Virgin Media - How Does it Work (Splinter thread) Is Digital Digital?

[Sephiroth]

QAM is analogue, and the bit streams are encoded into QAM channels. Pure digital / baseband has a spectral efficiency of 1 bit per Hz, 256QAM has an efficiency of 8 bits per Hz. No real competition.

Analogue TV is 8MHz per channel, digital TV is digital streams encoded into analogue for transport. Once the QAM has been decoded it will produce 1s and 0s.

http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation

This is a nice image.

https://www.cableforum.co.uk/images/local/2009/10/34.jpg

Each grouping in the lower right indicates 8 bits of data, there are 256 possible points in the constellation, 2 ^ 8 = 256. The signal that arrives at the decoder is actually 2 signals 90 degrees apart in phase, the decode splits them and decodes them, one into the horizontal axis on the constellation, one into the vertical axis, 16 possible values for each signal, that produces the 8 bits per Hz.

Out of respect for Digital Space and his thread at http://www.cableforum.co.uk/board/12/33656533-virgin-media-how-does-all-work-page-2.html#post34888326, I'm taking the deeper technical debate into a separate thread.

Broadbandings' Wikipaedia reference does not support his assertion that QAM is analogue. The first sentence in the article says:

Quadrature amplitude modulation (QAM) (pronounced /ˈkwɑːm/ or /ˈkæm/ or simply "Q-A-M") is both an analog and a digital modulation scheme.

The article goes on to describe the differences between the two types of modulation.

Now, it is a brave person who tells Broadbandings that he is totally wrong when he goes into technical depth.

So I'm not going to do that.

But I think that a lot of us are curious as to why, for a digital service, VM would use Analogue from the Head End to the modem (albeit a common view of the modem is to convert analogue to digital). Analogue is inefficient in terms of bandwidth - at least that's what we're taught.

The Wiki article refers to Quantized QAM and, from its description, I'm wondering whether or not this is the modulation used by VM.

So, why is what you say as you say? I'm puzzled.

[BenMcr]

Possibly because the DOCSIS standard for cable broadband was designed before the rollout of Digital TV services

Therefore it was designed to use analogue standards?

Just my guess

[Graham M]

My guess (and it is a complete guess) is that you can fit more digital signals encoded into an analogue waveform than you could fit into the available bandwidth if you just sent digital signals down the line, it multiplexes them together and is a more efficient way of encoding it? As I said just a guess :)

[Ignitionnet]

I'll post some more in response to this in a bit, but QAM on cable networks is digital I should have said, digital / baseband QAM being a bit of an anomaly and quite specific.

DOCSIS is from when digital cable was alive and well - it uses the same transport as digital cable, MPEG 2 frames with some bytes in the MPEG header indicating it's a DOCSIS frame.

Digital cable is also carried by QAM modulated waveforms, there is no baseband on your average cable network because, as mentioned above, it is spectrally inefficient.

Analogue TV requires 8MHz per channel according to the PAL standard we use, however a digital TV channel due to the MPEG compression can be crammed into as little as 2 - 4Mbit. This is then multiplexed into a 42Mbps transport stream along with other channels and this bit stream is then encoded into 2 analogue waveforms which are combined and used to modulate an 8MHz wide carrier.

To transport that 42Mbps using baseband / digital transport would require 42MHz, doing it by using QAM needs 8.

[Ignitionnet]

Have a look at http://www.cadcosystems.com/html/techdata/headend.pdf

Note the modulators, they are the things that package up the MPEG 2 streams into a series of QAM carriers for transport. If you check the lines going into them you'll see they receive baseband AV signals and pump out modulated RF.

The DOCSIS signal is combined with the TV signals a bit further along usually, at the combiner furthest down, next to the directional tap. This happens as the DOCSIS downstream is already modulated RF, either modulated within the CMTS itself or by an external QAM modulator in the case of the Cisco 10k, where a Gigabit Ethernet SPA (http://www.cisco.com/en/US/prod/collateral/modules/ps6267/product_data_sheet0900aecd804b424e.html) provides the baseband data which the External QAM (http://www.made-in-china.com/image/2f0j00fMsERugWVtVSM/CATV-Digital-QAM-Modulator-WDQ-3200-.jpg) then modulates into RF.

This is then transmitted using an amplitude modulated optical link.

The signal that leaves the headend / hubsite is digital information encoded into analogue, the signal that arrives in your home is digital encoded into analogue, the cable modem and set top box both demodulate the analogue signal to get the digital data out of it, and modulate digital data to transmit back to the hubsite or headend.

DSL and satellite are the same. Analogue transport of digital data.

The confusion I am guessing arises in that when people talk about analogue TV they are talking about your bog standard PAL signal, 8MHz of spectrum per channel. Everything going down the cable is analogue, just some of it happens to be modulated digital signals which get demodulated, some of it isn't :)

[altis]

Analogue TV uses an analogue signal to modulate an RF carrier.

Digital TV uses numerical values to modulate an RF carrier.

But as far as the transmission medium is concerned, both are RF signals - ie. analogue.

Don't know where you were taught that analogue is bandwidth inefficient but they were certainly wrong!

[Kymmy]

That's the nail on the head, RF in it's most efficient format is a sine wave, whether you use AM/FM/PM/SSB you're still transmitting a sine wave...

BUT is the cable transmissions sine wave or in essence a radio wave?? Or are they just pulsed binary transmitting at a desired frequency.. Just that you can send pure digital down a cable but NOT via true air as electromagnetic radiation... this needs an RF signal.. I think you're confusing the two

[Sephiroth]

Broadbandings and I sometimes read the same learned sources (e.g. http://cable360.net/ct/operations/techtalk/Digital-Transmission-Part-1_23792.html). Digital QAM modulation.

Digital modulation across the network is feasible starting with those new mother Motorola BSR 64000 CMTSs and ending with the right type of cable modem.

What appears to be true is that the broadband services < 50 Mbps are carried over the older infrastructure (soon if not now using the new CMTSs) which uses analogue modulation between head end and (I think) cable modem where ADC takes place.

For 50 Mbps, with different UBRs, are we still analogue between CMTS and UBR/Modem? For 200 Mbps?

[The-Darkside]

All broadcast digital TV is carried on analogue carriers. In order to send a digital signal (square wave) you need an infinite bandwidth from the frequency of the square wave upwards. Break a square wave up using a fourier transform (goes between the temporal domain (width timewise of the square wave) to frequency domain (the frequencies that combine together to produce the square wave) and its an infinite series. Coaxial cable has nothing like an infinite bandwidth. Reduce the bandwidth allowed to reconstruct the square wave and your square wave has round edges and a non flat peak.
In order to transmit a digital signal using coax or freespace, the phase of a carrier wave is varied. Simple satellite TV has only 4 positions available in the equivelent of the equalised constellation graph above. Using those 4 positions it can signal (i=0,q=0),( i=0,q=1) (i=1,q=1) or (i=1,q=0). So the phase of the received signal tells the receiver the values of i and q (1's or 0's), this means two bits of data can be sent for each symbol.
Cable is meant to be relatively noise free, so a more complicated method can be used with a far greater number of phase changes available. But the theory is the same, calculate the phase and work out the values of i and q, but due to the greater choice of positions in the constellation graph (256 for qam256) its now possible to send 8 bits per symbol.

So its an analogue carrier whose phase is varied to represent digital values.

[Sephiroth]

Fine. All cleared up now.

Thanks.

[The-Darkside]

Broadbandings and I sometimes read the same learned sources (e.g. http://cable360.net/ct/operations/techtalk/Digital-Transmission-Part-1_23792.html). Digital QAM modulation.

Digital modulation across the network is feasible starting with those new mother Motorola BSR 64000 CMTSs and ending with the right type of cable modem.

What appears to be true is that the broadband services < 50 Mbps are carried over the older infrastructure (soon if not now using the new CMTSs) which uses analogue modulation between head end and (I think) cable modem where ADC takes place.

For 50 Mbps, with different UBRs, are we still analogue between CMTS and UBR/Modem? For 200 Mbps?

All signals between the Modem and CMTS are analogue carriers. To get the higher bitrates DOCSIS 3 simply uses channel bonding, meaning a number of 8MHz channels are used at the same time.

[Ignitionnet]

For 50 Mbps, with different UBRs, are we still analogue between CMTS and UBR/Modem? For 200 Mbps?

Same holds true as before, some cable networks in some VM areas have about 500MHz of downstream bandwidth to carry everything, internet and all TV. To do 200Mbps baseband would require a minimum of 200MHz of bandwidth. Using 256QAM, which gives 8 bits per symbol, and EuroDOCSIS which has a symbol rate of 6.952Msym/s in each 8MHz channel that same 200Mbps can be delivered using 32MHz.

6.952Msym/s * 8 bits per symbol = 55.616Mbps * 4 channels = 222.464Mbps.

Take off 10% for overheads leaves about 200Mbps of IP throughput.

[REM]

Do you think this would all sound like complete fantasy in the days when a 75/1200 baud modem was the cutting edge technology? :)