A friend and I got into a debate about what a 'digital signal, sent over-the-air, as in when receiving OTA HDTV from broadcast towers' appears like electrically. Pardon me if this isn't the best place to ask but it does apply to music too and I thought solid state amplifiers would be suitable.
The friend said that the 'digital signal', where his understanding was that the digital signal was that of a square wave, was 'perfect' and meaning in its 'squareness and exactly SHARP corners'. The signal was never, ever 'in-between' and only always exactly electrically representing a '0' or a '1'.
I argued that there is no way possible to create an absolutely perfect square wave signal, which would represent 0's and 1's, because there is always a bit of overshoot on the, say, on the 'way up', and then some over compensation resulting in a bit of undershoot, and that repeats but that signal does quickly 'level off' at the 'flat top' which is supposed to be there.
My argument was based on the fact that NOTHING, except only in the realm of mathematics perhaps, when in the real world, can go from say '0 volts to 1 volt' instantaneously and without any over and/or undershoot before it settles to its 'correct/wanted' amount/voltage/etc. So, therefore, my argument was that even broadcast 'square waves' cannot possibly be 'perfectly digital' (in his vision of the signal) which he thought was so.
And, furthermore, there isn't anything such as a 'digital antenna' which are named that and sold with those words. Maybe that was his problem with me arguing the topic because he had bought a whole new $100+ 'digital antenna' when then I showed him a few minutes later on that a simple piece of wire could receive the signal just fine as it did with the former NTSC broadcast signal (which everyone called 'analog television' or something like that).
Which of us is correct? I am curious.
Thanks!
The friend said that the 'digital signal', where his understanding was that the digital signal was that of a square wave, was 'perfect' and meaning in its 'squareness and exactly SHARP corners'. The signal was never, ever 'in-between' and only always exactly electrically representing a '0' or a '1'.
I argued that there is no way possible to create an absolutely perfect square wave signal, which would represent 0's and 1's, because there is always a bit of overshoot on the, say, on the 'way up', and then some over compensation resulting in a bit of undershoot, and that repeats but that signal does quickly 'level off' at the 'flat top' which is supposed to be there.
My argument was based on the fact that NOTHING, except only in the realm of mathematics perhaps, when in the real world, can go from say '0 volts to 1 volt' instantaneously and without any over and/or undershoot before it settles to its 'correct/wanted' amount/voltage/etc. So, therefore, my argument was that even broadcast 'square waves' cannot possibly be 'perfectly digital' (in his vision of the signal) which he thought was so.
And, furthermore, there isn't anything such as a 'digital antenna' which are named that and sold with those words. Maybe that was his problem with me arguing the topic because he had bought a whole new $100+ 'digital antenna' when then I showed him a few minutes later on that a simple piece of wire could receive the signal just fine as it did with the former NTSC broadcast signal (which everyone called 'analog television' or something like that).
Which of us is correct? I am curious.
Thanks!
Maybe it's possible in the purely digital realm, but as soon as it's converted to analog you will hit the resolution limits of the system.
Speaking semantically, rather than as an engineer, I think the concept of a digital signal is the expectation that the digital information can be reconstructed exactly, even after being carried / modulated on an analogue medium such as radio waves or conducting wires, where - as you say - the actual waveform is analogue.
To get perfect square waves you need a circuit infinite band width and infinite slew rate. Also a capacitanceless and inductanceless circuit what clearly is impossible to meet.
Not exact at all but precise enough to receive and distinguish between 0 and 1 in 999999 out of 1000000 bits sent. And as there are bits sent to be able to do checksum and repair it is in reality no problems. Your friend needs to check again - you are correct. I spent 30 years in telecom / radio network development.
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The transmission of so called 'Digital' TV and radio is still actually very analogue in nature. The resulting data we can retrieve from the transmission is effectively a data stream but it gets there by analogue techniques modulating and phase shifting the 'analogue' carrier/s.
Look up Quadrature Modulation, Phase Shift Keying and Frequency Shift Keying. These are all techniques to transmit digital (1's and 0's) via analogue carriers.
The digital 'cliff edge' we experience when the (analogue) carrier signal becomes to weak is still real, either there is enough (analogue) carrier signal to receive and demodulate or there isn't. Unlike pure analogue demodulation such as FM or AM where the signal just gets progressively worse and more noisy, the digital receiver suddenly finds the incoming data has become meaningless as the (analogue) carrier falls below the reception threshold and so it either mutes or displays/reproduces garbage.
In practice a 'digital' system is more robust and will work properly down to lower received carrier levels than an analogue system simply because the data can still be extracted cleanly from the noisier low level received signal... up to a point... and then it all falls apart.
Impossible.
In real life, digital signals are dirty. Yet they can convey signals without error with these "dirty" signals.
In real life, digital signals are dirty. Yet they can convey signals without error with these "dirty" signals.
A perfect transition from 0 to 1 or vice versa would mean that it happens in no time. Even the smallest delay would mean the signal is not square any more...
Digital Broadcast TV uses a scheme utilising I/Q modulation/demodulation techniques. DVB uses Quadrature amplitude modulation (QAM) up to 4096QAM giving 12bits per symbol. Essentially its an analogue modulation scheme, and no it doesnt use square waves.
WiFi uses the same techniques, the same with Microwave RF links and high capacity data like HFC Cable networks.
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To expand on it: "Digital" signals are so, and depend on almost perfect squarewaves or pulses, because they are all the time Reconstructing the squarewave.
Not even thinking about it, just doing it at every stage.
Since early TTL times, when "everything above 3V was Logic 1, everything below 2V was Logic 0" ... not even need for Schmitt triggers or anything, the system worked that way down to the humblest component.
As others above are indicating, your friend is incorrect. Digital signal transmission does not require perfectly sharp waveform edges. Requiring such, would make digital the opposite of the robust (error resistant) signaling paradigm which it is. The advantage of digital communication, processing or storage is that they, as said above, reject noise. This inherent noise-rejection ability is not due to any noise-cancelling circuit as such, it's instead due to how the signal is coded and interpreted. In other words, with how the signal is interpreted as conveying some limited set of discrete values, rather than some range of continuous values.
There are digital signaling systems which feature far more than the basic 2-levels of binary. Especially with HDTV transmission, where there are at least 256 discrete levels. However, for simplicity, if we look at one of the old signal definitions utilized in digital logic signaling, a logic-1 digit was arbitrarily defined as any signal level above +2.4 volts, while a logic-0 was arbitrarily defined as any signal level below +0.7 volts. The real significance of those definitions is that any signal voltage between +0.7 and +2.4 volts is inherently interpreted as invalid (noise), and so is ignored. It doesn't matter to digit interpretation whether the transition edges between those only two valid signal levels is square shaped, or sine shaped, or what have you. The transition region is essentially being ignored anyhow by defined interpretation. What more rapid level transitions do afford is the ability to signal bits at faster rates, but otherwise plays no role in determining whether a bit is a one, or a zero.
There are digital signaling systems which feature far more than the basic 2-levels of binary. Especially with HDTV transmission, where there are at least 256 discrete levels. However, for simplicity, if we look at one of the old signal definitions utilized in digital logic signaling, a logic-1 digit was arbitrarily defined as any signal level above +2.4 volts, while a logic-0 was arbitrarily defined as any signal level below +0.7 volts. The real significance of those definitions is that any signal voltage between +0.7 and +2.4 volts is inherently interpreted as invalid (noise), and so is ignored. It doesn't matter to digit interpretation whether the transition edges between those only two valid signal levels is square shaped, or sine shaped, or what have you. The transition region is essentially being ignored anyhow by defined interpretation. What more rapid level transitions do afford is the ability to signal bits at faster rates, but otherwise plays no role in determining whether a bit is a one, or a zero.
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I still design and build TTL logic control and protection circuits. They work fine.
I still work pretty well too, even though I've been around all the way back to the 1950s.
For that matter, I just fired up my parent's old "hi-fi" tabletop radio from the 1950s. No silicon, no logic, no software subscriptions to expire. It still sounds good! It sat on top of the refrigerator. I woke up to that radio every day for 17 years.
I still work pretty well too, even though I've been around all the way back to the 1950s.
For that matter, I just fired up my parent's old "hi-fi" tabletop radio from the 1950s. No silicon, no logic, no software subscriptions to expire. It still sounds good! It sat on top of the refrigerator. I woke up to that radio every day for 17 years.
Most people will never accept this, because marketing liars have poisoned their brains with drivel. Stupid people make good customers.
Well said! Whatever waveform is used for digital communication (and there are a great many different ones used), it need not be "perfect" or ideal. It only needs to be good enough so that the receiver can correctly decode it back into the original data stream with an insignificant number of errors.
Very little in digital communications still involves "square waves" anymore. Your computer PCIe bus uses 10 or 130 bit serial chirps. See https://en.wikipedia.org/wiki/PCI_Express#History and revisions
ATSC TV in the USA uses 8VSB. See https://en.wikipedia.org/wiki/8VSB
All these things keep evolving and new and improved methods appear quietly behind the scenes, especially cell phone systems like 5G (and 6G).
Some may not understand that any signal exists forever, from the infinite past to the infinite future, and a "pulse" is simply where the sidebands come into phase. The same is probably true of mass.
ATSC TV in the USA uses 8VSB. See https://en.wikipedia.org/wiki/8VSB
All these things keep evolving and new and improved methods appear quietly behind the scenes, especially cell phone systems like 5G (and 6G).
Some may not understand that any signal exists forever, from the infinite past to the infinite future, and a "pulse" is simply where the sidebands come into phase. The same is probably true of mass.
Thank you! That is one of the other things I tried to say and was told "NO WAY", heh.