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wellnot Oh kitties and curious adults here we are with a Noah some tutorial rimmel cat time division multiplexing here now it's an advanced subject but I'm going to stay at a fairly introductory level now those of you who purchased my learning package consisting of the SC 500 kit snap circuits and the digital multimeter analog multimeter and the CD with all of the lessons Katie - lessons and the extra stuff this is from the extra stuff that's on that CD so TDM time use MUX what is that how does it work well even though this is introductory there are some background that you really should have and that would be understanding what a codec is you know what a codec is well if you don't I'll show you a little bit about it but do you really need more information that I'm going to show you here you also need to have some understanding of data streams so you might want to go look up some of those tutorials alright here we go this is from Kidd tricity - Katie - lesson number eight page 25 time revision bucks oldest and least complicated a serial or sequential way of putting multiple input streams onto a single output path that's what a MUX does puts many things onto one thing in this example I'm showing you here this is a telephone system the oldest ones there is in digital time division multiplexing it's known as T type carrier here in the United States so what I've done now is take a analog voice input right here voltage variation coming from a microphone put it into a codec a coder decoder converted it into digital right here an 8-bit byte representing periodic samples of this voice now this is the telephone system that I'm dealing with here so you need to know a couple of basic pieces the first is that this is always been limited to no more than four kilohertz that's because standard telephone lines always handle the voice at four kilohertz or less so the wizard to figure this out guy named Nyquist says you must sample the incoming analog frequency at least twice the highest rate or highest frequency of that signal so if we got a four kilohertz max coming in Nyquist says twice that the codex will sample at eight thousand times per second each one of the samples then will be turned into an 8-bit digital code representing that sample I'll show you this more detail in the next slide so now I've created an 8-bit sample here every 125 microseconds I take that 8-bit sample and this is where the time division multiplexing sequencer comes into play it takes this 8-bit puts it out here on this shared facility right here's the eight bits that were this guy's then I take the next codex 8-bit sample and I stick it on the line then the next one and I stick it on the line I've only shown three but there are really twenty four in this T type carrier system that we're going to be using as their example so if I'm doing something 8,000 times per second which I am back here that means I'm doing it every 125 microseconds so that's telling them that out here on the transmission facility each one of these channels 24 channels will gain access to that transmission facility every hundred twenty-five microseconds that's the way it works right so think of it this way now I've got an 8-bit byte that's 25 microseconds long here but I have to squish it down in time out here so that that eight bits now occurs in about five micro seconds rather than 125 microseconds you okay with that so effectively I squished the bits down in time so they're much shorter duration of on-off pulses or volts on volts off or high volts or low volts whatever the system happens to be working on and that's a little bit too much detail for our little tutorial here so let's move on to look at a little bit more detail on the codec so here's my encoding step sample quantized encode right so here's my sample right here little window opens up checks the voltage on the wire from the microphone is that little pulse of voltage over to a measuring device gets a number takes that number converts it into an 8-bit code that represents that number now normally in the CD that this stuff came from this slides over here so you'd be able to see the whole bit stream here the eight bits so that number value here - 125 or whatever it was I can't remember is coded in this bit sequence right here 8 bits long so this is the American tea system that we're looking at now and to make sure you understand that so I'm creating now an 8-bit byte right here that represents this instantaneous sample and they're going to be 8,000 of these samples coming in here so each one of the 8,000 is measured and an 8-bit byte is put out for each 8,000 per second sample so if you do this counting here you'll find that we have 8 bits per sample and we have 8,000 samples per second because Nyquist 8000 samples per second so we're going to create a signal known as ds0 digital signal level 0 it's usually called DSO even though that's a zero they call DSO that's a 64 kilobits signal per channel right so the next page will show you in the 24 channel or the American T type system ds1 serial bit stream because we're going to enter mean 20 for these guys 24 of these DSOs or 24 of these 64 kilobit signals to create what's known as a T type carrier system now I hate to burst anybody's bubble because I've seen this all over the place but I've been in the industry since 1961 started working on T type carrier systems about 1965 I've worked on hundreds hundreds hundreds of carrier systems in a telephone company all of them had letters on down there earliest ones back from the 1920s and 1930s started with like the letter B and C and like that I started working on these things when we were working on o n N 1 n 2 & 3 kay carrier work lots of K carrier all these just sounds letters the telephone company Ma Bell always name them with a letter well when they finally an abandoned digital carrier systems time division multiplexing next letter in the alphabet oh hey let's use the letter T so it doesn't stand for terrestrial it doesn't dam for twisted-pair doesn't stand for any other baloney that you see floating around out there it's just the next letter in the alphabet so there you go sorry to burst your bubble if you thought it was something else so I've got a digital carrier system a tea type carrier system I'm going to do this I'm going to interleave 24 of the codec outputs and stick them on out here on the line so I've got 125 microseconds for this 8-bit byte from channel 1 for that single sample but I've got to squish it down here at a time because if I got 125 here I don't have 125 for here because I got to share it inside that 125 window right here so here's channel 1 squished down channel 2 squished down in time channel 3 blah blah blah like that so I end up with 24 time slots or channel slots of 8 bits each which gives me a hundred two bits from here to here 24 channels is on there 92 bits but I'm going to add a frame bit right here which makes 193 bits per frame or per rotation now remember they're 8,000 rotations per second here right so I've got 8,000 frames per second rotations of 193 bits so 8,000 times right 8,000 times 193 bits gives me one point five four four million bits per frame or per rotation it's also known as a ds1 digital signal level one one point five four four million bits in T type systems is ds1 right but I didn't change the bit pattern by squishing it down time I just made it shorter in duration alright so here this one I've been trying to fix for years you can't do it so but I'll point out to you there is no such thing as high speed data the word speed means velocity how far something is moving that doesn't mean this at all here's our tea type carrier system letter doesn't stand for anything it's just tea so here's our 24 channel rotations right here right this little arm is going to come around here and touch these little buttons it's going to rotate 8,000 times per second because that's what Nyquist says you have to do for voice for killer's voice so here's a channel - it's the 8 bits from the coder process right here this arm picks up those 8 bits and sends them out right there in serial then it moves to this one sends out those 8 bits then it moves to this one sends out those 8 bits so it's serializes these 24 channels right here they're bitrate remember was 24 time slots of 8 bits each +1 frame bit that's the red guy right here gives us a hundred 9 3 bits per frame there's 8,000 frames per second because this is rotating 8,000 times per second gives us 1.544 megabit rate that's the rate of the bits per second look at this this is the e carrier European carrier it has 32 time slots all right you know that computer geeks got involved here because the time slot starts with 0 it's a dumbest thing I ever seen from a hardware perspective I can tell you how confusing this gets trying to tell somebody which piece of hardware to go look at starting counting to zero incredibly dumb but it's ok for computers works fine so we got 32 time slots here but these 32 times lots of 8 bits each also have to fit within this rotation this little arm is rotating 8,000 times per second its Nyquist so over here we got 32 time slots times 8 bits equals 256 bits per frame 8,000 frames per second same as over here gives us 204 8 megabits this is the rate not the speed these little bits over here are not physically moving down the wire any faster than these you can't speed them up you can increase the rate by decreasing the time for any individual bit but you can't make the it's the voltage pulses go faster stupid but you can't fix this so don't worry about it okay so this now is an exercise for you to go through we won't have time on my little video right here but this will allow you to be a digital receiver a digital demultiplexer because this is an example of how you can figure out where to start counting a particular set of bits for a particular time slot or channel so go ahead and work through this I think it's fairly self-explanatory I know that kids in a class that I've used this on many many many times over the years they figured it out although it sometimes took a few times to go back and forth and back before so I'm going to flip the slides here okay you got that one you might have to put your video on hold draw boxes on a little piece of paper like this then do this with that little piece of paper with the boxes you just did then do this and you should be able to figure out where to start counting the frame then here is the answer I'm going to give it to you by counting one two three so you can stop the video to try this okay all right here we go stop the video at one two three and here's the answers okay I'll see you on the next tutorial