[fpc-pascal] Modems etc.

Mark Morgan Lloyd markMLl.fpc-pascal at telemetry.co.uk
Mon Jul 20 11:49:12 CEST 2015

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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