Hi, Most people using iaxmodem + HylaFAX or spandsp from an Asterisk application get satisfactory results for both transmitting and receiving FAXes. However, quite a few people find FAX receive is reliable, but FAX transmit can be flaky. Its box dependent. The cause of this has been known since the early days of spandsp. Asterisk + zaptel/dahdi is unable to produce a consistent audio stream on transmit, even though it usually receives OK.
I suspected Digium might have finally done something about this, as they are releasing a FAX module similar to app_fax. It looks like they have. In chan_dahdi.c there is now code that extends the buffering inside dadhi when a FAX is detected, and puts the buffering back to normal at the end. This isn't really a cure - its more of a bandaid. However, I expect it has the desired effect if they have put it into the trunk code. You need to enable this feature in chan_dahdi.conf. The current default chan_dahdi.conf has the following line ;faxbuffers=>6,full You will need to remove the ";" to make the feature work. I doubt that 6 is a big enough number, though. 8 might be more successful, based on audio logs I have received in the past. Background --------------------- In the early days of developing spandsp, a number of people reported they could receive FAXes reliably, but could not transmit them reliably. As this did not happen on my test machines, I asked a few people to try a test, and send me the resulting audio files. The test was as follows: - Loop 2 E1/T1 ports together - Use patched app_rxfax and app_txfax apps, which records the audio in files - Set up a FAX call, so it dials out through one E1/T1 and back into the other - Send me the resulting audio files What I found was occasional gaps of 20ms, 40ms, 60ms, or sometimes even 80ms in the received audio. The gaps all appeared to be an exact multiple of 20ms. If these gaps were edited out by hand, the received signal precisely matched the transmitted signal. One would expect this problem to be on the transmit side, as a problem on the receive side would likely loose some of the signal, not inject silences. This was basically confirmed by tests between a flaky box and a reliable one. Reports said the problem happened on lightly loaded machines, so it does not appear to relate to heavily loaded machines. This problem was reported, but nobody seemed to care. Without a machine displaying the effect in my possession, it was impractical for me to investigate the root cause properly. Very interestingly, many people who have problems sending from app_txfax or app_fax have no problem sending from iaxmodem + HylaFAX. It seems Asterisk can feed a zaptel/dahdi channel much more reliably when passing a signal through, than when generating on in an app. After all these years it looks like Digium have finally had to face up to this problem to get a commercial FAX package out the door. Everyone should benefit. Regards, Steve _______________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users