km@mathcs.emory.edu (Ken Mandelberg) (03/07/91)
What is the norm for errors per page in Fax reception? I rarely see
fewer than 5 bad scan lines per page, and often substantially more.
The same telephone line seems to do fine for data modems, even high
speed ones (V.32, PEP).
Our fax hardware is a Digitan board for a PC, and I'm talking about
errors that show in the G3 output file, not added in any later step.
Its a very low cost board, and I wonder if maybe its just subpar in
doing the job.
--
Ken Mandelberg | km@mathcs.emory.edu PREFERRED
Emory University | {rutgers,gatech}!emory!km UUCP
Dept of Math and CS | km@emory.bitnet NON-DOMAIN BITNET
Atlanta, GA 30322 | Phone: Voice (404) 727-7963, FAX 727-5611tnixon@hayes.uucp (03/07/91)
In article <7120@emory.mathcs.emory.edu>, km@mathcs.emory.edu (Ken Mandelberg) writes: > What is the norm for errors per page in Fax reception? I rarely see > fewer than 5 bad scan lines per page, and often substantially more. > The same telephone line seems to do fine for data modems, even high > speed ones (V.32, PEP). CCITT T.30 leaves it totally up to the designer of the fax machine to answer the question "is the copy quality good enough?". This applies both to the initial measurements of the quality of the circuit (when the machines are deciding which modulation scheme to use) and to the actual transmission of the images. Thus, you'll see a wide variety of behavior between products from different manufacturers with regard to what speed they'll select on a given circuit, etc. You'll also see a significant difference between V.32 and 9600bps fax on the same quality phone line. Because of trellis coding, V.32 can tolerate about 4dB worse signal-to-noise ratio and achieve the same bit error rate as V.29. If your circuit is only fair, you might very well see good performance with V.32, and not so good with V.29 (the high-speed modulation scheme used in most Group 3 machines, which doesn't have trellis coding). The latest extension to Group 3 adds "V.17" modulation, which includes trellis coding at 7200, 9600, 12000, and 14400. When V.17 capability becomes more widespread, you'll see faxes with fewer bad lines in addition to being just faster. -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net
hot@integow.uucp (Roland van Hout) (03/08/91)
From article <3833.27d6468f@hayes.uucp>, by tnixon@hayes.uucp: > In article <7120@emory.mathcs.emory.edu>, km@mathcs.emory.edu (Ken > Mandelberg) writes: > You'll also see a significant difference between V.32 and 9600bps > fax on the same quality phone line. Because of trellis coding, V.32 > can tolerate about 4dB worse signal-to-noise ratio and achieve the > same bit error rate as V.29. If your circuit is only fair, you Does this mean we would be able to connect to a fax machine with V.32 and then we can send faxes if we have/make appropriate software? So we would use our T2500 to send faxes. bye Roland -- UUCP: ..!uunet!mcsun!hp4nl!integow!hot or hot@integow.UUCP Roland van Hout, Sr. software engineer, Integrity software consultants, Pelmolenlaan 2, 3447 GW Woerden, Netherlands, tel +31 3480-30131, fax +31 3480-30182
tnixon@hayes.uucp (03/11/91)
In article <1526@integow.uucp>, hot@integow.uucp (Roland van Hout) writes: > From article <3833.27d6468f@hayes.uucp>, by tnixon@hayes.uucp: >> In article <7120@emory.mathcs.emory.edu>, km@mathcs.emory.edu (Ken >> Mandelberg) writes: >> You'll also see a significant difference between V.32 and 9600bps >> fax on the same quality phone line. Because of trellis coding, V.32 >> can tolerate about 4dB worse signal-to-noise ratio and achieve the >> same bit error rate as V.29. If your circuit is only fair, you > > Does this mean we would be able to connect to a fax machine with > V.32 and then we can send faxes if we have/make appropriate software? > So we would use our T2500 to send faxes. No, not at all. I was simply comparing the performance of V.32 and V.29, and didn't imply that V.32 that you could send faxes with V.32. -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net
shaw@paralogics.UUCP (Guy Shaw) (03/18/91)
As long as changes are being made, the people who introduce new technology
and/or a new standard must overcome the inertia of all the existing fax modems.
So, then, why shouldn't this be the time to just switch to V.32 for fax?
What does V.29+V.17 have that couldn't be done with V.32?
There must be something I don't understand about how the transition can be
made to V.29+V.17 more easily than switching to V.32
Am I wrong in these assumptions:
1) the only reasons that fax modems and computer modems are different are
purely historical, and
2) if the proliferation of cheap personal computers had arrived earlier,
fax would just be some standard encoding format (a la uuencode) and
not have modem technology of its own.
If these are misconceptions, then do feel free to straighten me out.
If this is a topic in a frequently asked questions list, then a
referral would be good enough. But, I have been browsing this newsgroup
for a while and have not seen one. FAXFAQ, anyone?
Asking stupid questions is a dirty job, but someone's gotta do it.
--
Guy Shaw
Paralogics
paralogics!shaw@uunet.uu.net or uunet!paralogics!shawtnixon@hayes.uucp (03/19/91)
In article <427@paralogics.UUCP>, shaw@paralogics.UUCP (Guy Shaw) writes: > As long as changes are being made, the people who introduce new technology > and/or a new standard must overcome the inertia of all the existing fax modems. > So, then, why shouldn't this be the time to just switch to V.32 for fax? > What does V.29+V.17 have that couldn't be done with V.32? A lot of people in the _modem_ industry agree with you. In general, we believe that the performance of Group 3 fax could be significantly improved by switching to a high-speed full duplex modem (like V.32bis), and staying with it, rather than constantly switching between modulation schemes during the call. It's pretty ridiculous to send synchronous data using V.21 FSK modulation anyway, and to do it at 300bps -- arghhh! But there's a lot of inertia in the fax world. The best argument for sticking with half-duplex modulation schemes seems to be "we've always done it that way"!! It's unfortunate, but political squabbles and "egos" (which standards committee knows best how to send a fax) often get in the way. Proponents of half-duplex transmission also argue that echo cancellation is expensive (was, but isn't any more), and that half-duplex modems work better on poor circuits than full duplex (hasn't been demonstrated in a repeatable way). > Am I wrong in these assumptions: > 1) the only reasons that fax modems and computer modems are different are > purely historical, and > 2) if the proliferation of cheap personal computers had arrived earlier, > fax would just be some standard encoding format (a la uuencode) and > not have modem technology of its own. I think you're forgetting that the purpose of a fax modem is to be compatible with standalone fax MACHINES, not with computers. Group 3 fax predates PCs by a good bit. Faxes were using 9600bps transmission when the "standard" PC modem was still a 300bps Bell 103. When Group 3 standards were developed, there WASN'T a standard for dial-up, full duplex data modems above 1200bps. In fact, V.29 isn't even standardized for dial-up use; CCITT Study Group VIII (the fax standards committee) kludged it into the fax application without any cooperation from Study Group XVII (the modem standards committee). PC fax modems were definitely an afterthought, and had to be compatible with the (by then) huge installed base of fax machines; compatibility with high-speed data modems wasn't even a consideration. Study Groups VIII and XVII are now working on even faster modems ("V.fast") that will modulate at up to 24,000bps. XVII wants VIII to consider using the modem in a full-duplex mode, but nevertheless is developing a half-duplex mode if VIII decides to hold their ground. -- Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net
dww@stl.stc.co.uk (David Wright) (03/22/91)
In the referenced article shaw@paralogics.UUCP (Guy Shaw) writes:
#Am I wrong in these assumptions:
# 1) the only reasons that fax modems and computer modems are different are
# purely historical
FAX modems run at 9600 baud, with downgrade to 4800,2400,1200. (Some cheap
ones only manage 4800).
V32 modems also run at 9600. But they require much more processing =
higher cost.
The difference is that a FAX modem only runs 9600 baud in one direction at a
time (half-duplex). V32 is full duplex - data in both directions at once.
That means you need to do echo cancellation as well as forward equalisation
- in other words a LOT more digital signal processing.
That's why some FAX modems will support 9600 FAX rate but only 2400 for
computer links.
Talking of which, does anyone have experience of the SRM-2496 modem, and the
Z-FAX software that goes with it? This is a US-made FAX (9600 sync) and
V.21,V.22 (600 to 2400 duplex async) modem board for an IBM PC. It's sold
in the UK for "#299 list, #169 our price" by Datatech Systems (and no doubt
others), and comes complete with the Z-FAX software which allegedly supports
FAX send/receive, FAX mail-merge, transmission delay to cheap rate times,
FAX to list, print, view with zoom/rotate/clip, and supports PCX and TIF
image formats.
I'm thinking of buying one. What are the snags?
Regards, "None shall be enslaved by poverty, ignorance or conformity"
David Wright STL, London Road, Harlow, Essex CM17 9NA, UK
dww@stl.stc.co.uk <or> ...uunet!mcsun!ukc!stl!dww <or> PSI%234237100122::DWW