterminated, or not working into a load of some sort, these are measurements of voltage only, not power. When setting up your gear, it is important that every piece of equipment be calibrated to some sort of standard level, usually using one of these reference levels.

  Whew! That was a mouthful. Now lets get into some terms that you may have heard in passing or in a QSO on the air, and find out how they help or hurt us.

Dynamic range is the basis of what we all are trying to accomplish when we start processing our audio .The definition of dynamic range is defined simply in two ways:

1 – To describe the actual range of signal fluctuations that are going through the equipment and,

2 – To define the maximum allowable range of signal fluctuations that can be put through the equipment (3).

Basically saying, the dynamic range is the quietest sounds all the way to the loudest sounds going through the equipment. Lets for example say the noise floor (noise the piece of equipment makes in operation) is –100 dB. The highest level our equipment sees coming in is measured at +3 dB. Our dynamic range in dB’s is 103 dB. This now gets us to peak to average ratio. Peak levels are the highest level the audio goes at any given time. The average levels are where the audio stays around most of the time. A station that has high peak and low average audio has a lot of dynamic range, which has a more “open” or natural sound. A station that reduces the peak to average ratio of the audio increases loudness. Density is where the audio peaks are kept uniform throughout, but reduce the dynamic range significantly (4).