There is an article of faith among analog electronics engineers that, if you have to send a signal through difficult conditions, pumping up the power is a good, time-honored approach to doing so. But what about distortion? Somewhere power boosts have to encounter diminishing returns of signal quality. For this reason, most transmission systems have published maximum power levels for the signals they are expected to carry. By the same token, there is a noise floor that limits the low end of the power spectrum over which a signal’s content can be discerned.
For facsimile, the ITU-T T.30 protocol specifies that a T.30 message signal should be intelligible over a power band described in the T.4 specification. The T.4 document states that “The receiving terminal should be capable of functioning correctly when the received signal level is within the range of 0 dBm to –43 dBm.” So, it’s official: any PSTN telephony network should be able to carry and any fax machine should be able to accurately receive T.30 messages at a 0 dBm power level and should function correctly all the way down to -43dBm. A number of punch lines come to mind but the simple truth is that real-world PSTN networks just don’t support this range of operability.
High Power and High Distortion
Incoming fax signals will typically begin to suffer distortion at about -8dBm to -7dBm. Some networks can handle levels up to -5dBm but that is just about the maximum that you can find. Fortunately, the very attenuation that higher power levels are designed to combat, reduces the input levels to fax receivers down to no more than -15dBm and often much lower. Fortunately, because fax machines have an even lower tolerance for over-cooked signals than the network equipment does. And the other end of the spectrum is not nearly so forgiving as the specs would have us believe either. Most PSTN networks have a noise floor of about -35dBm. This is the level where the system noise rises up and swallows the transmitted signal making it completely indecipherable. What to do?
The older T.30 implementations (before V.34 – Super G3) used the Training Check Field (TCF) signal during capabilities negotiations and a scrambled ones Turn-on Sequence before each high speed data transmission in an attempt to cope with distortion. The TCF used a second and a half of zero bits transmitted at the desired modulation, data rate AND power level to test the PSTN connection to see if the call had a distortion problem. They didn’t have an adaptive power level control but they could knock back data rates and use slower, more distortion-immune modulations to compensate. The Turn-on Sequence was a white noise pattern sent just before a high speed transmission (page image or TCF) that gave a parametric amplifier in the answering terminal a chance to adjust its frequency response to help beat back distortion. Fax terminal designers have, in recent years, begun to either ignore or dramatically loosen the tolerances on both of these aspects of fax transmission in the belief that modern phone systems no longer require them. But what happens when one of their devices must be used from a location that has a rather less than modern telephony network?
The Super G3 or V.34 level of fax uses a modulation that pulls out all the stops to get the last half ounce of transmission speed out of a fax call. V.34 employs extensive line probing and testing to determine exactly what distortion is present and then employs a whole array of tricks to combat it. The V.34 modulation has a vast array of modulation symbols that allow it to shape its symbol ‘constellation’ to play distortion off against the power levels of groups of symbols. It can adaptively adjust overall power levels, change symbol rates and even use high frequency pre- and de-emphasis to pull back a readable signal from noise and distortion. Amazing as these coping mechanisms are, they still can’t deal with an over-driven signal.
The best T.30 performance comes from fax machines that output signals in the -9dBm to -12dBm range. This offers the greatest interoperability with the widest range of PSTN networks and will function well with new digital and VoIP systems as well. PSTN networks will typically put out fax signals in the -20dBm to -30dBm range so this is where designers of fax terminals need to focus their answering efforts. The -30dBm to -35dBm range is the fall-off area towards signal oblivion, so solid reception down to -30dBm will cover the signals that a fax system has a real chance of receiving.
Trying to bull through marginal phone connections with more signal power seldom pays off. Or do you have a different take?