Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.fax
Subject: Re: Fax Quality
Message-ID: <3833.27d6468f@hayes.uucp>
Date: 7 Mar 91 13:56:31 GMT
References: <7120@emory.mathcs.emory.edu>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 34

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