The Electrical Voice Channel?
I had a gnawing fear when I began to put this page together that I would turn 90% of
the readers off telecommunications for good.
Why?
It ain't easy to grasp unless there's a really good peg to hang it on!
John Mellencamp came to the rescue.
In a TV interview - he spoke quietly about how - all he wanted to do in life - was to write songs. To do that - he had to learn to play the guitar.
He wasn't interested in Hot Licks or any particular style. The ability to strum and knock-out a song
or two was the thing! He even admitted that he thought he couldn't sing but that wasn't going to stop him!
Persistence is a wonderful thing, right?
And so, driving downtown one night - I began to formulate a way to put this page together along
the lines of Mellencamp's thought process. I made a little leap to another American songwriter - Woody Guthrie and his beautiful song - This Land Is Your Land:
Here's the refrain:
This land is your land
This land is my land
From California to the New York Island
From the Redwood Forests to the Gulf Stream Waters
This land was made for you and me.
Guthrie penned this beautiful anthem back in the Thirties.
Muscially, it is very simple: three chords - G D A and if you want to add a little wrinkle - throw in
an D7th!
Like Mellencamp, you could buy a guitar and be playing and singing this song in a couple of days.
The question would be though - could you convey it's true meaning to an audience?
I think you would agree that the likes of a Mellencamp or a Springsteen certainly could and do - much to
their credit!
So it goes with the voice path!
I can give you a quick overview, but you have to put a few more chords into the mix in order to
fully understand the wrinkles of the technology.
I'm going to do my part on the basis that you are going to follow through and do yours.
Here we go:
So you bought headphones for your new CD Player - Koss, no less.
Probably never read the little pamphlet that came with it, right? Who cares, right? Wanna
hear Prince "..party 'til it's 1999!"
Somewhere within the pamphlet - you might find the following information:
Frequency Range 20 - 20,000 Hertz (Hz)
That range just happens to be the frequency range of the human ear.
Do CD's come close to either the lower or higher ranges? Not really, but who knows with all those
sublimal messages that everybody talks about, right?
An important concept to note:
The audio transmitted to your headphones goes only one way. This is known as Simplex transmission!
A Telephone Voice Channel comes in at 0 - 4000 Hertz (Hz). Is it all used? Sorta!
It's the frequency range available to you whenever you pick up a telephone handset from it's cradle.
Let's assume, you're at home getting up some courage to ask someone you care about out for a date!
You're looking at the phone - thinking about how you are going to ask!
Before you do it - let's re-discover Tip & Ring.
You've got stereo headphones, right?
Check out the plug that goes into your CD Player.
Tip is simply the
end of the plug while Ring is that little ring around the sleeve. Both in concert are used to establish a
physical connection for the audio path.
The terms Tip & Ring have been around from the days of the Switchboard Operator.
Do not blow them off as irrelevant. You will hear Tip & Ring over and over again, especially in high-speed data communications!
Back to the telephone.
An on-hook (idle condition) is drawing 0 Volts DC on the Tip (Green or Black Wire) and -48 Volts DC on the Ring (Red or Yellow Wire) from the Central Office.
The instant you go off-hook - the Central Office drops the voltage Tip to -20 Volts DC and the Ring to -28 Volts DC. Bingo! The Local-Loop.
Let's do a little math here:
28-20 = 8 Volts DC
All of a sudden - we have a Voice Channel (path) powered by 8 volts DC (20 milliamps) on which we can send your voice across the Public Switched Telephone Network along
with the signalling to setup and complete the telephone call!
You can emulate the voice-side (Talk Battery) of this concept with a 9 Volt Battery, a few feet of twisted-pair, and two standard handsets at home.
Now, of course, we have to have dial-tone before we can dial.
We have to be able to punch in the numbers on our Touch-Tone Phone.
The call on our Local-Loop:
- has to be accepted by the Central Office
- passed across Tie-Trunking to another Central Office
- sent out to the Local-Loop of the party we want to talk to
- send Ring Voltage if that party's phone is in an Idle state or condition.
- return a busy signal or error if the line is in use or you mis-dialed and so on.....
Whew, get outta here!
But wait - remember Mellencamp only wanted to strum the guitar when he first started in Rock 'n' Roll.
So let's strum our way through the Voice Channel.
Let's look at the frequency range of the telephone voice channel.
No need to solve for x - all the unknowns are givens:
- The bandwidth or frequency range on a Voice Channel is 0 - 4000 Hertz (Hz)
- The bandwidth or frequency range for voice signals (your voice) - is between
200 and 3400 Hertz (Hz) within the Voice Channel
- DTMF Dialed Digits, Control & Status Tones are spread throughout the frequency range.
- 20 milliamps of power must be present for a telephone call to take place.
- Ring Voltage at 20 to 47 Hertz - Hz (110 Volts AC)
- Finally, a telephone conversation is considered a full Duplex transmission technology. Conversations are two-way simultaneously unlike
listening to a CD, record or tape on your headphones or talking on a Two-way Radio.
Okay, so we learned the first two chords of This Land Is Your Land - G and D.
So far we've done it all unplugged.
Let's flip-on a little Voltage and crank up the
amps to try and get a handle on the A chord - the electrical component!
The Voice Channel employs electrical technology developed by both Edison and Westinghouse:
- Direct Current - Edison. The voice path and associated signalling.
- Alternating Current - Westinghouse. Ring Voltage
Remember the 8 Volts DC on the telephone line.
Think of it as a continual wave varying over time - cycling it's merry way from your telephone down the wires - all the way to the
Central Office.
After you have gone off-hook - dialed your intended date - connected and are ready to talk - audio in the form
of your voice agitates or excites that smooth undulating electrial wave in a predictable way within the frequency range of
the Voice Channel.
The microphone in your telephone handset reverberates with your voice.
This process interferes with the line voltage
as makes its way down the line - through to the Central Office - across tie-trunks to another Central Office and
out to the Local-Loop into the earpiece or earphone of your date's telephone handset where it is converted back into your voice.
Within the same bandwidth or frequency range of the voice channel are the signalling processes that perform
all the common functions associated with placing a telephone call.
The Voice Signal is carried within DC Voltage along with the signalling.
Depending where you are in the United States that signalling may take one of three formats:
- Rotary or Pulse
- DTMF (Dual Tone Multi-plexed Frequency)
- Digital
Right here is a good time to add a little wrinkle or an A7th Chord which will help us
get close to the end of this page!
Our telephone conversation started out on our Local-Loop - the connection between
your home phone and the Central Office and ended up on your intended date's phone - the
distant Local-Loop.
It would be safe to say that the technology used to perform this little feat is commonly known
as Analog, a continual but varying wave between the home phone and Central Office.
Like any good musician who wants to enhance the emotive quality of a song - he or she will throw
in a 7th chord which in the case of This Land Is Your Land - happens to be an D7th!
Guess what, the phone company does the same thing over what is commonly known as a Trunk.
What the heck is a Trunk?
In our example - the inbound call from the Local-Loop needs to be transported across town. To do this - it
probably has to travel between two Central Offices to be delivered to the distant Local-Loop.
Let's use the good old T1 technology.
And let's say both Central Offices are trunked together with a few of these network interfaces.
Now we know that modern-day T1 technology is digital, right? It employs the 0's and
1's of the Binary Numbering System.
One method of employing digital technology is commonly referred to as:
- PCM - Pulse Code Modulation
How does it work?
Our voice reaches the Central Office on the Local-Loop in Analog form. Remember - it is contained
within the 0 - 4000 Hertz (Hz) Frequency Range.
Analog to Digital Converters take this signal and multiply it by a factor of 2.
4000 - 4 KHz magically becomes 8000 - 8 Khz.
Digital technology samples the inbound signal at 8000 times per second.
This, in effect, allows the phone companies to clean-up the signal by removing unwanted Noise and stuff. Filtering, if you will!
It also allows them to multiplex your call with thousands of others!
But for the sake of sanity - let's pretend that there is just one T1 between both Central Offices.
Okay, here comes the Key to the Universe via T1 B8ZS signalling and
Time Division Multiplexing.
An Analog signal 0 - 4000 Hertz is doubled to 8000 Hertz, right?
Sampled & Quantitized at 8000 bps = 8 kbps.
So our call across town happens to be one of 24 calls. Do you remember that a T1 has 24 channels?
Good!
Channel One gets the first 8 Bit signal - the second channel the next 8 Bit signal and so on.
One voice channel is 8 Bits at 8000 times a second for a total of 64 Kbps
24 voice channels at 8 Bits + 1 framing bit (to synchronize) at 8000 times a second for a total
of 1.544 Mbps.
So our call that started out as an Analog tranmission:
- hit an Analog to Digital Converter
- was sent across a Tie-Trunk T1 between two Central Offices
- hit a Digital to Analog converter
- and then routed out to the distant Local-Loop in Analog form.
And all the while - the conversation continued!
That's it!
I close by getting back to This Land Is Your Land. Woody Gurthrie wrote about .. a ribbon of
highway.
Do think maybe he could have thinking about the wires dangling off telephone poles? Probably not, but
I like to think so!
You can choose to ignore that ribbon of highway and discard the poetic license of Guthrie, Mellencamp or Springsteen, but if you do -
you are really turning your back on what could lead you into a rewarding and fulfilling profession.
Like Guthrie's This Land Is Your Land, Mellencamp's Small Town or Springsteen's Badlands - Telecommunications technology paints vivid images.
Those images come in thousands of formats but a simple fact remains:
the technology hangs on just a few basic structures or chords.
To grasp it - you have to start out just like you would with a basic song:
understand it's chord structure, have an ear for melody and be
able to convey it's true meaning.
Of course, you'll need a guitar. Will it be a Martin or Yamaha?
Telecommunications is no different.
You have to be persistent to move from strumming to
conveying the truth found within any work of art!
Make no mistake - Telecommunication is an art form that
requires you to put yourself out to fully understand it!
I hope this four-part series on the Voice Path has helped you strum through at least a couple of chords of the technology.
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