Re: flexfax: RetrainNegative problem - not in FAQ

[David Woolley <david@djwhome.demon.co.uk>]

This question is *the* FAQ;  if it is not in the FAQ it should be.

> One client, i regularly send faxes to, reports a RetrainNegative,
> after transmission, BUT _every_ page got sucessfully transmitted
> and is perfectly legibly on the other side.

Are you sure you are getting RTN rather than ERROR from the modem?
ERROR from the modem is assumed to be RTN, but might be something 
else.  Please include a log next time.

Although technically RTN is due to bad analogue quality, in practice
it can be caused by coding errors in the digital data.  Most fax machines
have a very low threshold for reporting RTN based on coding errors.

There is a known cause of coding errors in an unpatched Hylafax to do
with the tag line and a patch to fix this.  There is also a patch to
slow down after RTN, which may cover some other causes.

Other causes, apart from the obvious bad phone line or modem, are overruns,
underruns and bugs in the serial driver (I don't entirely trust the
Linux drivers on some serial cards).

Limiting retries should be a simple matter of changing the configuration,
although I haven't checked.  The problem is that the fax will be abandoned.
I think a code change would be needed to make RTN be ignored completely,
and there is (reasonable) resistance to any such change away from correct
behaviour being put into the official version.

Ultimately though, if these are real RTNs, the problem is in the remote fax
machine for returning RTN on acceptable material.  (Hylafax will send
RTN if there are more than 5% malformed scan lines.)