[vorbis-dev] How many MIPS for Vorbis Decoder?

[John Ripley]

Dan Conti wrote:
> 
> On Fri, 19 Mar 2004, John Ripley wrote:
> 
> > It just occurred to me that a lot of the higher performance embedded CPUs
> > tend to have ethernet, USB or wireless interfaces built in at a cost saving
> > vs separate chips. That might make some of the otherwise expensive higher
> > performance controllers more attractive.
> 
> The other factor here is they may have enough on chip memory to avoid
> requiring an external chip. For example, the cirrus logic 7312 has about
> 48K of on chip memory available.
> 
> I do not know how much memory vorbis will require, but i believe i looked
> at lwip/uIP before and decided i could get either of those into 3-5K of
> ram. The only other factor is space for decoded audio buffers, the size of
> which sort of depend on your network performance and consistency of your
> decode times.

The 7312 is a fantastic embedded CPU, but it's quite dated, and 48KB is
way too little RAM. I believe the internal SRAM on the Cirrus 7312 is
also half speed, and very many cycles to access. I ran some benchmarks
on it a while back and discovered to my horror that it was about 5
cycles random access and 2-3 cycles sequential depending on core clock
speed. It wasn't actually worth the programming effort to put anything
in internal RAM because cache was way faster, and internal vs external
made very little difference to power consumption or performance.

If all you want is to play a low quality stream, then 48KB might shave
by, but you're looking at a LOT of optimisation and even then quality
tradeoffs. Let's put it this way - having only 288KB of RAM and an
arcane DSP architecure is what is preventing me from adding Vorbis
support to the remaining Rio platforms out there are the moment. That's
288KB before a rather complex UI eats a chunk. It's possible to fit the
rest, but it'll take way longer than my spare time :)

I think over the next year we may see a lot of "MP3" player chips which
have plenty of internal RAM, e.g 256KB+. Embedded DRAM (refreshed 1T)
seems to be the next big thing. Makes my life easier anyway :)

- John Ripley.
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