Frequency response of modern landline telephone connection - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: April 29, 2021, 12:27:19 am
content: Back in the bad old steam-driven days of rotary dials and carbon microphones the frequency response of a telephone connection was something like 300Hz-3.5kHz. Just now though I had to ring up my ISPand while I was on hold the music was playing and I could clearly hear bass notes in the 100-120Hz region. Never heard that on a landline phone before. What is the frequency response of a modern landline connection? This was an office phone so it connects to an ethernet cable then off to who knows where. Maybe interstate or even overseas.

author: ataradov
author_rank: Super Contributor
timestamp: April 29, 2021, 12:49:50 am
content: Modern VoIP stuff is 50 Hz to 7 kHz. I don't know if this is some sort of an actual standard, but that's what most codecs are designed for.

author: BrianHG
author_rank: Super Contributor
timestamp: April 29, 2021, 01:39:43 am
content: Quote from: Circlotron on April 29, 2021, 12:27:19 amAnalog relay contact telephone systems have a bandwidth of something like 5hz to over 25KHz. In fact, before the modern multi-band DSP encoded modem, in the really early 90's, USRobotics had a modem which would work on old analog relay telephone exchanges at 57,600 baud, bi-directional. I remember they used to be used in Europe with many Amigas BBS to distribute pirated games, however, whenever you crossed over to a modern digital phone line connection, the baud rate ended up being capped down to 9,600 baud (compatible with the industry standard...). The modems were useless when the 14,400, 28,800 standards came into being as they just didn't have the DSP horsepower under the hood to do the sophisticated multi-band encoding. (I think there was a rom update for the standard 14,400 alone on some units, but they only could do 57,600 on relay switched analog phone lines.)

author: Circlotron
author_rank: Super Contributor
timestamp: April 29, 2021, 01:57:17 am
content: Quote from: BrianHG on April 29, 2021, 01:39:43 amOkay, i didn't know that. When I did my apprenticeship in the late 70's crossbar was all the rage and step by step was still quite common. I never got the chance to do a frequency response test back in the day. Only going by what we were told then. I would imagine long distance frequency division multiplexed systems would have been much more limited of course. Sometimes sounded like talking down a long pvc pipe.

author: ataradov
author_rank: Super Contributor
timestamp: April 29, 2021, 02:05:11 am
content: POTS never had 25 kHz bandwidth. Old 56K modems used V.90/V.92 standard, which used the same 300 Hz - 3.5 kHz bandwidth.Also, it was never 56K bidirectional, upload was about half of that.

author: BrianHG
author_rank: Super Contributor
timestamp: April 29, 2021, 02:06:07 am
content: I doubt it was flat, but the modem most likely boosted the high frequency by quite a few db. Once your phone exchange went digital, you hit a brick wall at ~4KHz, plus, the audio was also compressed.

author: BrianHG
author_rank: Super Contributor
timestamp: April 29, 2021, 02:23:28 am
content: Hang on, need to double check. I think it was a feature in the first beta X2 modems, only the large black ones, the modems which were the ~size of LP record. All the newer smaller ones had the normal slow upload speed, then they went to the V.90 standard.

author: ataradov
author_rank: Super Contributor
timestamp: April 29, 2021, 02:33:07 am
content: I think X2 was the name of the standard before it got adapted by the ITU. So V.90 is either the same or extremely close to X2.

author: BrianHG
author_rank: Super Contributor
timestamp: April 29, 2021, 02:45:27 am
content: I thought it was the Lucent/Motorola K56Flex standard which was adopted into the V.90 standard.Those big black 2k$ USRobotic modems might have also been the same as the backbone the tel-co used to achieve the ISP download rate of 56k (in non-card form). Perhaps this is why with a non-sampling relay wired exchange, with 2 of these modems talking directly to each other at each end, users on these hard wire connected exchanges were able to achieve 56k baud in both directions between each other while everyone else connecting together on modern ADC/DAC phone lines were stuck at ~28k-33k depending on line quality.

author: ataradov
author_rank: Super Contributor
timestamp: April 29, 2021, 03:01:49 am
content: According to wiki: "USR developed its own 56k standard known as X2, while a consortium of other companies introduced its own K56flex. In contrast to the success of HST, neither X2 nor K56flex saw any real market uptake, as it was clear they would soon be followed by V.90 modems. After the introduction of V.90, USR abandoned support for X2".So I guess they all are somewhat different, but I doubt they were significantly different, since they all competed for the same market.And yes, I could never get 56K even on new-ish telephone station within the same city with the same 56K USR modems.

author: johnh
author_rank: Regular Contributor
timestamp: April 29, 2021, 03:19:36 am
content: Quote from: ataradov on April 29, 2021, 12:49:50 amG722, AMR-NB, AMR-WBhttps://en.wikipedia.org/wiki/G.722https://en.wikipedia.org/wiki/Adaptive_Multi-Rate_audio_codec

author: edpalmer42
author_rank: Super Contributor
timestamp: April 29, 2021, 03:24:16 am
content: 56K connections never happened in the real world. Maybe under lab conditions. Typical download speeds under real-world conditions were 45K - 50K bps. Upload speeds were typically 25K - 30K bps.These connections could go through a digital telephone switch with no problems as long as one end (typically the ISP/BBS end) was a pure digital connection, i.e. ISDN, T-1, or similar. 3db or 6db digital pads weren't a problem because the modem just considered them to be predictable bit scramblers that could be trivially unscrambled. Outside plant issues could mess things up really bad by degrading the frequency response of the cable pair.USRobotics modems had some undocumented commands that allowed you to measure the frequency response of the line and therefore predict the chances of getting a high-speed connection on any particular line. There was a program that did everything automatically, but I can't remember the name.I worked for a Canadian Telco when all this was happening. My title at the time was Supervisor - Network Standards (Switching). I did the investigation and testing to see what could and could not be expected from our network.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 29, 2021, 03:24:42 am
content: I wonder what would be the ultimate bandwidth limit of a residential phone line (the copper pairs themselves) at various line lengths. My guess is that either safety voltage limits or line heating would limit the maximum signal strength. Then it would be a question of the noise floor which depends on a lot of factors. Crosstalk could be managed using massive MIMO. I guess the main problem is the economics of it or lack thereof, everyone would rather invest in fiber but that leaves out rural areas. That might be helpful in implementation, however - use fiber in densely populated areas, use massive MIMO over copper in sparsely populated areas where the lower number of copper pairs makes the "massive MIMO" problem somewhat less massive.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 12:05:53 pm
content: Quote from: NiHaoMike on April 29, 2021, 03:24:42 amHereI found a chart. Only rough estimate, of course.The exchange (DSLAM) where my home is connected has a distance of ~2.5km. My modem shows a line capacity of 12544 kbits/s. See attached screenshsots.Just wondering, is ADSL/VDSL not commonly used in your country/region for providing consumer internet access?Or do you have larger distances than a handful of kilometers in mind?

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 12:28:07 pm
content: Quote from: johnh on April 29, 2021, 03:19:36 amEventuall the codec to be used is negotiated between the communication partners, when the SIP connection is established. The calling party proposes a list of supported codecs, and the called party responds which of them it suports as well.G.711(64kbit, 8kSa/s, 8-bit µ-law/A-law encoding, uncompressed) is basically supported by every VoIP endpoint. Other codecs may or may not be supported by the communication partners (or one of them).Hereist is list of some commony used codecs.

author: bsdphk
author_rank: Regular Contributor
timestamp: April 29, 2021, 12:30:22 pm
content: Quote from: ataradov on April 29, 2021, 02:05:11 amAs with everything else: It depends.If you called another phone on the same central and that central used metalic contacts, you had two pieces of copper and with suitable equalization and amplification, 1 MHz bandwidth were not a problem.I know people who have run ethernet through local phone calls in California.As soon as two or more centrals were involved, the bandwidth would almost always drop to less than 10kHz, even if the exchange connection were copper, because the inter-central connections ("exchange" and "toll") almost always were on a 4-wire basis, and the "hybrid coils", which convert from pair to quad limit the bandwidth.If any kind of multiplexing, be it carrier frequency (= radio over phone-cable) or PCM (= digital) gets involved, actual filters brutally removes anything above 4kHz with at margin which decreased from 1 kHz (ie: 3kHz) to 100Hz (ie: 3.9kHz) as technology progressed.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 12:45:10 pm
content: Quote from: bsdphk on April 29, 2021, 12:30:22 pmIf the plain subscriber line hadn't this bandwidth, then DSL were not possible (ADSL2+ for instance uses carriers up to ~2MHz).

author: CaptDon
author_rank: Super Contributor
timestamp: April 29, 2021, 01:27:16 pm
content: Back when I worked in radio broadcast dialup telco frequency response dropped like a brick over 4khz. We did have a pair of special dedicated equalized 'broadcast loops' between the studio and transmitter that were mostly flat 50hz to 15khz, they traversed around 10 miles point to point. We couldn't easily do a microwave shot due to a mountain in the way, but eventually did install an STL mountaintop microwave repeater. We also had a copper D.C. telco pair for transmitter remote control. I don't know how they did it but the loop resistance was only around 100 ohms!!!Must have been 2 or 3 parallel pairs??

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 29, 2021, 01:28:44 pm
content: Quote from: gf on April 29, 2021, 12:05:53 pmI was thinking just how well could DSL be made to perform if it was pushed to the limit of the wire itself. I'm under the impression current DSL standards operate well below the safety voltage limits due to the high cost of high bandwidth amplifiers able to output such voltages.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 02:17:09 pm
content: Quote from: NiHaoMike on April 29, 2021, 01:28:44 pmBandwidth does rather not depend on voltage. However, if SNR can be improved, this enables the transmission of more bits per baud. But I'm afraid that you won't win anyting ifall wire pairsin the cable increase their signal power - relative crosstalk is still the same then (and that's where a good deal of the noise comes from). I guess it can only help if you are the only subscriber in the cable who uses a stronger signal than the others (at the disadvantage of the others), or if the whole cable is exclusively reserved for you alone. But I think that DSL standards have their focus primarily on bunched cables, not on exclusive ones. VDSL2 vectoring is already an approach to reduce crosstalk and thus improve SNR, enabling higher speed, granted that the DSLAM has full control overallwire pairs in a cable.

author: jmelson
author_rank: Super Contributor
timestamp: April 29, 2021, 03:47:07 pm
content: Quote from: ataradov on April 29, 2021, 02:05:11 amSpeaking of 56K modems, does anybody know how to send faxes today? I have an Asterisk/FreePBX system that receives FAXes just fine, but now my Cardinal 56K modem will not work on my analog Charter cable VOIP modem OR my copper landline direct to the CO (no RT). Some time ago, it used to generally work on either. It still connects, tries to negotiate, but never seems to get to transmitting the document before it drops the connection.Thanks for any suggestions.Jon

author: ataradov
author_rank: Super Contributor
timestamp: April 29, 2021, 04:04:58 pm
content: Quote from: jmelson on April 29, 2021, 03:47:07 pmI think there are some online services that let you do it, but I have not tried them. Last time I needed to send a fax, I used a big multi-function printer/scanner thing in the office. I think it is connected over IP, so I don't know at what point it became an actual fax.

author: bsdphk
author_rank: Regular Contributor
timestamp: April 29, 2021, 04:34:56 pm
content: Quote from: NiHaoMike on April 29, 2021, 01:28:44 pmThe problem has never been the wire itself, (and by never I mean "since they started using copper instead of iron in 18xx").The problem is all the other wires, and the cross-talk they cause.These days xDSL uses "vectoring" which coordinates (for lack of a better word) the transmission between the individual pairs in the cable, in order to cope with cross-talk.If you want to learn more, I can recommend pretty much any article in BSTJ in the 1930ies relating to carrier system.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 04:47:02 pm
content: Quote from: jmelson on April 29, 2021, 03:47:07 pmI've seen analog fax via V.29 up to 9600 bits/s working reasonably over good VoIP channels.It is important to enforce use of an uncompressed codec (G.711), since lossy compression garbles the signal.Still - depending on quality of service (delay, jitter, and packet loss) - it may indeed not be reliable enough.There exists however theT.38 protocolfor transmitting the T.30 fax data digitally over a VoIP channel.Seems that Asterisk can also function asT.38 fax gateway. I don't know any details, though.

author: bw2341
author_rank: Regular Contributor
timestamp: April 29, 2021, 08:05:53 pm
content: Quote from: Circlotron on April 29, 2021, 12:27:19 amI’ve experienced the same thing on my Polycom VoIP phone that I tinker with. The sound seems richer and deeper than my analog phones. You’d think that a lower frequency response would be better but I find it fatiguing sometimes. It seems to make environmental noise from the call centre more prominent, worsening voice intelligibility.So if G.711 is supposed to be 300-3400, is it the phone’s responsibility to filter out the low frequencies? Obviously, the phone’s handset DAC and ADC are much better than that. I suspect that modern phone designers have a more consumer electronics and information technology emphasis and traditional telecom knowledge is given less attention now.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 08:49:44 pm
content: The bandwidth needs to be limited before a digital audio signal with a higher sampling rate gets downsamped to 8kSa/s, or alternatively, already the analog signal needs to be bandlimited to <4kHz when the initial sampling happens with only 8kSa/s. Otherwise the Nyquist Shannon sampling theorem is violated. So yes, a VoIP phone has to do it when it uses G.711 codec. Other codecs can have different limits.

author: bw2341
author_rank: Regular Contributor
timestamp: April 29, 2021, 09:26:57 pm
content: Yes, I understand that a low pass filter is needed before decimation. I’m talking about having a high pass filter to block 20-300 Hz. In my opinion, the low frequencies aren’t helpful for voice communications.

author: gf
author_rank: Super Contributor
timestamp: April 29, 2021, 09:51:26 pm
content: I agree. In order to make voice better understandable, keeping deep bass tones is certainly not helpful.

author: David Hess
author_rank: Super Contributor
timestamp: April 29, 2021, 11:34:29 pm
content: That is not how the various 56k standards worked.By the time of the faster standards including USR HST and V.32, all phone lines were digitized to 64 kbits/second at 8 bits and 8 kHz (see below) so once the quantization error, asynchronous sampling, and the Nyquist rate was taken into account, 33.6 kbits/second was the practical limit with V.34.The 56k standards synchronized with the sampling of the telephone system allowing PCM at 56 kbits/second. The reason it was 56 kbits/second instead of 64 kbits/second is thatrobbed-bit signallingactually only preserved 7 bits instead of 8 bits.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 30, 2021, 12:26:26 am
content: Quote from: gf on April 29, 2021, 02:17:09 pmHence the "massive MIMO" bit, learn which neighboring pairs get crosstalk and how much in order to inject opposing signals to reduce it. I suppose it's actually an example of "MU-MIMO" when there are multiple users involved. The biggest challenge with that is likely the sheer complexity of massive MIMO.Also, using a higher voltage should help if there's noise from an external source and/or if the line is really long. It could work really well for boosting upstream bandwidth in sparsely populated areas assuming the lines are in a tree topology where there's a significant length of dedicated pairs between the customer building and the junction box where the connection to shared cables is made. In that case, the ISP end could check how much crosstalk there is to neighbors and let the customer end modem know how much voltage it can output without causing excessive interference to neighbors.

author: Miyuki
author_rank: Frequent Contributor
timestamp: May 03, 2021, 07:18:53 am
content: Quote from: NiHaoMike on April 30, 2021, 12:26:26 amEvery DSL modem measures line response and adjusts gains. It is a pretty smart device. Both sides adjust their power to have the required SNR.The real limitation of output power is more radiation from lines to the environment as it goes to pretty high MHzYou cant push tens of wats of power into kilometers of wires. They can easily form perfect antennas for long and medium wavebands.So you are not allowed to use more than like 20 dBmFrequency bands used

author: paul8f
author_rank: Regular Contributor
timestamp: May 03, 2021, 09:36:40 am
content: Regarding the hold music, I think some modern systems use very heavy audio compression to reduce their bandwidth overhead. Also, it's possible the quality depends on how many others are on hold at the same timeJust my 2 cents worth.

author: mansaxel
author_rank: Super Contributor
timestamp: May 03, 2021, 10:03:44 am
content: 1. Listen to phk. He's telling the truth.2. G722 and similar are 8KHz bw, true. Much nicer. Not nice enough to do transmission of hifi on, of course.3. The idea that a limited frequency response is better for speech recognition is mostly true. As long as distortion is kept low, that is. What happens now is that the limitation on bandwidth is moved from transmission medium (the wire, at length, and either the analog carrier frequency system filters or the PCM brick wall) to the microphone, and since phone microphones are essentially pressure receivers (ie. non-directional) the limitations of having a greedy microphone in a bad acoustic environment start to show up.What needs to be done is to limit what the microphone hears. Some modern headsets use microphone arrays and some clever DSP to limit their hearing. A more traditional approach could be a pressure gradient receiver coupled with a high-pass filter (the proximity effect of most pressure gradient receivers is a great start on limiting LF interference, if it is properly tailored). The important thing is that this IMNSHO should happen separately from the limitations of the transmission medium.

author: jmelson
author_rank: Super Contributor
timestamp: May 03, 2021, 03:52:54 pm
content: Quote from: jmelson on April 29, 2021, 03:47:07 pmNeeded to send another FAX today, and decided to play around with options (there are LOTS of them). I have a Cardinal56K faxmodem, and use efax under linux to transmit. So, I added -o 0 (that's dash oh zero) before the -t phonenumber, and it connected and sent the fax! Amazing, I don't know why I didn't try setting options before.This was using the analog port of my Charter VOIP modem.Jon