I'm curious to know why the 300 Hz low frequency limitation exists on POTS. Since fundamentals of the male human voice can easily extend below this, it would seem beneficial to accomodate low frequencies, at least down to say 150 Hz. Is there some historical reason that the low frequencies are limited? I tried searching the net, but only found info on why the upper frequencies are limited (which is quite understandable), not on the lower. This message was sent using the Comp.DSP web interface on www.DSPRelated.com
Why the low frequency limitation on POTS?
Started by ●September 9, 2005
Reply by ●September 9, 20052005-09-09
jharris wrote:> I'm curious to know why the 300 Hz low frequency limitation exists on POTS. > Since fundamentals of the male human voice can easily extend below this, > it would seem beneficial to accomodate low frequencies, at least down to > say 150 Hz. Is there some historical reason that the low frequencies are > limited?My guess would be to avoid 60Hz pickup problems. Among others, as I understand it early telegraphs systems used ground (real earth) return. Maybe not for telephones, though. Still, with open wire phone lines, probably near power lines, the 60Hz pickup could have been a problem. Otherwise, how hard was it to make amplifiers work down to lower frequencies? It requires larger coupling capacitors, for example. -- glen
Reply by ●September 9, 20052005-09-09
glen herrmannsfeldt wrote:> jharris wrote: > >> I'm curious to know why the 300 Hz low frequency limitation exists on >> POTS. >> Since fundamentals of the male human voice can easily extend below this, >> it would seem beneficial to accomodate low frequencies, at least down to >> say 150 Hz. Is there some historical reason that the low frequencies are >> limited? > > > My guess would be to avoid 60Hz pickup problems. > > Among others, as I understand it early telegraphs systems used > ground (real earth) return. Maybe not for telephones, though. > > Still, with open wire phone lines, probably near power lines, > the 60Hz pickup could have been a problem. > > Otherwise, how hard was it to make amplifiers work down to > lower frequencies? It requires larger coupling capacitors, > for example. > > -- glen >Originally all that was transformer coupled, and low frequency, high fidelity transformers are expensive. A POTS line still has to supply 48VDC along with audio, and it has to deliver 96V at 20Hz to make the ringer go -- those are both things that will drive the lowest audio frequency higher in a cost-effective system. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●September 10, 20052005-09-10
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:542dnZEbj-t_gr_eRVn-1w@web-ster.com...> glen herrmannsfeldt wrote: > >> jharris wrote: >> >>> I'm curious to know why the 300 Hz low frequency limitation exists on POTS. >>> Since fundamentals of the male human voice can easily extend below this, >>> it would seem beneficial to accomodate low frequencies, at least down to >>> say 150 Hz. Is there some historical reason that the low frequencies are >>> limited? >> >> My guess would be to avoid 60Hz pickup problems. >> >> Among others, as I understand it early telegraphs systems used >> ground (real earth) return. Maybe not for telephones, though. >> >> Still, with open wire phone lines, probably near power lines, >> the 60Hz pickup could have been a problem. >> >> Otherwise, how hard was it to make amplifiers work down to >> lower frequencies? It requires larger coupling capacitors, >> for example. >> > Originally all that was transformer coupled, and low frequency, high fidelity > transformers are expensive. A POTS line still has to supply 48VDC along with > audio, and it has to deliver 96V at 20Hz to make the ringer go -- those are > both things that will drive the lowest audio frequency higher in a > cost-effective system.I figured it had something to do with the analog properties of the phone line. Someone I spoke with recently claimed that it was a psychoacoustic issue: that since the high frequency end was limited, the low frequencies had to be similarly limited in order to "sound right". I had never heard of anything like that, but thought I would investigate his claim (he was a stranger, but in the audio industry in some form). Has anyone else heard of that?
Reply by ●September 10, 20052005-09-10
Jon Harris wrote:> "Tim Wescott" <tim@seemywebsite.com> wrote in message > news:542dnZEbj-t_gr_eRVn-1w@web-ster.com... > >>glen herrmannsfeldt wrote: >> >> >>>jharris wrote: >>> >>> >>>>I'm curious to know why the 300 Hz low frequency limitation exists on POTS. >>>> Since fundamentals of the male human voice can easily extend below this, >>>>it would seem beneficial to accomodate low frequencies, at least down to >>>>say 150 Hz. Is there some historical reason that the low frequencies are >>>>limited? >>> >>>My guess would be to avoid 60Hz pickup problems. >>> >>>Among others, as I understand it early telegraphs systems used >>>ground (real earth) return. Maybe not for telephones, though. >>> >>>Still, with open wire phone lines, probably near power lines, >>>the 60Hz pickup could have been a problem. >>> >>>Otherwise, how hard was it to make amplifiers work down to >>>lower frequencies? It requires larger coupling capacitors, >>>for example. >>> >> >>Originally all that was transformer coupled, and low frequency, high fidelity >>transformers are expensive. A POTS line still has to supply 48VDC along with >>audio, and it has to deliver 96V at 20Hz to make the ringer go -- those are >>both things that will drive the lowest audio frequency higher in a >>cost-effective system. > > > I figured it had something to do with the analog properties of the phone line. > Someone I spoke with recently claimed that it was a psychoacoustic issue: that > since the high frequency end was limited, the low frequencies had to be > similarly limited in order to "sound right". I had never heard of anything like > that, but thought I would investigate his claim (he was a stranger, but in the > audio industry in some form). Has anyone else heard of that?Yes. It is generally accepted -- and listening tests indicate -- that poor low- and good high-frequency response (or vice-versa) sounds less "natural" than the two being balanced. For low-frequency cutoffs between 20 and 100 Hz, the recommended product is -- surprise! -- 400,000 Hz^2. Personally, I think that calls for restricting the highs too much above about 60 Hz, but you might not agree. Telephones run from 300 to 4000, giving a product of 1,200,000 Hz^2. (Score one for me.) 100 to 4,000 would meet the "hifi" norm, but (when I could hear that high) I preferred 100 to about 8,000. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 10, 20052005-09-10
"Jerry Avins" <jya@ieee.org> wrote in message news:432255B0.50803@ieee.org...> Jon Harris wrote: >> "Tim Wescott" <tim@seemywebsite.com> wrote in message >> news:542dnZEbj-t_gr_eRVn-1w@web-ster.com... >> >>>glen herrmannsfeldt wrote: >>> >>> >>>>jharris wrote: >>>> >>>> >>>>>I'm curious to know why the 300 Hz low frequency limitation exists on POTS. >>>>> Since fundamentals of the male human voice can easily extend below this, >>>>>it would seem beneficial to accomodate low frequencies, at least down to >>>>>say 150 Hz. Is there some historical reason that the low frequencies are >>>>>limited? >>>> >>>>My guess would be to avoid 60Hz pickup problems. >>>> >>>>Among others, as I understand it early telegraphs systems used >>>>ground (real earth) return. Maybe not for telephones, though. >>>> >>>>Still, with open wire phone lines, probably near power lines, >>>>the 60Hz pickup could have been a problem. >>>> >>>>Otherwise, how hard was it to make amplifiers work down to >>>>lower frequencies? It requires larger coupling capacitors, >>>>for example. >>>> >>> >>>Originally all that was transformer coupled, and low frequency, high fidelity >>>transformers are expensive. A POTS line still has to supply 48VDC along with >>>audio, and it has to deliver 96V at 20Hz to make the ringer go -- those are >>>both things that will drive the lowest audio frequency higher in a >>>cost-effective system. >> >> >> I figured it had something to do with the analog properties of the phone >> line. Someone I spoke with recently claimed that it was a psychoacoustic >> issue: that since the high frequency end was limited, the low frequencies >> had to be similarly limited in order to "sound right". I had never heard of >> anything like that, but thought I would investigate his claim (he was a >> stranger, but in the audio industry in some form). Has anyone else heard of >> that? > > Yes. It is generally accepted -- and listening tests indicate -- that poor > low- and good high-frequency response (or vice-versa) sounds less "natural" > than the two being balanced. For low-frequency cutoffs between 20 and 100 Hz, > the recommended product is -- surprise! -- 400,000 Hz^2. Personally, I think > that calls for restricting the highs too much above about 60 Hz, but you might > not agree. Telephones run from 300 to 4000, giving a product of 1,200,000 > Hz^2. (Score one for me.) 100 to 4,000 would meet the "hifi" norm, but (when I > could hear that high) I preferred 100 to about 8,000.That is really interesting, Jerry. Do you know if this is primarily for the speaking human voice or if it applies to music as well? It would seem to me that in the case of music, if you were restricted in high frequency range by some physical limitation (i.e. sample rate, channel bandwidth), all things being equal having an extended low frequency response would sound better than limiting the lows to "match" the highs. Would you agree? Do AM radio stations (with highs limited to around 7kHz) correspondingly limit the bass? Also, it seems there isn't consensus on the exact high frequency limit of POTS. I've seen 3kHz, 3.3kHz, 3.4kHz, and 4kHz. Nowadays, I know that the audio is sampled at FS=8kHz, so that limits you to _at most_ 4kHz. But because of the need for a finite filter transition band, the true bandwidth will be somewhat less than 4kHz.
Reply by ●September 10, 20052005-09-10
Jerry Avins wrote:> Jon Harris wrote: > > "Tim Wescott" <tim@seemywebsite.com> wrote in message > > news:542dnZEbj-t_gr_eRVn-1w@web-ster.com... > > > >>glen herrmannsfeldt wrote: > >> > >> > >>>jharris wrote: > >>> > >>> > >>>>I'm curious to know why the 300 Hz low frequency limitation exists on POTS. > >>>> Since fundamentals of the male human voice can easily extend below this, > >>>>it would seem beneficial to accomodate low frequencies, at least down to > >>>>say 150 Hz. Is there some historical reason that the low frequencies are > >>>>limited? > >>> > >>>My guess would be to avoid 60Hz pickup problems. > >>> > >>>Among others, as I understand it early telegraphs systems used > >>>ground (real earth) return. Maybe not for telephones, though. > >>> > >>>Still, with open wire phone lines, probably near power lines, > >>>the 60Hz pickup could have been a problem. > >>> > >>>Otherwise, how hard was it to make amplifiers work down to > >>>lower frequencies? It requires larger coupling capacitors, > >>>for example. > >>> > >> > >>Originally all that was transformer coupled, and low frequency, high fidelity > >>transformers are expensive. A POTS line still has to supply 48VDC along with > >>audio, and it has to deliver 96V at 20Hz to make the ringer go -- those are > >>both things that will drive the lowest audio frequency higher in a > >>cost-effective system. > > > > > > I figured it had something to do with the analog properties of the phone line. > > Someone I spoke with recently claimed that it was a psychoacoustic issue: that > > since the high frequency end was limited, the low frequencies had to be > > similarly limited in order to "sound right". I had never heard of anything like > > that, but thought I would investigate his claim (he was a stranger, but in the > > audio industry in some form). Has anyone else heard of that? > > Yes. It is generally accepted -- and listening tests indicate -- that > poor low- and good high-frequency response (or vice-versa) sounds less > "natural" than the two being balanced. For low-frequency cutoffs between > 20 and 100 Hz, the recommended product is -- surprise! -- 400,000 Hz^2. > Personally, I think that calls for restricting the highs too much above > about 60 Hz, but you might not agree. Telephones run from 300 to 4000, > giving a product of 1,200,000 Hz^2. (Score one for me.) 100 to 4,000 > would meet the "hifi" norm, but (when I could hear that high) I > preferred 100 to about 8,000.Interesting. It ought to be easy to test: Make some vice recording with the PC, filter it with different filters, and play it back. A nice little hobby project over the weekend... Rune
Reply by ●September 10, 20052005-09-10
Much of the telephone system is dc coupled. Using ISDN equipment it is possible to send anything from dc to almost 4kHz (bit exact) across Europe and often to the USA. (I have a large amount of experimental data to prove this.) The 300Hz (or often about 150 Hz) hp filtering on analogue lines comes from filters in the codecs at the telephone exchange. Even the GSM mobile phone system can transmit dc under some circumstances. In particular, the high pass filter specified for the standard and half rate codecs is broken and passes dc with a few dB attenuation. They got it right in the enhanced codecs. This can cause problems because dc offsets are sometimes interpreted as audio activity in half duplex echo cancellors. John Walliker
Reply by ●September 10, 20052005-09-10
Jon Harris wrote: ...>> It is generally accepted -- and listening tests indicate -- that poor >>low- and good high-frequency response (or vice-versa) sounds less "natural" >>than the two being balanced. For low-frequency cutoffs between 20 and 100 Hz, >>the recommended product is -- surprise! -- 400,000 Hz^2. Personally, I think >>that calls for restricting the highs too much above about 60 Hz, but you might >>not agree. Telephones run from 300 to 4000, giving a product of 1,200,000 >>Hz^2. (Score one for me.) 100 to 4,000 would meet the "hifi" norm, but (when I >>could hear that high) I preferred 100 to about 8,000.> >> That is really interesting, Jerry. Do you know if this is primarily for the > speaking human voice or if it applies to music as well? It would seem to me > that in the case of music, if you were restricted in high frequency range by > some physical limitation (i.e. sample rate, channel bandwidth), all things being > equal having an extended low frequency response would sound better than limiting > the lows to "match" the highs. Would you agree?The testing was done primarily with music program, mostly by Harry Olsen and his crew at RCA Labs. It was part of an attempt to discover why some systems which measured better according to intuitive criteria sounded worse than those with poorer measurements. Two general conclusion came from those studies: frequency balance is necessary for "naturalness", and freedom from intermodulation distortion is a better measure of subjective quality than is harmonic distortion. (Both measure nonlinearity, but the IM numbers correlate better with perceived sound quality than THD. The need for wide-range low-distortion reproducers to use in the study led to the development of the LC1-A loudspeaker. The studies were conducted with various reproducers and live musicians behind a scrim curtain, with audiences of about 20 to 30 knowledgeable listeners (including, I was told, Toscanini and some orchestra members.)> Do AM radio stations (with highs limited to around 7kHz) correspondingly limit > the bass?Most AM radios wouldn't reproduce below about 150 Hz. That saves money on filter capacitors in the power supply.> Also, it seems there isn't consensus on the exact high frequency limit of POTS. > I've seen 3kHz, 3.3kHz, 3.4kHz, and 4kHz. Nowadays, I know that the audio is > sampled at FS=8kHz, so that limits you to _at most_ 4kHz. But because of the > need for a finite filter transition band, the true bandwidth will be somewhat > less than 4kHz.You know as much about that as I do! Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 10, 20052005-09-10
jrwalliker@gmail.com wrote:> Much of the telephone system is dc coupled. Using ISDN equipment it is > possible to send anything from dc to almost 4kHz (bit exact) across > Europe and often to the USA. (I have a large amount of experimental > data to prove this.) > > The 300Hz (or often about 150 Hz) hp filtering on analogue lines comes > from filters in the codecs at the telephone exchange. > > Even the GSM mobile phone system can transmit dc under some > circumstances. In particular, the high pass filter specified for the > standard and half rate codecs is broken and passes dc with a few dB > attenuation. They got it right in the enhanced codecs. > > This can cause problems because dc offsets are sometimes interpreted as > audio activity in half duplex echo cancellors. > > John WallikerTry sending audio down to DC on an analogue PSTN pair and you have two serious problems. First, if you go below about 180Hz (US) or 150Hz (most other places) you get hum problems from the 3rd harmonic of the mains. If you go below 60Hz or 50Hz that gets a lot lot worse. Second, DC is used for signalling and power feed on analogue pairs. You definitely wouldn't want the audio response going so low it interfers with that. Those factors defined the minimum for the PSTN audio band as being around 300Hz. A third problem with DC reponse affects many things on the PSTN - isolation. An isolated interface with a response down to DC was not a trivial device until recently. ISDN and GSM systems have no DC response at the communications channel level. However, they carry digital data, and what you convey in that digital data is completely open. If you carried audio in a modem signal across the analogue PSTN, that could also have response down to DC. Regards, Steve
