Part 8 – Input Basics

Now that we have looked at software we need to now ask how should we interface with that software? If you thought software choices were many, then wait until you look at interfaces. So again we begin with our newly adopted motto for everything, “begin with the end in mind.” What do you want to record? How many simultaneous tracks do you need? And what type of connections do you require? But before we get there we need to get some more terminology and concepts to better our understanding to better answer our questions.

If you have been following along with trying your systems basic soundcard with recording software you may have noticed a few things. If you plug your mic into the “line in” you hear nothing, if you used the “mic in” chances are you heard very little or it was noisy. Many times basic soundcards just don’t cut the mustard for audio recording, what you need is “more power” to steal a phrase from the “Toolman”.

There are 2 types of signal inputs found on most soundcards that we have been referring to. Those inputs are “line-in” and “mic-in”.

A device that produces a line signal produces a voltage. A balanced line will be upwards of 1.23 volts +4dBm and an unbalanced line 0.136 volts of –10dBv (dBm and dBv refer to decibles related to mili-watts and volts respectively and beyond the scope of our discussion here). Balanced refers to a cable/connector having 2 signal conductors and a sheild as opposed to unbalanced having only 1 conductor and a shield that also carries signal. Because of impedance differences in the conductor and the shield carrying the signals hum is introduced and amplified and depending on that difference you can have a lot or a little. The 2 conductors that carry signal are identical therefore there is no impedance difference to amplify any hum that is present on the line. Most consumer equipment has unbalanced inputs of the ¼” phono jack or RCA jack variety though much today’s recording gear for home studios contain balanced ¼” phono and/or XLR jacks.

In contrast a standard mic signal, also a voltage,  is produced by a microphone, which converts an analog signal to voltage by means of a transducer. This voltage is extremely low around 2 milivolts or .002 volts. Because that voltage is so low, it needs to be pushed up by way of a preamp to line voltages to be processed. Most basic soundcards will not increase the voltage up to full line level as they have very weak and feeble preamps. Needless to say a good way to blow out your soundcard is to put a line signal into the mic in.

Now the question comes what if you only have 1 jack that is line level only? Then you would need an external preamp – which we will look at later. What if the converse is true, you only have a mic-in and not a line-in? Then you would need a PAD switch aka an attenuator which will reduce the voltage. A good tool to have is a DI aka Direct Injection Box.  See http://www.whirlwindusa.com/dirbox.html .

Here is a great reference on the 2 most common connectors you will deal with and some technical discussion on balanced and unbalanced differences: http://www.nullmodem.com/Audio.htm .

Also a technical discussion relating to high and low impedance (which we didn’t cover) also some more on balanced and unbalanced lines: http://www.whirlwindusa.com/tech03.html .

Next Installment: Inline Preamps

Basic Home Recording ©2003 by Jim Goodman. All rights reserved. You may download and reproduce entire document only for non-commercial purposes providing this copyright exists on all copies. All other uses require a written request to jgoodman@alltel.net or thru http://psalm149.com

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