On 27 May 2012 12:56, Bruce Perens <[email protected]> wrote:
> David,
>
> Even without the AX.25 headers, the TNC frames its own packets.
> Unfortunately, TNCs buffer the packets so the delay is horrible. And the
> throughput is somewhat less than 1200 Baud, so long transmissions
> accumulate an additional delay. Still, it's useful for a demo.
>
>
My understanding is that the KISS TNC framing is just for the serial link
(a way for the computer to talk with the TNC and keep in sync) from what
I'm reading this is not sent over the air. So for that, you must either use
AX25 and deal with the lower bitrate. With a lower bitrate of course if you
buffer up what cannot be sent you will be able to complete the over, but it
will take some time after the talking is over to complete the transmission.
That's fine, if a little inconvenient. However upon reception it's another
story. You cannot play it in real time else there will be gaps between
frames, so it will sound.. Well bad. Of course you could buffer the input
some. But without knowing the length of an over, it'd be problematic
knowing how much to buffer to be able to end on time. It'll also make weird
gaps in simplex conversations.
For 1200bps audio over a 1200bps link there's, 3 choices as I see it. Two
need to exploit the codec. The remaining one, is going to result in
fidelity loss.
1: Basic.. Every x frames, some of the codec audio is dropped in order to
send a sync pattern. This means that there will be some fidelity loss on
those frames with sync on.
2: Using the method David speaks of, using the codec2 frames to establish
sync by using the ordered LSP's as a sync method. As he says this is not
100% yet. But it does have no overhead and thus no loss in fidelity... On
the 87% or so frames it syncs fine
3: Another suggestion I saw David mention. It would need some way of
detecting "silence" in a frame. Or when audio drops to a point considered
silent. When these are detected a sync frame could be sent instead. Of
course when there's no silence for some time, if sync is lost it could be
some time before sync is re-established. But, I guess some combo of this
with option 2 could be ideal.
Not sure what everyone thinks though.
Best regards,
Peter.
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