They could well both be right on different lines. One has old-fashioned copper from the DSLAM in the exchange to the street box, the other has fibre to a DSLAM in the box. When they finally roll out fibre to the home, then it'll be digital all the way. Inside your current Modem/Router is an analogue line driver, though.
The feed into your house consists of a number of modulated narrowband HF carriers, which are phase and/ or amplitude modulated, I can't remember which, and an analogue baseband signal for your normal phone.
Basically the HF band from 10Khz up to about 1.5Mhz is split onto bands, each one of which has a (suppressed? ) carrier with some weird modulation imposed. Not all are usable, and these are discarded.
IIRC some adjustment of bit rate on each is also possible.
The broadband bit comes from the fact its not a single carrier frequency.
Or did. Like modem now means 'something between a computer and an exchange' it now means 'high speed internet'
I get the impression someone is losing the argument and is getting desperate. You can remind him that ultimately there is no such thing as analogue as it comes down to discreet levels in quantum physics. That is if you believe the theory of quantum physics. He won't know I have said this as he likes to add people that disagree with him to his killfile.
Of an analogue carrier. The signal on the line is *not* a simple on/off, it uses a combination of phase and amplitude modulation of multiple carriers. What controls the modulation is digital, the modulated carrier is analogue.
To aid in your understanding, *all* signals passing along the local loop (AKA the "Last mile") are analogue.
Digital signals are transmitted by encoding onto a carrier, and then decoded by comparing the transmitted signal with a stable reference, as in a FM or AM radio set. The output from this decoder to your computer is digital, the input to the decoder from the line is analogue. In the days of 75 baud modems, the digital data was transmitted by switching an audio tone on and off, and later, the tone was switched between two frequencies, which allowed faster transmission. Later still, advances in design and manufacture permitted the use of combinations of level and phase variation of the carrier to allow the transmission of more than one bit of data per cycle of carrier. Broadband basically just uses a number of these phase and amplitude modulated carrier signals coming down the same line.
Now, either you're trying to wind me up, or you *really* don't understand the difference between the analogue and digital parts of a modem. If the latter, then I'd suggest studying the Wiki reference given, and follow the links in the article to the bits you don't understand. You might also like to study amplitude, frequency and phase modulation schemes for radio transmission.
I draw it at the point where the digital data is turned into an analogue signal by the modulator and vice versa.
Though, I might *call* it digital all the way through, if I were talking to someone who doesn't know a lot about it......
To draw a (Maybe not perfect) analogy, the "digital" sound recording I made the other day was analogue from the microphone up to the point where the signal met the ADC, then it was digital all through the chain until it met the DAC just before the amplifier. Then it was analogue again. If I published it, it would be classified as DDD on a CD, as all the recording, editing and distribution was done in the digital domain. Both ends of the reproduction chain must be analogue, though. Unless you've got digital vocal chords and ears. It's still *called* a digital recording, though.
Exactly so. xDSL uses discrete multi-tone modulation (DMT), a variation on the theme of OFDM used for digital radio and TV. These are undoubtedly *digital* modulation schemes.
Of a large number of (suppressed) carriers, in fact. The resulting electrical signal has a time variation of amplitude that looks similar to white Gaussian noise, but you shouldn't think of it as being analogue. The amplitude is not a direct representation of a continuously varying quantity an the way that the analogue speech signal on the line represents sound pressure.
So what? A digital representation doesn't have to be binary. Digital data - that could, for example, represent an audio signal after sampling and A/D conversion - is frequently transmitted over wires or radio channels as a sequence of 'symbols' each symbol representing one *or more* bits. Increasing the number of bits per symbol generally allows more data to be sent over a given spectral bandwidth but the signal remains digital - each symbol represents one of a finite number of discrete states.
The speech signal is analogue. The xDSL signal is digital.
Both sides of a modem can be digital.
And you might like to try these:
formatting link
last one's interesting. Like you it starts by referring to an "analog carrier", but then goes on to say:
"According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission."
No, the phase and amplitude of the (suppressed) analogue carrier compared to a reference are controlled by the digital signal. Taking the simplest case, is an FM or AM radio signal modulated by a square wave analogue or digital?
I know about that. It's how a 56Kbps modem gets its data along a phone line that can't carry an audio frequency of more than about 4KHz. The more steps you can distinguish the faster the data can be transferred, relative to the bandwidth. The more steps you try to distiguish, the greater the error rate. The faster you transit, the greater the error rate. The Nyquist limit dictates how much information you can shift through a particular link.
? MODulator/ DEModulator implies an analogue carrier in the transmission medium. If both sides are digital, then surely it's just a line driver?
Thanks, I have.
The last two articles both mention conversion of digital data to and from analogue form for transmission.
"In digital modulation, an analog carrier signal is modulated by a digital bit stream" is a quote from the first paragraph of the Digital Modulation article.
The discrete multitone modulation article block diagram shows an ADC/ DAC pair in the chain, before and after the transmission medium. Part of the DAC function is to convert inexact analogue levels in the transmission into steps for digital decoding.
I would say that the *signal* on the line is analogue, with the levels partly dictated by analogue mechanisms such as AGC and interference, while the information transmitted is digital.
I suspect we're looking at the same thing from different angles.
Several symbols (carriers) are usually used simultaneusly (together with the baseband audio signal) The signal is analogue -- any amplification etc. has to be (analogue) linear or the quality (BER) is reduced by crosstalk and intermodulation.
It may represent binary digits, but it must be treated as an analogue signal.
In my book an "interface" can have more than one physical connection, each phyical connection is a "port", as in the ports of an interface card in a PABX or comms matrix.
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