On 24 May 2012 10:43, Jon K Hellan <hel...@acm.org> wrote:
> There should be some bits identifying the codec, and/or whatever else
> somebody may want to change. If D-Star had that facility, we could have
> just changed the ID.
>
> Ihe ID can be done in many ways. Some bits in each frame, some bits
> every N frame, or even a metadata frame once in a while.
>
>
Well, in my bigger D-star like specification (for what was 2500 and now
2400 speed) I'm implementing a "mode" flag in the header. But, the header
is only transmitted once (and then usually slowly over the slow data
channel over a course of a second or so). Actually now the codec is 2400bps
there's no reason at all we couldn't copy the d-star spec entirely and just
change the sync patterns. Of course to be sensible we'd need to change
which parts of the codec have FEC applied (so that the less important parts
aren't protected).
I guess we could place a codec identifier in the header and then in the
data segment from time to time. Or make the voice sync 20 bits and make the
last 4 bits a codec ID (16 possible codecs). There's no real reason that
the same framework can't be used and just the sync pattern changed for
other vocoders.
For the smaller voice only modems we're working on right now. It's really
just sync, codec data and FEC of one kind or another. I think making
multiple vocoder possibilities at this level is overkill. In this case it's
meant to be as simple as possible.
73
Peter - M6DGI.
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