Lukasz Sokol wrote:
Hi Mark,
On 18/07/15 17:43, Mark Morgan Lloyd wrote:
Has anybody used FPC to decode (and generate) FSK modem tones on a
VOIP connection? Strictly V.21, don't need the complexity of QAM
etc.
i haven't ;) but it's interesting: are you just assembling/decoding
the ulaw/alaw/choose$codec data into/from
RTSP and sending/receiving through a SIP voice connection?
How real-time does this need to be, to start with?
Real-time enough to simulate a V.21 modem as the backend of a comms
program running a proprietary telemetry protocol. Before anybody
criticises use of an obsolete modem type, I suggest they consider how
long it takes to set up a V.21 call and transfer a few dozen bytes
compared with the training time of something more sophisticated.
Leaving aside the call control for the moment, my current thoughts are
to accumulate G.711 over UDP into a buffer ensuring this is a multiple
of four samples, use x86 MMX facilities to extract the mark/space
frequencies, and something like a rolling average to measure which of
those is dominant.
--
Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk
[Opinions above are the author's, not those of his employers or colleagues]
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