Subject: Re: [linux-audio-dev] MTC, SMPTE, etc.
From: Maarten de Boer (maarten.deboer_AT_iua.upf.es)
Date: Thu Jul 20 2000 - 20:31:09 EEST
> 2) SMPTE
>
> This is more complex, but Takashi noted that Maarten gave him a
> SMTPE decoder, which would be very nice to have as a library or
> something.
I did indeed. If more people are interested, and can make the code
public, but I thought it would be better that Takashi adds alsa audio
input and alsa sequencer output on it first.
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.
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.
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...
Maarten
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