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FAST Protocol
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Re: FAST Complaints
David Rosenborg / Pantor Engineering AB
26 Jan 2010 5:06AM ET The tid appears after the pmap so it can be optimized away most of the time using the implicit copy operator. It not only saves you bytes on the wire, it also allows you to save table lookups and similar operations depending on your implementation.
Not sure what you mean by reading the pmap twice though. Every implementation I've seen so far do the stop bit scanning once, store the bits in some way or other, then access the stored bits in order. So if you mean memory/register accesses I'd say there is at least one for every bit in the pmap. Even if you don't use intermediary storage for the decoded pmap, you should get away with finding its length once. And I don't see how the placement of the tid would change that.
/David
> After seeing a lot of complaints about FAST on this forum, I'd first
> like to say to the creators that I personally like FAST encoding. At
> first I was taken aback by the complexity, which I think is the source
> of most complaints. However once I had a full understanding of the
> specification, I realized the beauty of FAST and without it's complexity
> it would never be able to accomplish what it does.
>
> I do have one comment or question though... Why does the template id
> field come after the PMAP? Is there an implementation of FAST which does
> not use templates?
>
> The reason I ask is because in my FAST decoder there is no way I can
> figure to decode without first looping through the PMAP to see if the
> template id is present, looking for the stop bit, extracting the
> template id, then starting all over (now that I know the template id)
> and reparsing the PMAP along with skipping over the template id.
>
> To me this seems inefficient to loop through the same bytes twice.
> Everywhere else my decoder never touches the same bytes twice. I really
> can't figure there is a solution to this, but maybe I'm wrong? Anyway,
> if there was one change I could make, that would be to have the template
> id be a mandatory field which precedes the PMAP and also remove the
> template id bit from the PMAP.
Re: FAST Complaints
David Rosenborg / Pantor Engineering AB 26 Jan 2010 5:06AM ET
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