fundamentals series signals -...

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Fundamentals SeriesSignals

© Polycom, Inc. All rights reserved. 2

Fundamentals Series

Signals

Analog vs. Digital

Defining Quality

Standards

H.323

SIP

Network

Communication I

Network

Communication II

Welcome to Signals, the first module in the Polycom Fundamentals series. This module is approximately 10 minutes long.


Introduction

In order to understand how videoconferencing works it’s important to understand the underlying technologies at work behind the scenes. In this short module we will talk about some audio and video basics.

We won’t get too in depth here, the goal is just to give you an understanding of how these basic technologies work.

To start, let’s define what a signal is – the mechanism by which a picture or sound is transmitted from one place to another, either down a wire or wirelessly.


Audio Signals

One of the most important things that happen in a video conferencing call is actually the audio. Electronic communication has been using audio for over 100 years, both through the use of the telephone and radio transmissions. So let’s talk first about what sound is and how we measure it.


Sound Waves

Like waves in a swimming pool are caused by disturbing the water, sound is generated by disturbance of air. These are also called waves, only these waves travel through the air.


Sound Waves

Waves of any kind can change in two ways. The first is the size of the wave vertically; with sound waves we can think of the size as being the strength of the sound, or how loud it is. The size of a wave is known as its amplitude. The second way a wave can change is how often it occurs in a second; this is known as the frequency, which is measured in Hertz. The frequency of the wave is linked to the length of the wave, a low frequency wave is said to be long, and a high frequency wave is short.

Our ears are sensitive to sound waves and how they change, these sounds are picked up by our ears and interpreted by our brains, enabling us to hear.


The Human Ear

How that happens is fascinating. The ear and its inner mechanism is one of the most complicated and intricate things in your body. Here is a picture of what is going on that enables you to hear.

The sound waves come into your ear and vibrate the eardrum, sending signals to your brain. Then, your brain decides what the sound is, and, as you have two ears, which direction it’s coming from. This enables us to hear a wide range of sounds from low rumbling to high pitched tones.


Measuring Sound

One of the things we geeks like to do is measure things. And sound level is a good thing to measure. When it’s too loud it can be painful and harmful and when it’s too quiet it can be unintelligible. Especially when we’re talking across a video or audio conference. As sound waves are caused by varying air pressure, when discussing sound level we refer to it as sound pressure. When we’re talking about sound we have to remember that there is an ambient pressure level in the air. So the practical way to measure sound is to figure out the difference between the ambient air pressure and the particular sounds we’re measuring. This is referred to as sound pressure level (SPL) and it is measured in decibels (notated dB). Decibels tell us how loud or quiet a sound is when compared to a reference level, referred to as 0dB, when we can hear absolutely no sound. At about 120dB damage occurs to the human ear (referred to as the threshold of pain). So 0 to 120dB is called the dynamic range of human hearing.


Sound Reference

As a reference for what loud and quiet are, here is a chart showing some relative levels in dB of some common sounds. The higher the dB the larger the amplitude of the sound wave, the louder the sound and the more potential damage to the ears. But, remember that decibels are always in reference to something… Doubling the amplitude doesn’t double the perceived sound, it only increases it by about 3 dB. We need to raise the sound level by 10 dB to give us an approximate doubling of the perceived sound.


Audio Frequency

When we’re talking about audio and sound, the frequency of a wave gives how high or low the sound is in pitch... Human hearing is usually said to have a frequency range from 20 Hz to 20,000 Hz (or 20 kHz).

We’ll talk more about this when we discuss how analog audio is turned into digital, but for now it’s good to know that there is a range for hearing. From very low, bass sounds all the way up to very shrill treble sounds. Those bass sounds are actually very long sound waves and below 20 Hz we don’t hear them (this is why you “feel” strong bass sounds… a 20 Hz sound wave is about 20 meters or 56 feet long). Above 20 kHz (in the dog whistle range) we don’t hear things either. Average people are somewhere in between with the focus for most around the 1 kHz frequency (which has a wave about 30 centimeters or 1 foot long). This is where most of the sound in a normal speaking voice is centered. The range that a person can hear is referred to as their frequency range.


Microphones

As we have seen, our ears perform an amazing trick of turning sound pressure changes into signals that our brains can interpret, allowing us to hear sound waves. To be heard further away than our immediate environment (and without the need to shout!), we use a microphone, which takes sound waves and turns them into electrical signals for us. A microphone uses a membrane that vibrates when sound waves hit it, just like your eardrums. This creates variations in an electrical current that then travels down a cable for us to use. The variations in the signal voltage match the frequency and amplitude of the original sound waves. The best microphones can handle the whole human frequency range to make sure that all the information the ear senses is picked up, to provide the most realistic reproduction of the sound.


Amplifiers

Due to the natural electrical resistance of the audio signal moving through the wire, the signal is susceptible to two things. The first is called noise, and is when signals other than the one we want to hear at the other end get into the cable and introduce imperfections. The second is that due to the resistance of the cable the signal is travelling through, the signal gets weaker while it travels; this is called attenuation.

Because the signal attenuates as it travels we may have to amplify it (make it bigger) to make it usable. This is done by a device perhaps unsurprisingly known as an amplifier. All speakers need an amplifier of some kind to take an audio signal from a microphone or other device (like my MP3 player) and make sound. There is an amplifier built into your TV if it has internal speakers, for example.


Speakers

So now we have an electrical representation of our sound waves. This is an analog signal, meaning it is constantly variable. The amplitude and frequency values go up and down constantly and our voltage along with it.

And how do we turn this voltage into something we can hear? Well, by using speakers of course! Speakers turn the electrical signals originally created by our microphone into sound pressure waves again. By reversing the original process the electrical signals interact with a magnet to make a membrane move back and forth in complex ways to generate the sound waves for our ears. If the audio signal is not good or the frequency range of the microphone or speakers is not very wide it will not sound very natural.


Electromagnetic Spectrum

Digging a bit deeper into frequencies and moving naturally on from audio, here might be a good time to talk briefly about the electromagnetic spectrum, which is how we see things as well. Electromagnetic describes the relationship between electricity and magnetism, and a spectrum is just a range of frequencies, the results of which can be seen using this great diagram created by NASA. EM energy has some properties familiar to us, for example, it can be seen as waves, which is what we will take a look at specifically here. The different frequencies of the waves determine what properties each has and therefore what we refer to it as. See if you can find radio waves on the diagram. These are created by many things, but specifically by man when electricity and magnets are manipulated in the right way. The sun emits EM across the spectrum, but the portion we call visible light, which is how we see, is actually a mixture of different frequencies. These are the frequencies that our eyes are sensitive to.


Video Signals

Sight is a huge part of human experience, and a significant portion of our brain power is geared towards processing visual information. It is why human interaction is so much more effective when we can see who we’re talking to. The non-verbal information we’re getting helps us to understand what the other person is really thinking and giving us information about how best to communicate with them. It’s part of what makes video conferencing so invaluable today. To see, our eyes take in light and process it into impulses for our brain to deal with. Just like with audio, your body takes in a physical phenomenon and turns it into a signal. In the late 1920’s and 1930’s engineers were exploring ways to send information electrically, both across wires and wirelessly. Telephony and radio had shown that sound could be transmitted over distances by using a microphone. Now they wanted to send images too, but needed to figure out how to turn a “picture” into a signal. It was found that light could be converted by using a device that represents the values of light levels and colors as electrical signals. This is more complex than converting an audio signal, but essentially works the same way – a gadget at one end of the connection makes the signal into something which can be transmitted, and another gadget at the far end converts the signal into a representation of its original form.

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