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How can digital be more spectrum efficient than analog ?

It's common knowledge that digital technology gives more telephone [4 Khz wide] channels than analog technology - for the same spectrum width. This even with the steep-sided filters and SSB technology.

Since the input and output need to be in analog form, I find digital's claim very unintuitive -- almost doubting it.

Can someone offer an intuitive explanation ?

I guess a 4 HHz bandwidth telco signal contains a lot of redundancy which is not needed to convey the inteligence ?

Speech can even be transmitted as a sequence of 'predetermined sylables' - what are they called again ?

So it seems to me that the improvement over analog methods can't be acheived merely by 'modeming', which would only add entropy. But once it's in the digital format, standard compression [by removing redundancy] can make for more efficiency ?

Thanks for any info,

== Chris Glur.

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Lot of information is already on line on the topic; Goggle for it.

See

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Should get you up to speed so you can search intelligently.

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Reply to
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I disagree with what you say is "common Knowledge." What is your reference for this? Digital transmission provides a much better grade channel WRT noise, but at the cost of bandwidth.

Telecommunication systems put 24 SSB nominal 4 kHz circuits in 96 kHz. The D4 digital systems require a 1.544 Mbit/s channel for their 24 nominal 4 kHz channels.

Speaking for Digital for the moment, "Digital" has not claimed any such ridiculous thing.

I guess I wouldn't guess about important things; I'd do some study.

What were they called the first time? What's the relevance to your main issues?

Don

Reply to
Don Bowey

Compression is one. The type of multiplexing could be another. It appears that you're assuming the analog approach using frequency multiplexing where each channel uses up 4kHz of bandwidth. However, other multiplexing methods are available with a digital stream. Example: N channels of voice might be time domain multiplexed and silent passages not sent at all in favor of serving other channels, etc. etc. The possibilities are clear enough - I don't know how they do it.... One can well trade bandwidth for computing load and fidelity *up to a point*. More computing load (coding/decoding), more channels in the same bandwidth. Less fidelity, more channels in the same bandwidth.

Fred

Reply to
Fred Marshall

This can be a difficult subject. One place to start is to investigate quadrature amplitude modulation (QUAM) and learn how several binary numbers can be encoded into each chunk of a transmission stream.

Reply to
Charles Schuler

Or look at the required SNR fro audio and the required SNR for a GMSK or BPSK signal. And then take into account the influence of neigbouring cells.

Thomas

Reply to
Zak

QAM can be entirely analog. Digital signals are more tolerant of moderate amounts of noise, but that's another (long) story.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
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Reply to
Jerry Avins

Comparing bandwidth to bit rate is nonsense! 96 KHz does not compare to 1.5 Mbps in any productive way.

Keep in mind that a telco voice channel is 4 KHz analog, but over the same bandwidth one can easily send a 64 Kbps digital PAM signal (i.e., v.90) using the exact same codec in the telephone switch line card.

Bandwidth does not use more bitrate, it provides more!

Shannon showed channel capacity to be relative to both bandwidth and signal-to-noise ratio. Anything you do to change one

*requires* a change in the other to maintain the same maximum capacity. Different carrier systems do exactly that, balancing between SNR and bandwidth.

Bandwidth is not what makes digital more effective than analog. Above a minimum SNR digital systems are essentially error free while analog system have additive noise. For that reason digital is usually prefered. The second most significant reason is the ease with which digital carrier systems can be designed to trade SNR for bandwidth by using multi-level encoding, which means well matched adaptions to anything from very noisy fiber optics to very quiet coax cable.

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Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

It is quite common any where that bandwidth is at a premium. Satellite circuits are the most obvious, but there are others.

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

Hmm. Your statement reminds me of getting full-power transfer by matching impedances. Could information transfer be viewed similarly, i.e., by matching the "impedance" of the transmitter to the "impedance" of the channel?

--
%  Randy Yates                  % "Remember the good old 1980's, when 
%% Fuquay-Varina, NC            %  things were so uncomplicated?"
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Reply to
Randy Yates

I doubt it. You get as much power as you can when you match the load to the source, but you get it at 50% efficiency.

Consider a 12V battery with a .01-ohm internal resistance. With There is

6 volts across a matched load, and the total power power is 7.2 KW divided equally between the load and the battery. (For a short while, anyway.) Moral: matching load to source is a good way to avoid reflections. Matching source to load is a poor way to deliver power.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
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Reply to
Jerry Avins

Yes. That is an interesting comparison.

That is *not* an impedance match. It discusses DC resistance, not impedances. It is not a valid desciption of what happens with matched vs. unmatched imedpances, and does not compare well with what was being discussed.

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

Impedances can be pure resistances. You can easily show that a generator with complex impedance Z will deliver maximum power into a load of impedance Z*, where Z and Z* are complex conjugates.

Early dynamos were so inefficient that it became common laboratory practice to match the load to them. The electrical load on a dynamo had only slight influence on the load on its prime mover. Dynamos became more powerful and efficient over time, but the practice was retained, and they remained laboratory instruments. Several learned papers showed that wide distribution of electric power was infeasible because the losses in the distribution system far exceeded the deliverable energy.

Thomas Edison's first great insight into generation and distribution amounted to discarding the "matched generator" mindset, and that was, to put it aptly, the electrifying departure from received wisdom that made central generating stations possible. (The equalizer bus, which enabled parallel operation of overcompounded -- hence negative-resistance -- dynamos was another example of his genius, but a mere implementation detail.)

To get maximum power from a given generator if you dare, match the load to it. To best use energy from the prime source -- steam engine or chemical cell -- make the generator impedance as small as possible.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
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Reply to
Jerry Avins

That is not the point. With DC there is no such thing as impedance. The above discusses DC, it is *not* discussing impedance matching.

DC load matching and AC impedance matching are not the same...

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com
Reply to
Floyd L. Davidson

Would you care to defend that erroneous statement?

Reply to
Richard Owlett

I'm sure that you have something in mind with at least a germ of validity. Instead of my asking questions based on guesswork, it might be simpler and quicker if you simply explained why you think not.

A 100 kilowatt generator and a 100 kilowatt alternator are surely different, but the methods of matching their loads don't differ much. The criteria for matching power and communications loads differ, but the principles are the same for both. Ohm's law and all that.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

See what I started? :)

--
%  Randy Yates                  % "Ticket to the moon, flight leaves here today 
%% Fuquay-Varina, NC            %  from Satellite 2"
%%% 919-577-9882                % 'Ticket To The Moon' 
%%%%            % *Time*, Electric Light Orchestra
http://home.earthlink.net/~yatescr
Reply to
Randy Yates

Jerry Avins wrote: snip Moral: matching load to source is a good way to avoid

Matching source to load is good when you are not paying for the power dissipated in the source. For example, when the source is a receiving antenna or a solar cell.

John

Reply to
John Monro

*Snip*

Well, we could have a facinating debate about what constitutes a "real" tellco & I could name names but mom always told me- "Don't bite the hand that feeds you"

Or maybe that was the dog she was talking to- either way as I don't post under a psuedonom the guilty, who occaisionaly pay me to do stuff for them, shall remain nameless to protect my income :).

That being said there are revenue generating calls from subs being carried on service provider circuits using ADPCM, not as many as there were say 10 years ago but they're still some out there. Personally I always hated the darn things.

H.

Reply to
Howard Eisenhauer

There is the the reflection coefficient at the junction, which is another matter. As for power delivered, when you can use a transformer to effect the match, it is worth while. On the other hand, I've seen designs with series resistors supposedly to effect a match, and that's nuts. A step-up transformer (or its equivalent) between a relatively low-impedance antenna and a high-impedance FET input boosts not only receiver sensitivity, but also SNR. (Noise power rises directly with impedance, hence voltage, but signal power rises with the square. There's a limit, though. When the transformer or other impedance raising circuit begins to load down the antenna, that advantage diminishes.

Jerry

--
        "The rights of the best of men are secured only as the
        rights of the vilest and most abhorrent are protected."
            - Chief Justice Charles Evans Hughes, 1927
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply to
Jerry Avins

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