Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: Paul Barton-Davis (pbd_AT_Op.Net)
Date: Fri Jul 21 2000 - 04:55:45 EEST
>The code is pretty straightforward: the LTC biphase signal is extracted
>from the audio signal (using a hi/lo state threshold), the biphase signal
>is converted into binary, and whenever a SMPTE sync word is encountered,
>the decoder has read a SMPTE frame. I tested with some pretty weird
>SMPTE (from a TASCAM multitrack recorder, moving the tape speed up and down)
>and the decoder finds almost all frames.
Great!
>I am not sure what would be the best way to have this decoder. A library,
>as Paul suggests, might be a good idea, but in the end I think a stand-
>alone application, which shows SMPTE time, and has error indicators,
>and maybe can regenerate SMPTE, would make most sense. The alsa sequencer
>can be used to connect it to other applications.
I know that my applications could/would not use it via the
sequencer. I don't like the sequencer much at all, not because of the
implementation but because of the concept.
I would much rather have a C++ (or C) function that accepted
audio data in, and returned SMPTE time, and another than accepted
SMPTE time and returned a buffer of audio data.
>About jitter: of course the audio input buffer size adds jitter to the
>SMPTE. I am not sure if this is a problem. The SMPTE time could be
>adjusted accordingly...
I don't think you necessarily mean jitter, do you ? More like latency?
Unless you're referring to times when the SMPTE signal crosses the
fragment (interrupt) boundaries ?
--p
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