Floyd L. Davidson wrote:> DC load matching and AC impedance matching are not the same...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 ���������������������������������������������������������������������
How can digital be more spectrum efficient than analog ?
Started by ●October 27, 2006
Reply by ●October 28, 20062006-10-28
Reply by ●October 28, 20062006-10-28
Jerry Avins <jya@ieee.org> writes:> Floyd L. Davidson wrote: > >> DC load matching and AC impedance matching are not the same... > > 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.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' %%%% <yates@ieee.org> % *Time*, Electric Light Orchestra http://home.earthlink.net/~yatescr
Reply by ●October 29, 20062006-10-29
Jerry Avins wrote: snip Moral: matching load to source is a good way to avoid> reflections. Matching source to load is a poor way to deliver power. > > JerryMatching 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 by ●October 29, 20062006-10-29
On Fri, 27 Oct 2006 21:38:33 -0700, Don Bowey <dbowey@comcast.net> wrote:>On 10/27/06 8:36 PM, in article kmg5k21ljhik3sgfecg6br34u2llkko0cn@4ax.com, >"Howard Eisenhauer" <howarde@REMOVECAPShfx.eastlink.ca> wrote: > >*Snip*>> >> Telcos sometimes cut the 64 Kbit/VC rate down to 32 Kbits for a 2-1 >> improvent in transmission capacity, This doesn't really sound any >> worse for voice but don't try to send high speed data over it :(. > >Name even one real telco that does that. I can't think of one. ADPCM is >used, however, in many private networks. >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 by ●October 29, 20062006-10-29
John Monro wrote:> Jerry Avins wrote: > snip > Moral: matching load to source is a good way to avoid >> reflections. Matching source to load is a poor way to deliver power. >> >> Jerry > > 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.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 by ●October 29, 20062006-10-29
Jerry Avins wrote: ...> 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.Ignore that nonsense. The antenna's noise power scales directly with the impedance, as does the signal power. All three figures scale with the square of the turns ratio. If the receiver has 0 dB noise figure, SNR is unaffected. Otherwise, the step-up transformer is like adding noise-free gain at the front end -- a good thing. 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 by ●October 29, 20062006-10-29
On Fri, 27 Oct 2006 20:38:37 -0800, floyd@apaflo.com (Floyd L. Davidson) wrote:>Howard Eisenhauer <howarde@REMOVECAPShfx.eastlink.ca> wrote: >>On Fri, 27 Oct 2006 10:29:06 -0500, no-top-post wrote: >> >>>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. >>> >>T'aint nesessarily so. As another poster pointed out FDM could >>squeeze 24 channels os SSB 4kz voice into 96 KHz. where a T1 takes >>over 1.5 megabits/sec to do the same thing. > >Comparing bandwidth to bit rate is nonsense! 96 KHz does not >compare to 1.5 Mbps in any productive way.Lets define "Bandwidth"- Bandwidth properly refers to spectrum & is measured in Hz. If you're talking "bitrates" then you're refering to a system with a fixed transmission capacity. Whether or not it's being used efficiently is measured by what % of the data stream consists of overhead messages & empty data octets. The OP was talking about "efficiency", more or less relating to voice circuits. Without a doubt the standard reference of a 4 KHz voice channel it's more efficient spectrally to use FDM than to move 24 voice channels than to use a T1. In fact, as the T1 spec requires a pulse shape that's more or less square that implies at least 3rd order harmonics @ 4.5 MHz. You can fit 600 FDM voice channels in that space.> >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.Well, that sort of figures as a standard digital voice channel is 64K/sec anyhow. You certainly can't squeeze a higher bitrate from your modem through there. & how much you can send depends on the card, if its using robbed bit signalling (yes, its still in use here & there) you're limited to 56K. If you have a crappy phone line with poor S/N your rates going way down. A pointless quibble perhaps but its late & I'm bored.> >Bandwidth does not use more bitrate, it provides more!From the OP's question I'd say you're putting the cart before the horse. Bitrate uses Bandwidth, spectrally speaking. How much depends on the modulation method, QAM, QPSK, FSK, FM, CW yadda yadda. Bandwidth is not an unlimited resource, not everybody has the luxury of multiple dark fibers in the ground between here & there. There are very real- er, um, real world constraints with systems such as satellite, terrestrial M/W & the previously mentioned wireless systems.> >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.I could have sworn thats what I was getting at by way of the examples of cellular transmission standards, but what the hell do I know ???> >Bandwidth is not what makes digital more effective than analog.Agreed. Whole heartedly.>Above a minimum SNR digital systems are essentially error free >while analog system have additive noise.Unfortunately bandwidth is never unlimited & the noise level can occaisionally be out of your control. Digital systems are far less tolerent of noise when it *does* exceed the critical level. Analog systems get "hissy", digitas ge& ..or_de^5T3+...>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.True, although as I also alluded to encoding & error protection schemes eat into the actual traffic throughput for a given system at layer 1. You can over come a shit load of noise with the right setup, but if you have to send a million bits to get one bit of payload through to the customer you're better off hand delivering the message. H.
Reply by ●October 29, 20062006-10-29
Jerry Avins wrote:> Jerry Avins wrote: > > ... > >> 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. > > Ignore that nonsense.Duly ignored, Jerry. :=) The antenna's noise power scales directly with the> impedance, as does the signal power. All three figures scale with the > square of the turns ratio.Caveat: Only if the FET impedance is very high compared with the source impedance. This would imply an operating frequency well below VHF. If the receiver has 0 dB noise figure, SNR is> unaffected. Otherwise, the step-up transformer is like adding noise-free > gain at the front end -- a good thing. > > Jerry
Reply by ●October 29, 20062006-10-29
Howard Eisenhauer <howarde@REMOVECAPShfx.eastlink.ca> wrote:>On Fri, 27 Oct 2006 20:38:37 -0800, floyd@apaflo.com (Floyd L. >Davidson) wrote: > >>Howard Eisenhauer <howarde@REMOVECAPShfx.eastlink.ca> wrote: >>>On Fri, 27 Oct 2006 10:29:06 -0500, no-top-post wrote: >>> >>>>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. >>>> >>>T'aint nesessarily so. As another poster pointed out FDM could >>>squeeze 24 channels os SSB 4kz voice into 96 KHz. where a T1 takes >>>over 1.5 megabits/sec to do the same thing. >> >>Comparing bandwidth to bit rate is nonsense! 96 KHz does not >>compare to 1.5 Mbps in any productive way. > >Lets define "Bandwidth"- Bandwidth properly refers to spectrum & is >measured in Hz. If you're talking "bitrates" then you're refering to >a system with a fixed transmission capacity. Whether or not it's >being used efficiently is measured by what % of the data stream >consists of overhead messages & empty data octets.Hence comparing bandwith (spectrum) to bitrate (capacity) is nonsense. Added prattle doesn't change that.>The OP was talking about "efficiency", more or less relating to voice >circuits. Without a doubt the standard reference of a 4 KHz voice >channel it's more efficient spectrally to use FDM than to move 24 >voice channels than to use a T1. In fact, as the T1 spec requires a >pulse shape that's more or less square that implies at least 3rd order >harmonics @ 4.5 MHz. You can fit 600 FDM voice channels in that >space.T1 does *not* require a square pulse shape. It requires 750 Khz bandwidth. It was not designed for efficient spectrum utilization, because that is not required on a twisted pair cable. However, since it was designed to replace a single 4 KHz voice circuit with 24 each 4 KHz voice circuits (granted with lower SNR), it is hard to accept your argument that a T1 represents bandwidth inefficency as such. Since nobody puts 600 FDM voice channels on a twisted pair, I can't see where you can claim that many will fit, eh?>>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. > >Well, that sort of figures as a standard digital voice channel is >64K/sec anyhow. You certainly can't squeeze a higher bitrate from >your modem through there. & how much you can send depends on the >card, if its using robbed bit signalling (yes, its still in use here & >there) you're limited to 56K.Line cards know nothing about robbed bit signaling. And therefore robbed bit signaling has nothing to do with the bit rate for a PAM digital signal on the wire side of a line card.>If you have a crappy phone line with >poor S/N your rates going way down. A pointless quibble perhaps but >its late & I'm bored. > >> >>Bandwidth does not use more bitrate, it provides more! > >From the OP's question I'd say you're putting the cart before the >horse. Bitrate uses Bandwidth, spectrally speaking.Which is why bandwidth *provides* more bitrate. You are confused.>How much depends >on the modulation method, QAM, QPSK, FSK, FM, CW yadda yadda. >Bandwidth is not an unlimited resource, not everybody has the luxury >of multiple dark fibers in the ground between here & there. There are >very real- er, um, real world constraints with systems such as >satellite, terrestrial M/W & the previously mentioned wireless >systems.So?>>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. > >I could have sworn thats what I was getting at by way of the examples >of cellular transmission standards, but what the hell do I know ???Swear all you like, but that isn't what you said.>>Bandwidth is not what makes digital more effective than analog. > >Agreed. Whole heartedly. > >>Above a minimum SNR digital systems are essentially error free >>while analog system have additive noise. > >Unfortunately bandwidth is never unlimited & the noise level can >occaisionally be out of your control.A useless statement of no significance.>Digital systems are far less >tolerent of noise when it *does* exceed the critical level. Analog >systems get "hissy", digitas ge& ..or_de^5T3+...Wrong. A digital system can function error free at an SNR so low that analog systems cannot function at all, much less simply get hissy.>>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. > >True, although as I also alluded to encoding & error protection >schemes eat into the actual traffic throughput for a given system at >layer 1. You can over come a shit load of noise with the right setup, >but if you have to send a million bits to get one bit of payload >through to the customer you're better off hand delivering the message.More prattle? -- Floyd L. Davidson <http://www.apaflo.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
Reply by ●October 29, 20062006-10-29
Howard Eisenhauer <howarde@REMOVECAPShfx.eastlink.ca> wrote:>On Fri, 27 Oct 2006 20:38:37 -0800, floyd@apaflo.com (Floyd L. >Davidson) wrote: > >>Howard Eisenhauer <howarde@REMOVECAPShfx.eastlink.ca> wrote: >>>On Fri, 27 Oct 2006 10:29:06 -0500, no-top-post wrote: >>> >>>>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. >>>> >>>T'aint nesessarily so. As another poster pointed out FDM could >>>squeeze 24 channels os SSB 4kz voice into 96 KHz. where a T1 takes >>>over 1.5 megabits/sec to do the same thing. >> >>Comparing bandwidth to bit rate is nonsense! 96 KHz does not >>compare to 1.5 Mbps in any productive way. > >Lets define "Bandwidth"- Bandwidth properly refers to spectrum & is >measured in Hz. If you're talking "bitrates" then you're refering to >a system with a fixed transmission capacity. Whether or not it's >being used efficiently is measured by what % of the data stream >consists of overhead messages & empty data octets.Hence comparing bandwith (spectrum) to bitrate (capacity) is nonsense. Added prattle doesn't change that.>The OP was talking about "efficiency", more or less relating to voice >circuits. Without a doubt the standard reference of a 4 KHz voice >channel it's more efficient spectrally to use FDM than to move 24 >voice channels than to use a T1. In fact, as the T1 spec requires a >pulse shape that's more or less square that implies at least 3rd order >harmonics @ 4.5 MHz. You can fit 600 FDM voice channels in that >space.T1 does *not* require a square pulse shape. It requires 750 Khz bandwidth. It was not designed for efficient spectrum utilization, because that is not required on a twisted pair cable. However, since it was designed to replace a single 4 KHz voice circuit with 24 each 4 KHz voice circuits (granted with lower SNR), it is hard to accept your argument that a T1 represents bandwidth inefficency as such. Since nobody puts 600 FDM voice channels on a twisted pair, I can't see where you can claim that many will fit, eh?>>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. > >Well, that sort of figures as a standard digital voice channel is >64K/sec anyhow. You certainly can't squeeze a higher bitrate from >your modem through there. & how much you can send depends on the >card, if its using robbed bit signalling (yes, its still in use here & >there) you're limited to 56K.Line cards know nothing about robbed bit signaling. And therefore robbed bit signaling has nothing to do with the bit rate for a PAM digital signal on the wire side of a line card.>If you have a crappy phone line with >poor S/N your rates going way down. A pointless quibble perhaps but >its late & I'm bored. > >> >>Bandwidth does not use more bitrate, it provides more! > >From the OP's question I'd say you're putting the cart before the >horse. Bitrate uses Bandwidth, spectrally speaking.Which is why bandwidth *provides* more bitrate. You are confused.>How much depends >on the modulation method, QAM, QPSK, FSK, FM, CW yadda yadda. >Bandwidth is not an unlimited resource, not everybody has the luxury >of multiple dark fibers in the ground between here & there. There are >very real- er, um, real world constraints with systems such as >satellite, terrestrial M/W & the previously mentioned wireless >systems.So?>>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. > >I could have sworn thats what I was getting at by way of the examples >of cellular transmission standards, but what the hell do I know ???Swear all you like, but that isn't what you said.>>Bandwidth is not what makes digital more effective than analog. > >Agreed. Whole heartedly. > >>Above a minimum SNR digital systems are essentially error free >>while analog system have additive noise. > >Unfortunately bandwidth is never unlimited & the noise level can >occaisionally be out of your control.A useless statement of no significance.>Digital systems are far less >tolerent of noise when it *does* exceed the critical level. Analog >systems get "hissy", digitas ge& ..or_de^5T3+...Wrong. A digital system can function error free at an SNR so low that analog systems cannot function at all, much less simply get hissy.>>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. > >True, although as I also alluded to encoding & error protection >schemes eat into the actual traffic throughput for a given system at >layer 1. You can over come a shit load of noise with the right setup, >but if you have to send a million bits to get one bit of payload >through to the customer you're better off hand delivering the message.More prattle? -- Floyd L. Davidson <http://www.apaflo.com/floyd_davidson> Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
