Jeff, I'm more than happy to collect captures from the "good" one, and re-collect captures from the "bad" ones. The captures I provided were just the ones I took when asked to by someone else a few months back, and he said he didn't particularly care about the captures from the "good" one, so I assumed he already had something specific in mind that he suspected might be the problem that perhaps he could identify/confirm easily just by looking at the failing captures. (Also, in my dealings with Yeastar, Grandstream, Flyingvoice, and even Adtran support, they always have asked for straight-off-the-DSP taps. Though I agree: you can't detect every problem with those. Like, what if something's going wrong with the DAC and/or amplification circuitry?)
When I have a few minutes sometime in the next few days, I'll reproduce the issue again and re-capture it, this time with analog taps. -- Nathan -----Original Message----- From: Jeff Brower [mailto:[email protected]] Sent: Tuesday, February 17, 2026 17:07 To: Nathan Anderson Cc: [email protected] Subject: Re: [VoiceOps] Re: Bizarre T.38 gateway/DSP modem interop problem Hi Nathan, If you want help, you might be making it a tad hard. You need to have audio output both from the Mot ATA and at least one failing ATA. Getting that from the ATA output (actual RJ-11 line) is the only way -- I wouldn't trust "debug outputs" further than I can throw a rock. I used to work on a wide range of DSP based devices, and debug/test outputs were never a high priority, did not include everything on the actual I/O lines, and only got updated from time-to-time. But from your subsequent mails it seems you're making progress, so things are looking good :-) -Jeff Quoting Nathan Anderson via VoiceOps <[email protected]>: > Jeff, > > Yes, correct: in my test environment, I simply swap out one of the > many "bad" ATAs for the Moto ATA (or vice-versa), and have it > register to the exact same SIP proxy while plugged into the exact > same fax modem, receiving test faxes from the same remote machines. > When using a specific fax modem, the Moto succeeds, the others fail. > If I change out the fax modem for a different one (different > model), the success rate of the non-Moto ATAs goes up. > > I have taken captures from ATAs by 3 vendors while using the "sus" modem: > > Flyingvoice > Yeastar > Grandstream > > In each case, I have captured: > > 1. A packet capture of the SIP, RTP, and (after re-INVITE) UDPTL > payloads (packets.pcap) > 2. Audio recordings of the FXS port, from the ATA's perspective (voice*.raw) > 3. Debug/console logs from the DSP captured during the session (console.log) > > In the case of Flyingvoice: both the TX and RX channels are included > in a single 2-channel PCM recording, 16-bits @ 16kHz > > In the case of both Yeastar and Grandsteam: TX and RX are separate, > 1-channel PCM recordings (voice_r for RX, voice_t for TX), 16-bits @ > 8kHz > > There are no WAV headers, but you should be able to import the raw > PCM into e.g. Audacity by specifying the above characteristics > manually during import. > > The files are available at > http://projects.fsr.com/ata-fax-captures.zip for anybody who is > interested/curious. > > I don't have any audio captures of successful fax reception from the > Moto. All of the audio captures I made for these other ATAs was > from the perspective of the ATA (both what it was hearing on the FXS > port, as well as what audio the DSP was generating and putting out > on the FXS port), using debug tools built into the ATAs themselves > for capturing them. I have not found a way to get the Motorola to > do the same thing, so to capture its audio, I'd have to put an > analog tap in place in between the ATA and the fax modem and > re-digitize it (which I could do, if anybody is curious to see > that). I do have pcaps of successful fax reception via the Moto > (both with and without ECM), though, if people care to see those. > > -- Nathan > > -----Original Message----- > From: Jeff Brower [mailto:[email protected]] > Sent: Monday, February 16, 2026 07:37 > To: Nathan Anderson > Cc: [email protected] > Subject: Re: [VoiceOps] Re: Bizarre T.38 gateway/DSP modem interop problem > > Hi Nathan, > > What captures do you have ? Are those wav or other audio format files > ? How long are the captures and do you know or can guess how far into > the capture is the first sign of trouble ? > > I assume when you say working that's the Mot ATA output and > non-working is one of the failing ATAs, with no changes in > transmission conditions or settings. I.e. everything is exactly the > same but somehow the Mot ATA output is a tiny tad better / different. > > -Jeff > > Quoting Nathan Anderson via VoiceOps <[email protected]>: > >> Raising this thread from the dead to see if anybody else who >> might've missed it the first time has any bright ideas. >> >> Shortly after I made the original post, a very kind gent with some >> actual DSP and fax-specific experience responded off-list, asking >> for some captures of working and non-working sessions. I sent those >> along, but unfortunately he seems to have dropped off the face of >> the earth. :-( Not that I really blame him...he was graciously >> volunteering his free time and expertise, and life is busy. But it >> just means I effectively lost one of the only leads I had. >> >> I'm desperate enough that now I'm willing to start naming names in >> public. At this point, I've run into nearly-identical T.38 >> receive-specific problems with products I've tested from all of >> these vendors: >> >> * Grandstream >> * Yeastar >> * Flyingvoice >> * (HP/)Poly(com) (f/k/a Obihai) >> * ...even Adtran >> >> It is mind-blowing to me that the only ATA I have ever found that >> works reliably with T.38 *reception* regardless of what modem I hook >> up to it is the freaking ancient Motorola model that I can't get >> anymore. The modes of failure across all of the newer ATAs that >> don't work are so strikingly similar that either I'm consistently >> doing something wrong without realizing it, or all of the engineers >> behind these products made the same wrong assumptions in their fax >> DSP code that do not hold true across all fax modems (or perhaps >> they share some [bad] code in common with each other! ...I do have >> reason to believe that at least 2 of the above vendors are using the >> open-source SpanDSP project/library to implement their T.38 gateway >> stack in their firmware!!) >> >> With the modem I've been testing against, the Grandstream just fails >> to receive entirely. The Flyingvoice adapter, on the other hand, >> will eventually succeed, but only after it trains all the way down >> to 2400-4800bps. I have had tickets open with both Grandstream and >> Flyingvoice for months now; they seem to be going nowhere, though to >> their credit they haven't given up (or at least the front-line >> support people updating the ticket continue to put on a brave face). >> Yeastar (which also just fails entirely) threw in the towel within >> days when I tried to ask them. I had forgotten that I ran into an >> extremely similar problem with Adtran a few years back that their >> support people also never solved. >> >> I have not yet tested Cisco ATA19x models. The only Poly/Obi one >> I've tried is a 300-series, which is now discontinued & replaced >> with the 400-series, so HP/Poly support won't touch it. I have >> considered acquiring a Poly 400 and a Cisco ATA192 and opening up >> tickets with both, but I just know I'm in for a bad time with both >> company's TACs if I do. >> >> Help me, Obi-Wan Kenobi... >> >> -- Nathan >> >> -----Original Message----- >> From: Nathan Anderson >> Sent: Thursday, November 13, 2025 23:21 >> To: Voiceops >> Subject: Bizarre T.38 gateway/DSP modem interop problem >> >> (...or, "Any currently-manufactured ATAs with a T.38 gateway >> implementation worth a damn?") >> >> Perhaps some will find this shocking, but for the longest time, we >> have been using Motorola VT1005 as our basic, low-port-count TA. We >> had lucked into a large source of overstock, still-new-in-box units >> for cheap some time ago, but that source is now gone. So we are >> shopping around for a new model to take its place. >> >> Part of the reason we stuck with the venerable Moto for so long was >> because our wish list looked like this: >> >> 1. Reasonable price point >> 2. Good performance for price >> 3. Solid T.38 implementation >> >> More to the point, we preferred a single TA that could fulfill all >> requirements, rather than having to stock multiple different models >> (e.g. one for voice-only, another for customers who actually cared >> about fax, etc.). And for the residential/SOHO crowd, it struck me >> as ridiculous that some 1-2 port count TAs out there often have >> MSRPs that are higher than the routers they're going to be sitting >> behind (I'm looking at you, Cisco...). >> >> The thing about the VT1005 is that not only did it have a solid T.38 >> gateway feature, but it was hands-down the MOST bullet-proof >> implementation I have EVER run across, period. It "just works". >> Even if I was okay with stocking a special model for our fax-using >> customers, and even if price was no object, I seemingly CANNOT buy >> another TA with as good an implementation for love nor money. It >> was the same story every time: every couple of years, I'd order >> another TA model and/or pull out some previous eval units we'd >> acquired before & update their firmwares, re-test them, and still >> run into tons of issues. So as long as the Moto was still >> available, I just kept kicking the can down the road. >> >> I'm going through that same hell again now, and I have realized over >> the last few weeks of opening tickets with hardware vendors & >> tearing my hair out that there is a common thread to my failing fax >> tests. >> >> 1. Fax TRANSMISSION always works fine (T.38 gatewaying kicks in, the >> modems train with each other at 14400bps, pages are sent >> successfully). >> 2. Fax RECEPTION is what breaks down (T.38 gatewaying kicks in, but >> the receiving modem -- the one plugged into the TA on our side -- >> always Fails To Train at virtually any speed) >> 3. ...though #2 is only true with CERTAIN fax modems, while others >> can receive faxes with non-Moto ATAs JUST FINE, at speeds up to >> 14400bps >> >> The fax modem I usually run my tests through is a cheap little >> USB-based hardware modem, but one with only Class 1.0 fax support. >> It's based on what seems to be a fairly ubiquitous Conexant chipset, >> the CX93010. When paired with Windows Fax & Scan and connected to a >> Motorola VT1005, receiving faxes via T.38 works just *fine*. But >> when paired with literally any other ATA with T.38 support that I've >> tried, it will either attempt but fail to train at 14400bps all the >> way down to 2400bps, or (with one ATA in particular) it will finally >> successfully train and send CFR after training all the way down to >> 4800bps, or 2400bps at the worst. >> >> As far as I can tell, this is not strictly speaking a T.38 problem >> per-se. This is an issue seemingly with the DSP on the ATA that's >> emulating the remote modem, and there is something about most of >> these DSP implementations that at least this particular >> Conexant-based modem does NOT like. It can send faxes through these >> ATAs all day long, but whatever tones these TAs are generating, the >> Conexant just isn't having it. >> >> If I sub in a different fax machine in its place (e.g. big HP >> multifunction Laserjet), fax reception (mostly) works great through >> a lot of these same ATAs. And similarly, if I just put the Moto >> back in service with the Conexant modem, that also works just fine. >> >> Now, sure, blaming the modem is fair game. And I don't discount the >> possibility that there is something that it's doing wrong. The >> thing is...the Moto VT just freaking works with it. And the fact >> that there is at least one modem model out there -- one with a >> common enough chipset -- that virtually all currently-manufactured >> TA models out there spouting T.38 support cannot interop with makes >> me concerned that I'm likely going to run into more such interop >> problems in the field with customers' own fax equipment, after we >> start deploying & the TA we choose to go with is suddenly exposed to >> a much more, erm, diverse crowd of fax machine models. >> >> What on earth could this modem could be so sensitive to that it >> doesn't work with any of the TAs I've tested with it (other than the >> Moto)...? >> >> -- >> Nathan Anderson >> First Step Internet, LLC >> [email protected] >> >> _______________________________________________ >> VoiceOps mailing list -- [email protected] >> https://lists.voiceops.org/postorius/lists/voiceops.voiceops.org/ >> To unsubscribe send an email to [email protected] > > > _______________________________________________ > VoiceOps mailing list -- [email protected] > https://lists.voiceops.org/postorius/lists/voiceops.voiceops.org/ > To unsubscribe send an email to [email protected] _______________________________________________ VoiceOps mailing list -- [email protected] https://lists.voiceops.org/postorius/lists/voiceops.voiceops.org/ To unsubscribe send an email to [email protected]
