Episode 815 – Pulse Code Modulation

This time on the show we get our phone phreaking on. No, not free long
distance, rather we dive into the world of signal modulation, coding
and digital signalling.

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  • Chris

    I got a question to this voltage thing.
    I understand when your saying 2 following 1-Bits gets to +3 and directly to -3, because when the signal is degrading you cant recognize them as 2 bits.
    But whats about 2 following 0-Bits?
    How they get recognized after a long distance?
    Greetings from austria

    • Quantum

      I think the system being used is On-Off-Keying (OOK). The case of 2 or more zero bits I believe could be calculated by using the pulse width of the set bits. The duration of zero bits divided by the average pulse width of the set bits would give you the number of zero bits.

    • molys

      i don’t watch this video, but i think i can answer you question.

      usually each interface has a timer and knows the bit rate. so it can count the bits (also 2 and more following 0-bits).

      some systems sends now and again alternate 1 and 0 bits (10101010101…) to synchronize the timers. other systems sends 101010101 before or after a message.

      also there are specific codes which avoids 2 same bits in succession. they also can correct some transmission errors.

      i can’t say some about the distance only that each system have a specific max. distance.

      I hope this can help you.

      I should watch the clip to better answer you question. but 30min is a lot of time :/

      • flashdrive

        I’m not 100% sure about the timer synchronization you speak of, but there are defintely clocks involved. In the next episode, he hints at the maximum run length constraint, which is used to make sure the clocks are synchronized and that it is looking for the voltage at the correct point in time for that signal run.

    • Ryan

      Not many electronic engineers in the hak5 crowd then?

      At the receiver you will remove the message signal from the carrier by demodulating the carrier frequency, so you’re left with the encoded bit message (see 21:52). This message signal is then decoded with a clock frequency of the same as used at the receiver.

      I’m thinking back 4 years ago when I lost touched on this method.
      It’s an old technique that is rarely used in modern communications systems, as QAM and PSK amongst others are more suited. Then combining them with OFDM, forward error correction and puncturing techniques allow more carriers on a small bandwidth with robust error protection.

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  • B362

    Rectifier: Converts AC to DC.
    Repeater: Amplifies the degraded signal received (noise and all) and puts it back on the line.
    Regenerator: Converts the degraded signal received back to a clean digital signal and puts it back on the line.

  • Dude

    Is your digital signal right? You say the analog signal gets converted to a series of numbers which are between 0 and 255 each, then this gets converted to binary and send, so then there is only ON and OFF. So you kind of drew the lower resolution anaglog curve that reaches the traget, in transmission the binary steps don’t form a curve, right?. Its just a stream of on and off.

    Very interesting stuff though!


  • ATT-Tween

    Wow does all this bring back memories! Multi-plexing / digital stuff.

    I was doing the same stuff… but, I had an insider that brought all the proprietary books to me… It was my father!

    AT&T Bell Labs in Chicago… and it’s the 5ESS (not ESS5)

    I learned Unix System V (from Bell Labs of course) on a 3b20..
    (Can you believe they gave me an account to it at Bell Labs… AND YES it was like ENCOM (tron referrence) inside that place… Lots of rack-level-workspaces… neat and whacky stuff…) This was when I was a tween propeller-head.. BBS’s and … FREE PHONE calls… compliments of Ma Bell.

    Too bad we outsource all our innovations now… 🙁

    Thanks for the retro!

  • Tom Knudsen

    Here is an easy way to convert from DEC to BIN

    In the segment example we have the number 173 that we want to convert to binary

    ok lets devide and here is the goody feature, for very result that has a ,digit answer such as 3,5 the binary code is 1 if the answer comes out with e.g 4 the digit number will be 0


    173:2 = 86,5 (answer has a leftover ,5) = 1
    86:2 = 43 (answer does not have a leftover) = 01
    43:2 = 21,5 (answer has a leftover ,5) = 101
    21:2 = 10,5 (answer has a leftover ,5) = 1101
    10:2 = 5 (answer does not have a leftover) = 01101
    5:2 = 2,5 (answer has a leftover ,5) = 101101
    2:2 = 1 (answer does not have leftover) = 0101101
    1:2 = 0,5 (answer has a leftover ,5) = 10101101
    0:2 = 0


    Se the answer on the right?

    = 10101101 is the correct binary for 173

    Fun right? Well you could also use a calculator, DEC to BIN converter or something like that, but hey, where is the fun in that 🙂

  • FatSean

    I’m just curious if a stream has to start at the beginning in order to keep track of the Timing bit thing. If it’s just one positive pulse, and the processor didn’t start at the very beginning of the 24 channels, it would seem that it could be confused as the beginning or end of a channel’s bit stream.

    Is the Timing bit unique in some way, so that the processor can “repickup” the timing after some kind of failure?

  • Zinahe

    Thanks so much.

    It brought back all the good memories from my EE college years. I say we need more of these. But I suggest you use more stuff other than just a whiteboard such as DSO or even cool animations. I think this series can grow in to a book that you guys can market.

    I’d definitely be the first one to buy it.


    Zinahe A.

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