This presentation is brilliant. This lesson was clear and concise and to the point. It's a shame there are not awards for educational videos of this type. Please keep sending outstanding educational videos going. Got to smile "right said fred". That was a flash from the past. Thanks
This video is so wonderful. When I heard that classic accent (Geordie or Yorkshire) "Why, this cable's rubbish," said Fred I absolutely doubled over in laughter.
A practiced my english transcription abilities. The text below is what can be heard in the video. I hope that somone will find it useful. Over a hundred years ago, telegraph and telephone engineers realized that different types of cable performed differently. Some were better at carrying signals of long distances than others. It became necessary to quantify this performance, so two (fictional) engineers, the Wilco (will comply) Luke and Fred set out to do some measurements. Luke set up a signal generator to send a signal with a power of 1000mW into a piece of cable, whilst Fred went down to stores with a wheelbarrow to fetch a one-mile-long reel of cable. Luke, then, connected the signal generator to one end of the cable, and Fred used his signal meter to see what was coming out of the other end. Now, it would seem that the cable that Fred had selected from stores was not well suited to long runs, because only 100mW of power was available at the other end of he run. "Why! This cable's rubbish" said Fred. "It shoo's 900mW on the first mile! Another couple of hundred yards and it be nothing left at all!" "Maybe it's a faulty reel" said Luke. "Let's try another reel to see if that's bad as well". "Right" said Fred, and he set off back to stores with his wheelbarrow to get another reel of the same type of cable. On his return, he looked at the first reel of heavy cable on the floor and said: "I'm sure this second reel of cable is going to measure ok. We can connect it to the end of the first rail to save me lifting it out of the way. A hundred milliwatts of signal is enough to do a test." "Very well, make it so!" said Luke. This time, when Fred connected up his meter, he found that there was 10mW of signal available at the end of the second reel. "Well, this really is better, but it's still losing 90mW in a mile" said Luke. "I think we're going to have to try another reel to find out what's going on. "Right" said Fred, and he set off again with his wheelbarrow. When the third reel of cable had been attached tot the end of the chain, the reading on Fred's meter was only 1mW. "Now in milliwatts of loss on this reel" said Fred. "I think we're going to have to send the whole batch back. "Hmmmmm" said Luke, scratching his head. "Funny. We've lost 90% on each reel. That seems a bit of a coincidence. "Maybe the position in the chain determines the loss for each reel. We could swap the position of each reel in the chain to check that" said Fred. "Brilliant idea" said Luke. "Make it so!" "Right" said Fred, starting to shift the heavy reels of cable and wishing he'd kept his mouth shut. As it turned out, swapping the order of the reels of cable, didn't make any difference. Each reel lost 90% of the energy it was given. "This is an important discovery" said Luke. "We now know that the loss in a cable can be quoted as a percentage per mile." "That's a pain" said Fred. "It means that to find a loss on, say 25% per mile cable, over 42 miles, I've got to multiply by knot 0.75 42 times! No one's invented the electronic calculator yet" "You're right" said Luke. "There must be a better way. How about we quote the loss in knots?" "What do you mean "knots"?" said Fred. "Well, look" said Luke. "This cable here loses exactly one knot per mile, so two miles is two knots. Three miles is three knots. Of course, this is a rather poor cable, so perhaps it would be more sensible to split each knot up into some smaller units. Perhaps we could call them sentinels or milliknots, to make it easy to do the calculation with better quality cable". "Right" said Fred, "but I think we all need to split each knot up into ten subunits, call it a deci-knot. We only need to know wether someone on the other end of the phone line can hear what's being said. 10% more or less signal doesn't make any difference." "This is starting to sound like a really good idea" said Luke, "but I'm feeling a bit daft calling the unit knots. We need to name them after someone famous, like Voltaire of Faraday". "They've both been used already" said Fred. "How about Alexander Graham Bell, the inventor of the telephone?" "You can't have a unit called a deci Alexander Graham Bell!" said Luke. "No, call it the deciBel, you fool!" said Fred, who wasn't too good at spelling. "Excellent!" said Luke. "Make it so!" "I'm still not quite sure what happens in between the full knots... Bells I mean" said Fred. "How do we describe a cable that loses, say 50% of its signal per mile?" "Ok" said Luke. "We'll draw up a table and try to fill in some gaps. We know where we are with 1, 10, 100, 1000. Just count the knots to find the bels, and multiply by 10 to turn that into decibels. Know let's have a look at the multiples of 2 which fit in between 1 and 10. Now, 2 times 2 times 2 makes 8, which doesn't quite reach our first knot. Since another times 2, would take us to 16, which is quite a long way past, I'm going to call it knot 0.9 of the way to the first full knot or Bel. That makes it 9 decibels. Now, we know that each times-two is equally spaced, so the other two entries are easy to fill in. 3dB's and 6dB's. Now that we know how to handle times-two's, we can fill them in on the rest of the table. We can also draw up a table for divisions. It's exactly the same as the multiplier table, but has a minus sign in front of the bell figures." "Right" said Fred. "That's brilliant! We'd better alert the authorities to the existence of our new unit." "Well, actually no" said Luke. "What we've been describing isn't actually a new unit at all. It's really just a different way of writing down a common or garden number, which turns multiplied and divided operations into add an subtract operations". Fred was still clearly excited about having found a new branch of maths that will be useful to telephone engineers around the world. He said: "Well, this is brilliant! It ought to be taught in schools instead of all that useless stuff that no one ever needs!" But Luke sighed and said: "I just realised... this is already taught in school. We all did it, except in those days it was called logarithms. Having seen that decibels are actually just a logarithmic way of expressing simple numbers, let's just have a quick look at a couple of applications that should help clarify how they are used in the real world. Firstly, sound. You may have heard the loudness of sound quantified in decibels, for instance, a jet aircraft at 140dB's, or a quiet office of 40dB's. Now, we know that decibels are just numbers, so what are the units of sound? Well, in this context, the unit that has been chosen is the quietest thing that an average person can hear. This sound level is called "A", and is written 0 dBA. Remembering that 0dB is just the number 1, we could read this as 1 times louder than the quietest thing an average person can hear. The onset of pain in a noisy environment is usually considered to be about 120 dBA. This means that a human ear can cope with sounds up to 12 knots louder than the quietest thing it can hear. That's a range of 1 million million. Pretty good sense of the human ear. Let's look after it. Signals. Now, Luke and Fred were investigating signal losses in cable. They found a loss of 90% per mile in their cable. Now, this is a factor of 10, each mile, which they ended up calling 1 Bel, or 10 decibels (dB) per mile. This is the correct way of expressing the loss. There are no units involved because a factor is just a pure number. However, when they started out, they were measuring milliwatts (1000mW = 30dBm). It's actually quite common to use decibels this way, especially when describing the input sensitivity or the output power of radio devices. The convention is marked by an "m", following the dB. For instance, a standard Wi-Fi device for use in the wireless networking of computers might have an output power of 100mW, often expressed as 20dBm. this is convenient because when the losses associated with cables, and the gains associated with antennas need to be considered, there can be just added is, to find the actual radiated power and eventually the signal available at the receiver. This can be compared with the sensitivity of the receiving device to check that a particular system will work. This calculation is called a link budget and daily fair for anyone connecting buildings for computer networking using Wi-Fi. If you need to know more about wireless networking and Link budgets, check out the relevant product sections and learning center on the Solarize website.
@@solwise thank You for answer. I really admire You work and.. being honest it is hard to convey knowledge in a simple way - you did it masterfully!!!!
I realise that it gets a bit vague here and one day I may do some more work on it to make it clearer. The point I am trying to make is that once we have established a scale where each multiple of ten is equally spaced then we need to find some sub-divisions. I try to do this in a hand-waving way without doing any real maths. Let's pick multiples of two and see what happens.: 1,2,4,8. That's three equally spaced steps to get from 1 to 'nearly' 10. Now 9dB is also 'nearly' 10dB, ie three steps of 3dB gets us to 'nearly' 10dB so we can make an approximation and say that: x2 ≈ 3dB , in words: "times two is about 3dB" Actually log2 = 0.301029995 so "times 2" = 3.0102995dB so our guess was pretty close! I hope this helps.
Generally you will find the sensitivity of a receiver expressed in dBm anyway so the question is somewhat moot. If a receiver says in can receive 10Mbps at -90dBm, then -80dBm of signal should work just fine. However, since you ask, -80dBm is the same as -8Bm, or eight zeros (noughts) to the right, so 1x10^-8, I think that makes -80dBm = 0.000 000 01 mW (is that 10pW picoWatts?)
@@SolwiseMD I was meaning to the end of the video,when there is the example that a transmit start to 100 mW (20dbm) and to the end i will find -80 dbm.I was trying to understand how many mW i will find,maybe like you say i will find 10 pW.Wait if someone explain or agree.Thanks.
Actually this is an approximation, just close enough. Each doubling of a value is represented as adding 'about 3dB'. So doubling 3 times (2x2x2) is multiplying by 8, adding 3dB three times (3+3+3) is 9dB. A closer aproximation is that doubling (x2.00000) equates to adding 3.01029995 dB (10 x log(2) ). Hope this helps.
Hi Ocky, We all have different learning styles and maybe my teaching style doesn't meet your needs. Although the feedback on this video is overwhelmingly positive I'm aware that it won't suit everone. Have a look at my Antenna Gain videos, for some people the approach in those is clearer.
I don't care how old this is, it's a great explanation! Thank you
one of the Best Videos to understand Decibels, all the other ones are like talking the Matrix language.
The best explanation of dB on You Tube. Thanks.
This presentation is brilliant. This lesson was clear and concise and to the point. It's a shame there are not awards for educational videos of this type. Please keep sending outstanding educational videos going. Got to smile "right said fred". That was a flash from the past. Thanks
This video is so wonderful. When I heard that classic accent (Geordie or Yorkshire) "Why, this cable's rubbish," said Fred I absolutely doubled over in laughter.
"Right" said Fred. Lol.
+tapiwakay i scrolled down to read the comments during the video, and EXACTLY as i read your comment, he said it XD
+tapiwakay "excellent", said Luke. "make it so".
Climbing up the ladder
A practiced my english transcription abilities. The text below is what can be heard in the video. I hope that somone will find it useful.
Over a hundred years ago, telegraph and telephone engineers realized that different types of cable performed differently. Some were better at carrying signals of long distances than others. It became necessary to quantify this performance, so two (fictional) engineers, the Wilco (will comply) Luke and Fred set out to do some measurements.
Luke set up a signal generator to send a signal with a power of 1000mW into a piece of cable, whilst Fred went down to stores with a wheelbarrow to fetch a one-mile-long reel of cable. Luke, then, connected the signal generator to one end of the cable, and Fred used his signal meter to see what was coming out of the other end.
Now, it would seem that the cable that Fred had selected from stores was not well suited to long runs, because only 100mW of power was available at the other end of he run.
"Why! This cable's rubbish" said Fred. "It shoo's 900mW on the first mile! Another couple of hundred yards and it be nothing left at all!"
"Maybe it's a faulty reel" said Luke. "Let's try another reel to see if that's bad as well".
"Right" said Fred, and he set off back to stores with his wheelbarrow to get another reel of the same type of cable. On his return, he looked at the first reel of heavy cable on the floor and said:
"I'm sure this second reel of cable is going to measure ok. We can connect it to the end of the first rail to save me lifting it out of the way. A hundred milliwatts of signal is enough to do a test."
"Very well, make it so!" said Luke.
This time, when Fred connected up his meter, he found that there was 10mW of signal available at the end of the second reel.
"Well, this really is better, but it's still losing 90mW in a mile" said Luke. "I think we're going to have to try another reel to find out what's going on.
"Right" said Fred, and he set off again with his wheelbarrow.
When the third reel of cable had been attached tot the end of the chain, the reading on Fred's meter was only 1mW.
"Now in milliwatts of loss on this reel" said Fred. "I think we're going to have to send the whole batch back.
"Hmmmmm" said Luke, scratching his head. "Funny. We've lost 90% on each reel. That seems a bit of a coincidence.
"Maybe the position in the chain determines the loss for each reel. We could swap the position of each reel in the chain to check that" said Fred.
"Brilliant idea" said Luke. "Make it so!"
"Right" said Fred, starting to shift the heavy reels of cable and wishing he'd kept his mouth shut.
As it turned out, swapping the order of the reels of cable, didn't make any difference. Each reel lost 90% of the energy it was given.
"This is an important discovery" said Luke. "We now know that the loss in a cable can be quoted as a percentage per mile."
"That's a pain" said Fred. "It means that to find a loss on, say 25% per mile cable, over 42 miles, I've got to multiply by knot 0.75 42 times! No one's invented the electronic calculator yet"
"You're right" said Luke. "There must be a better way. How about we quote the loss in knots?"
"What do you mean "knots"?" said Fred.
"Well, look" said Luke. "This cable here loses exactly one knot per mile, so two miles is two knots. Three miles is three knots. Of course, this is a rather poor cable, so perhaps it would be more sensible to split each knot up into some smaller units. Perhaps we could call them sentinels or milliknots, to make it easy to do the calculation with better quality cable".
"Right" said Fred, "but I think we all need to split each knot up into ten subunits, call it a deci-knot. We only need to know wether someone on the other end of the phone line can hear what's being said. 10% more or less signal doesn't make any difference."
"This is starting to sound like a really good idea" said Luke, "but I'm feeling a bit daft calling the unit knots. We need to name them after someone famous, like Voltaire of Faraday".
"They've both been used already" said Fred. "How about Alexander Graham Bell, the inventor of the telephone?"
"You can't have a unit called a deci Alexander Graham Bell!" said Luke.
"No, call it the deciBel, you fool!" said Fred, who wasn't too good at spelling.
"Excellent!" said Luke. "Make it so!"
"I'm still not quite sure what happens in between the full knots... Bells I mean" said Fred. "How do we describe a cable that loses, say 50% of its signal per mile?"
"Ok" said Luke. "We'll draw up a table and try to fill in some gaps. We know where we are with 1, 10, 100, 1000. Just count the knots to find the bels, and multiply by 10 to turn that into decibels.
Know let's have a look at the multiples of 2 which fit in between 1 and 10. Now, 2 times 2 times 2 makes 8, which doesn't quite reach our first knot. Since another times 2, would take us to 16, which is quite a long way past, I'm going to call it knot 0.9 of the way to the first full knot or Bel. That makes it 9 decibels. Now, we know that each times-two is equally spaced, so the other two entries are easy to fill in. 3dB's and 6dB's. Now that we know how to handle times-two's, we can fill them in on the rest of the table.
We can also draw up a table for divisions. It's exactly the same as the multiplier table, but has a minus sign in front of the bell figures."
"Right" said Fred. "That's brilliant! We'd better alert the authorities to the existence of our new unit."
"Well, actually no" said Luke. "What we've been describing isn't actually a new unit at all. It's really just a different way of writing down a common or garden number, which turns multiplied and divided operations into add an subtract operations".
Fred was still clearly excited about having found a new branch of maths that will be useful to telephone engineers around the world. He said:
"Well, this is brilliant! It ought to be taught in schools instead of all that useless stuff that no one ever needs!"
But Luke sighed and said:
"I just realised... this is already taught in school. We all did it, except in those days it was called logarithms.
Having seen that decibels are actually just a logarithmic way of expressing simple numbers, let's just have a quick look at a couple of applications that should help clarify how they are used in the real world.
Firstly, sound.
You may have heard the loudness of sound quantified in decibels, for instance, a jet aircraft at 140dB's, or a quiet office of 40dB's. Now, we know that decibels are just numbers, so what are the units of sound?
Well, in this context, the unit that has been chosen is the quietest thing that an average person can hear. This sound level is called "A", and is written 0 dBA.
Remembering that 0dB is just the number 1, we could read this as 1 times louder than the quietest thing an average person can hear.
The onset of pain in a noisy environment is usually considered to be about 120 dBA. This means that a human ear can cope with sounds up to 12 knots louder than the quietest thing it can hear. That's a range of 1 million million. Pretty good sense of the human ear. Let's look after it.
Signals.
Now, Luke and Fred were investigating signal losses in cable. They found a loss of 90% per mile in their cable. Now, this is a factor of 10, each mile, which they ended up calling 1 Bel, or 10 decibels (dB) per mile. This is the correct way of expressing the loss. There are no units involved because a factor is just a pure number. However, when they started out, they were measuring milliwatts (1000mW = 30dBm). It's actually quite common to use decibels this way, especially when describing the input sensitivity or the output power of radio devices. The convention is marked by an "m", following the dB.
For instance, a standard Wi-Fi device for use in the wireless networking of computers might have an output power of 100mW, often expressed as 20dBm. this is convenient because when the losses associated with cables, and the gains associated with antennas need to be considered, there can be just added is, to find the actual radiated power and eventually the signal available at the receiver. This can be compared with the sensitivity of the receiving device to check that a particular system will work. This calculation is called a link budget and daily fair for anyone connecting buildings for computer networking using Wi-Fi.
If you need to know more about wireless networking and Link budgets, check out the relevant product sections and learning center on the Solarize website.
Thanks i found it useful
Thank you!
really excellent! Well done!
3:59 - I am confused - what is nought? Noughts=0 ?
Hi, nought is Zero. Sorry for the confusion.
@@solwise thank You for answer. I really admire You work and.. being honest it is hard to convey knowledge in a simple way - you did it masterfully!!!!
And that's how 'Right said Fred' started.
its 2018 and I still find this explanation as an amazing one! :D
"fred, who wasn't too good at spelling" thank you for making me laugh
At 5:40 I am confused what he means by "not .9" or "nought .9" of the way, how is the relationship between 8 and 10 related to 9 to 10 decibels?
I realise that it gets a bit vague here and one day I may do some more work on it to make it clearer.
The point I am trying to make is that once we have established a scale where each multiple of ten is equally spaced then we need to find some sub-divisions. I try to do this in a hand-waving way without doing any real maths.
Let's pick multiples of two and see what happens.: 1,2,4,8. That's three equally spaced steps to get from 1 to 'nearly' 10.
Now 9dB is also 'nearly' 10dB, ie three steps of 3dB gets us to 'nearly' 10dB so we can make an approximation and say that:
x2 ≈ 3dB , in words: "times two is about 3dB"
Actually log2 = 0.301029995 so "times 2" = 3.0102995dB so our guess was pretty close!
I hope this helps.
Thanks, I know the 10log conversion I just didn't get what he said. That cleared it up
Nought as in zero?
im still confuse..
Tin Matulin and you the most to call another one stupid
Excellent technique in making something quite tricky, easier to understand. There's a bit of the younger Sean Bean coming from the presenter.
Excellent! Helped a lot thx 😂👍🏼😂
Decibel unit well explained. Thank you !
At the end how many milliwats will find to the receiver??How calculate it from -80 dbm??
Generally you will find the sensitivity of a receiver expressed in dBm anyway so the question is somewhat moot. If a receiver says in can receive 10Mbps at -90dBm, then -80dBm of signal should work just fine.
However, since you ask, -80dBm is the same as -8Bm, or eight zeros (noughts) to the right, so 1x10^-8, I think that makes -80dBm = 0.000 000 01 mW (is that 10pW picoWatts?)
@@SolwiseMD I don't know,i am italian and had some difficult to understand for the language too!!
Can anyone help with this issue in Italian?
@@SolwiseMD I was meaning to the end of the video,when there is the example that a transmit start to 100 mW (20dbm) and to the end i will find -80 dbm.I was trying to understand how many mW i will find,maybe like you say i will find 10 pW.Wait if someone explain or agree.Thanks.
@@guevaradelaserna85 -80dBm = 1mW/100,000,000 it's a very small number!
waw understood really easily
Super explanation
Great video. It was always confusing to me hoe you could measure sound in a unit that isn't a unit, but now I know sound is measured in A's :)
...on a logarithmic (dB) scale.
Hhgff
good video..
sir can you explain me the words at 6:00
"Now that we know how to handle 'times twos', we can fill them in on the rest of the table."
aah brilliant : funny + helpful > thanks!
Luke and Fred line enginneers who work for Wallace & Grommit Telcom Inc.
Excellent! Very funny.
heart-warming as hell
a decibel is a music from AC(lightning bolt)DC.
wow just in time. Thanks!
Excellent!
HELP, why is a signal of 8 equals 9 dBs
Actually this is an approximation, just close enough. Each doubling of a value is represented as adding 'about 3dB'. So doubling 3 times (2x2x2) is multiplying by 8, adding 3dB three times (3+3+3) is 9dB. A closer aproximation is that doubling (x2.00000) equates to adding 3.01029995 dB (10 x log(2) ). Hope this helps.
Love the fictional engineers. I can empathize with the problem. pefect.
Thanks sir
Sir can you me the words
Who work for who Bro
Great video ,it's Volta ,not Voltaire
Fred & Luke are hopelessly slow engineers.
But we stand on the shoulders of giants you see.
'And here comes Thomas' :-)
this is a riot. i love red coats.
Love. it. To
so smart!
Luke should have used the force.
Your video should be compulsory for all 4th year Physics class.
I'm lost.... :(
Hi Ocky, We all have different learning styles and maybe my teaching style doesn't meet your needs. Although the feedback on this video is overwhelmingly positive I'm aware that it won't suit everone. Have a look at my Antenna Gain videos, for some people the approach in those is clearer.
this is lit
Oh, poor Luke. 😀
Ah damn. You had me up until noughts and then my ADD kicked in.
ok
It's 'deciBel' not 'Decibel".
I would prefer "its a logaritmic system" instead to having to hear 5 min of talk, and maybe "its called after Alexander Bell"... didnt know that!
confusing
wtf
@ Paul Stephenson Thank you for your answers in my messages but I accidentally erased all the messages....