Hi, I am interested in a baseband digital communications system (Gigabit Ethernet). The signal spectrum is baseband and there are isolation transformers at both ends of the channel (cat5 cable). So one of the channel impairments that the transceiver must deal with is baseline wander. ie if there are a long string of 0's (or 1's) the signal begins to look like DC (0Hz) and this will not pass through the (highpass) transformers. So I would like to know what the typical algorithms for cancelling this baseline wander effect are and what the pros and cons for each algorithm is. So I consulted a few textbooks on DSP and on digital comms but I cant find any entry in the index for Baseline Wander. There are references for all the other blocks you will typically find in a baseband digital transceiver; equalization, echo cancellation, timing recovery, crosstalk cancellation, but nothing on BLW ! So I am wondering is this because: (i) Baseline Wander is too obscure a topic (it would not be relevant for wireless systems for example) and so typical texts dont cover it ? (ii) There is some other more common name for this effect that it will be listed under in the index ? If (i) is the case, does anyone know of any online reference that might list some of the approaches typically used to compensate for baseline wander in baseband digital comms ? Thanks, Usjes. --------------------------------------- Posted through http://www.DSPRelated.com
Is there another name for Baseline Wander ?
Started by ●April 7, 2015
Reply by ●April 7, 20152015-04-07
On Tue, 07 Apr 2015 16:55:58 -0500, Usjes wrote:> Hi, > > I am interested in a baseband digital communications system (Gigabit > Ethernet). The signal spectrum is baseband and there are isolation > transformers at both ends of the channel (cat5 cable). So one of the > channel impairments that the transceiver must deal with is baseline > wander. ie if there are a long string of 0's (or 1's) the signal begins > to look like DC (0Hz) and this will not pass through the (highpass) > transformers. So I would like to know what the typical algorithms for > cancelling this baseline wander effect are and what the pros and cons > for each algorithm is. So I consulted a few textbooks on DSP and on > digital comms but I cant find any entry in the index for Baseline > Wander. There are references for all the other blocks you will typically > find in a baseband digital transceiver; equalization, echo cancellation, > timing recovery, crosstalk cancellation, but nothing on BLW ! So I am > wondering is this because: > (i) Baseline Wander is too obscure a topic (it would not be relevant for > wireless systems for example) and so typical texts dont cover it ? > (ii) There is some other more common name for this effect that it will > be listed under in the index ? > If (i) is the case, does anyone know of any online reference that might > list some of the approaches typically used to compensate for baseline > wander in baseband digital comms ?Check the specification -- all the earlier versions of Ethernet use a physical-layer coding scheme that eliminates DC content specifically to allow for transformer coupling. I'd be surprised if Gigabit left this out. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●April 7, 20152015-04-07
On 4/7/2015 2:55 PM, Usjes wrote:> Hi, > > I am interested in a baseband digital communications system (Gigabit > Ethernet). The signal spectrum is baseband and there are isolation > transformers at both ends of the channel (cat5 cable). So one of the > channel impairments that the transceiver must deal with is baseline > wander. ie if there are a long string of 0's (or 1's) the signal begins > to look like DC (0Hz) and this will not pass through the (highpass) > transformers. So I would like to know what the typical algorithms for > cancelling this baseline wander effect are and what the pros and cons > for each algorithm is. So I consulted a few textbooks on DSP and on > digital comms but I cant find any entry in the index for Baseline > Wander. There are references for all the other blocks you will typically > find in a baseband digital transceiver; equalization, echo cancellation, > timing recovery, crosstalk cancellation, but nothing on BLW ! So I am > wondering is this because: > (i) Baseline Wander is too obscure a topic (it would not be relevant for > wireless systems for example) and so typical texts dont cover it ? > (ii) There is some other more common name for this effect that it will > be listed under in the index ? > If (i) is the case, does anyone know of any online reference that might > list some of the approaches typically used to compensate for baseline > wander in baseband digital comms ? > > Thanks, > > Usjes. >Ethernet uses 8b/10b encoding to remove DC offsets. http://en.wikipedia.org/wiki/8b/10b_encoding Rob.
Reply by ●April 7, 20152015-04-07
Usjes <59494@dsprelated> wrote:> I am interested in a baseband digital communications system (Gigabit > Ethernet). The signal spectrum is baseband and there are isolation > transformers at both ends of the channel (cat5 cable). So one of the > channel impairments that the transceiver must deal with is baseline > wander. ie if there are a long string of 0's (or 1's) the signal begins > to look like DC (0Hz) and this will not pass through the (highpass) > transformers.It is called baseband, as opposed to broadband, though that doesn't mean that it actually goes to 0Hz. Broadband ethernet fits into a TV (6MHz) channel (or two or three) on 75 ohm coax cable. There is plenty of room for other signals below the ethernet signal.> So I would like to know what the typical algorithms for > cancelling this baseline wander effect are and what the pros and cons > for each algorithm is. So I consulted a few textbooks on DSP and on > digital comms but I cant find any entry in the index for Baseline > Wander. There are references for all the other blocks you will typically > find in a baseband digital transceiver; equalization, echo cancellation, > timing recovery, crosstalk cancellation, but nothing on BLW ! So I am > wondering is this because: > (i) Baseline Wander is too obscure a topic (it would not be relevant for > wireless systems for example) and so typical texts dont cover it ? > (ii) There is some other more common name for this effect that it will > be listed under in the index ? > If (i) is the case, does anyone know of any online reference that might > list some of the approaches typically used to compensate for baseline > wander in baseband digital comms ?It is carefully coded to avoid DC, and also to keep the bandwidth as low as possible. 1000baseT uses all four pairs in a Cat-5 cable, going both directions at the same time. There are echo cancelers and such to make sure that the signal gets through. -- glen
Reply by ●April 7, 20152015-04-07
Usjes <59494@dsprelated> wrote:> I am interested in a baseband digital communications system (Gigabit > Ethernet). The signal spectrum is baseband and there are isolation > transformers at both ends of the channel (cat5 cable). So one of the > channel impairments that the transceiver must deal with is baseline > wander.Reminds me of a discussion once on comp.dcom.lans.ethernet. UTP ethernet uses balanced signals to avoid common mode interference. As long as it is balanced, it won't radiate and most signals that would be picked up by a long cable also won't bother it. The transformers are supposed to allow either 500V or 1000V of common mode on the cable. It was suggested, and maybe even tested, that a 120VAC common mode voltage could be added, and it would still work. Just be careful where you touch it. -- glen
Reply by ●April 7, 20152015-04-07
On Tue, 07 Apr 2015 16:55:58 -0500, "Usjes" <59494@DSPRelated> wrote:>Hi, > >I am interested in a baseband digital communications system (Gigabit >Ethernet). The signal spectrum is baseband and there are isolation >transformers at both ends of the channel (cat5 cable). So one of the >channel impairments that the transceiver must deal with is baseline >wander. ie if there are a long string of 0's (or 1's) the signal begins >to look like DC (0Hz) and this will not pass through the (highpass) >transformers. So I would like to know what the typical algorithms for >cancelling this baseline wander effect are and what the pros and cons >for each algorithm is. So I consulted a few textbooks on DSP and on >digital comms but I cant find any entry in the index for Baseline >Wander. There are references for all the other blocks you will typically >find in a baseband digital transceiver; equalization, echo cancellation, >timing recovery, crosstalk cancellation, but nothing on BLW ! So I am >wondering is this because: >(i) Baseline Wander is too obscure a topic (it would not be relevant for >wireless systems for example) and so typical texts dont cover it ? >(ii) There is some other more common name for this effect that it will >be listed under in the index ? >If (i) is the case, does anyone know of any online reference that might >list some of the approaches typically used to compensate for baseline >wander in baseband digital comms ? > >Thanks, > >Usjes.There are a number of techniques that are used to mitigate DC-offset, like scramblers, but ethernet and many optical systems use 8b/10b encoding as Rob pointed out. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com
Reply by ●April 8, 20152015-04-08
On 4/7/2015 3:35 PM, Rob Doyle wrote:> On 4/7/2015 2:55 PM, Usjes wrote: >> Hi, >> >> I am interested in a baseband digital communications system (Gigabit >> Ethernet). The signal spectrum is baseband and there are isolation >> transformers at both ends of the channel (cat5 cable). So one of the >> channel impairments that the transceiver must deal with is baseline >> wander. ie if there are a long string of 0's (or 1's) the signal begins >> to look like DC (0Hz) and this will not pass through the (highpass) >> transformers. So I would like to know what the typical algorithms for >> cancelling this baseline wander effect are and what the pros and cons >> for each algorithm is. So I consulted a few textbooks on DSP and on >> digital comms but I cant find any entry in the index for Baseline >> Wander. There are references for all the other blocks you will typically >> find in a baseband digital transceiver; equalization, echo cancellation, >> timing recovery, crosstalk cancellation, but nothing on BLW ! So I am >> wondering is this because: >> (i) Baseline Wander is too obscure a topic (it would not be relevant for >> wireless systems for example) and so typical texts dont cover it ? >> (ii) There is some other more common name for this effect that it will >> be listed under in the index ? >> If (i) is the case, does anyone know of any online reference that might >> list some of the approaches typically used to compensate for baseline >> wander in baseband digital comms ? >> >> Thanks, >> >> Usjes. >> > > Ethernet uses 8b/10b encoding to remove DC offsets. > > http://en.wikipedia.org/wiki/8b/10b_encoding > > Rob.Re-read the original question. One of the simplest (and earliest) ways to achieve a DC balance is to use Manchester encoding. http://en.wikipedia.org/wiki/Manchester_code Manchester encoding ensures that the average DC value is zero over any bit. 8b/10b ensures that the average DC value is zero over a sequence of bits. Therefore 8b/10b encoding has more low-frequency content and requires a transformer with more iron (bigger?) than a Manchester encoded signal. Manchester encoding has more high-frequency content (for fixed data rate). BTW I think 8b/10b was developed (and patented) by IBM for use in their disk drives where the magnetic flux transitions need to balanced. I think it's still part of the SATA disk standard. One of the many places where data communications and data storage technologies overlap. Rob.
Reply by ●April 8, 20152015-04-08
On Tue, 07 Apr 2015 15:35:03 -0700, Rob Doyle wrote:> On 4/7/2015 2:55 PM, Usjes wrote: >> Hi, >> >> I am interested in a baseband digital communications system (Gigabit >> Ethernet). The signal spectrum is baseband and there are isolation >> transformers at both ends of the channel (cat5 cable). So one of the >> channel impairments that the transceiver must deal with is baseline >> wander. ie if there are a long string of 0's (or 1's) the signal begins >> to look like DC (0Hz) and this will not pass through the (highpass) >> transformers. So I would like to know what the typical algorithms for >> cancelling this baseline wander effect are and what the pros and cons >> for each algorithm is. So I consulted a few textbooks on DSP and on >> digital comms but I cant find any entry in the index for Baseline >> Wander. There are references for all the other blocks you will >> typically find in a baseband digital transceiver; equalization, echo >> cancellation, >> timing recovery, crosstalk cancellation, but nothing on BLW ! So I am >> wondering is this because: >> (i) Baseline Wander is too obscure a topic (it would not be relevant >> for wireless systems for example) and so typical texts dont cover it ? >> (ii) There is some other more common name for this effect that it will >> be listed under in the index ? >> If (i) is the case, does anyone know of any online reference that might >> list some of the approaches typically used to compensate for baseline >> wander in baseband digital comms ? >> >> Thanks, >> >> Usjes. >> >> > Ethernet uses 8b/10b encoding to remove DC offsets. > > http://en.wikipedia.org/wiki/8b/10b_encodingIs it used in Gigabit? Gigabit uses a multi-level coding scheme, so just encoding the raw data as 8b/10b wouldn't necessarily result in a zero-DC signal. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●April 17, 20152015-04-17
Rob Doyle <radioengr@gmail.com> wrote:> On 4/7/2015 3:35 PM, Rob Doyle wrote:(snip on gigabit ethernet and transformer coupling)>> Ethernet uses 8b/10b encoding to remove DC offsets.>> http://en.wikipedia.org/wiki/8b/10b_encoding(snip)> Re-read the original question.> One of the simplest (and earliest) ways to achieve a DC balance is to > use Manchester encoding.> http://en.wikipedia.org/wiki/Manchester_code> Manchester encoding ensures that the average DC value is zero over any > bit. 8b/10b ensures that the average DC value is zero over a sequence of > bits. Therefore 8b/10b encoding has more low-frequency content and > requires a transformer with more iron (bigger?) than a Manchester > encoded signal. Manchester encoding has more high-frequency content (for > fixed data rate).Yes, but the transition rate is higher, so the bottom of the spectrum also moves up. It isn't how many zeros (or ones) are in a row, but how long they take to send. For 10baseT, they fit the transformers into a DIP package, smaller than a 74LS00 used to be (and still is). Note that Manchester is an inefficient code. It requires a bandwidth up to the data rate, which is twice as high as Nyquist figured out. For those who forgot, the actual problem Nyquist solved was the inverse of the one commonly named after him. Keeping the lower frequency high enough for transformer coupling is important, but, at higher data rates, keeping the high end down is also important. With 8B10B, the transition rate is 1.25 times the data rate, and the bandwidth half of that. At 250Mb/s/pair, that would still be higher than the 100MHz spec for Cat 5 cable. The 8B10B is used for fiber ethernet, which doesn't have the same problem with high frequencies, but does still have a DC problem. 1000baseT uses the PAM5 coding, which reduces the upper frequency even more, though requiring a higher S/N than simpler coding. PAM5 uses five voltage levels, so allows log2(5) bits per symbol (transition), and so a lower bandwidth. log2(5) is about 0.3, and even with the more complicated coding gets the bandwidth below 100MHz.> BTW I think 8b/10b was developed (and patented) by IBM for use in > their disk drives where the magnetic flux transitions need to balanced. > I think it's still part of the SATA disk standard.> One of the many places where data communications and data storage > technologies overlap.-- glen
Reply by ●April 17, 20152015-04-17
Tim Wescott <seemywebsite@myfooter.really> wrote: (snip, someone wrote)>> Ethernet uses 8b/10b encoding to remove DC offsets.>> http://en.wikipedia.org/wiki/8b/10b_encoding> Is it used in Gigabit? Gigabit uses a multi-level coding scheme, so just > encoding the raw data as 8b/10b wouldn't necessarily result in a zero-DC > signal.The fiber ethernets use it, 1000baseSX and such. They use codes with either 5 ones and 5 zeros, or pairs of codes with 4 ones and six zeros, or the other way around. That, and a running disparity, allows one to keep close enough to balanced. They also don't use codes that will result in more than four consecutive ones or zeros. After the code pairs for 256 data values are taken, there are still some left for other uses, such as an idle code when no data is being sent. There are 252 codes with five 0s and five 1s, 210 pairs of codes with four and six, enough to remove those with too many zeros or ones in a row. -- glen
