ANSam vs CED
Thought I should share these.
First a waveform view of two call start tones sent by two different fax machines. The top one is a SuperG3 fax, it sends an ANSam tone (specified in ITU V.8); the first giveaway is the 15Hz amplitude modulation which shows up as little bumps and valleys on the waveform. The bottom fax is a regular G3 fax machine, it sends a CED tone (specified in ITU T.30); the main signal is also at 2100Hz but there is no modulation.
Second a spectrum view of the two calls. The traces are VoIP traces so they aren't completely clean, but looking at the bottom trace it is clear that there aren't regular interruptions in the spectrum of the first tone sent out. On the contrary the top row shows regular bumps; these are actually phase reversals every 450ms (again, per V.8) which indicate to potential echo cancellers along the way that they should take themselves out of the path for the duration of this call.
Analysis done using Wireshark's RTP Payload export, and Audacity.
posted: 2011-02-02 22:54 tags: fax, modem, analysis