Audio for Video: Basics

While performing our daily work we must understand each other with others, using technical words to explain complex ideas. Below you will find a list of some of the most commonly used terms in the audio field. It is necessary to know the meaning of these words so that when a comment is made to other people, they understand what exactly we mean.

Amplifier (Figure 1). A device that increases the level of a signal (increasing the voltage or current). Some amplifiers are used as buffers to isolate signals. Other amplifiers are used to control the signal and not increase it, but decrease it.

Dimming (Figure 2). Reduce the level of an electrical signal, usually with a volume control.

Block diagrams (Figure 3). A schematic drawing illustrating the main circuit blocks and signal flow in a system or electronic device. This schematic does not show the specific wiring details.

Channel separation. The degree to which the signal from a channel is not picked up by an adjacent channel, electrical insulation. Separation is measured in decibels. A good separation implies little filtration or cross talk. The higher the number of decibels, the better the separation.

Clipping (Figure 4). Clipping occurs when the capabilities of an amplifier are exceeded. The result is a highly audible distortion that is also visible on the oscilloscope.

Cross Talk. Undesirable signal that appears in one channel, resulting from the filtration of another channel, usually specified in –dB (negative number). The lower the number (more negative) the better the performance. The cross talk increases with the higher frequencies.

Distortion (Figure 5). Distortion is an undesirable change that occurs in the audio signal, causing the appearance of frequencies in the output that were not present in the input signal. Distortion has different types such as clipping, cross over, slewing, etc. They are usually expressed in percentages. The lower the number the better.

Dynamic range (Figure 6). The difference, in decibels, between the highest and softest parts of a piece of music (or between the maximum signal level and the noise floor of an electronic equipment).

Equalizer. Electronic device that will amplify (boost) and/or attenuate (cut) certain portions of the spectrum of audio frequencies. Equalizers bring together a large group of equipment such as tone control, graphic, parametric.

Frequency response. The frequency range that an audio device or system will let through. The frequency response has no meaning unless a tolerance is specified; example, from (x) Hz to (x) KHz, and more importantly, +-(x) dB.

Head Room (Figure 6). The Head Room refers to the difference between the nominal operating level and the maximum level at any point in an audio system. Usually 10-14 dBs is a good average.

Noise. Any undesirable signal, such as hum, buzz, rumbles, etc.

Noise Floor (Figure 6). The basic noise developed by the thermal noise of the equipment. This is a minimum number that cannot be eliminated or reduced.

Another type of noise is that of the tape, which is usually heard as a "HISS". It is a negative number and the units are in dBs. The lower the number, the less noise.

Nominal operation levels (Figure 6). The average signal level at which a circuit is made to be operated, either at the input or output of an electronic equipment, (better performance).

Signal-to-noise ratios. The difference between the maximum or nominal operating level and the floor, specified in dBs. Sometimes equivalent to dynamic range.

Next we will briefly deal with the topic of systems architecture, that is, the way many small components come together to make a great system. Here are the main steps to follow if you are looking to get quality audio.

The sound must be free of hums, noise and distortion, with an adequate frequency response. You can probably understand the dialogue or appreciate the music. This is the sole or ultimate purpose of an audio system.

Considering that all audio systems are composed of many devices connected in some way, it is very important that each device in the system is implemented in a way that obtains its best performance. The problem is that we have to deal with different levels of impedances (Figure 7). In modern systems there will surely be a mix of professional and semi-professional video and audio equipment, as well as different brands. This can be a real threat to getting good audio. However, never be afraid to mix equipment in your system. In Figure 8 there is a system with different pieces of equipment and their signal levels. In the second (Figure 9) I made changes to show how to correct those kinds of problems. (See Figure 7, 8 and 9).

Now that you have the perfect fit between the system, the signal-to-noise ratio problem will be solved and the overall performance will improve.

Each piece of equipment must be conditioned to its optimum operating limit. The latter is of great importance.

Well, so far we have dealt with these ideas only on paper. In a future edition I will continue with the second part of this series and discuss "simple audio tests". The main topics will be: audio sources, for example, discs, tapes (audio/video), computers and test instruments. I will present a simple project to build a test oscillator.

Until the next edition and good luck with their audio systems.