[vorbis-dev] Fixed vs. Floating Point primer. was: Implementi ng Vorbis in hardware

Borgerding, Mark A. MarkAB at xetron.com
Tue Jan 23 09:23:45 PST 2001

> -----Original Message-----
> From: owner-vorbis-dev at xiph.org [mailto:owner-vorbis-dev at xiph.org]On
> Behalf Of Steve Underwood
> Sent: Tuesday, January 23, 2001 10:58 AM
> To: vorbis-dev at xiph.org
> Subject: Re: [vorbis-dev] Fixed vs. Floating Point primer. was:
> Implementing Vorbis in hardware
> "Borgerding, Mark A." wrote:
> > [...] 
> > So I guess my point is this:  If we just want to say that Vorbis is
> > available for a DSP chip, then start with a floating point 
> platform (I've
> > heard good things about ADI's Sharcs).  But if we want to 
> make it most
> > attractive for portable player companies, then a 
> fixed-point DSP algorithm
> > will probably be better received.
> I think that is a really bad idea. With many very cheap MP3 players
> already in the market, the only thing anyone would be 
> interested in is a
> competitively low cost Vorbis platform. Put Vorbis on any 
> floating point
> DSP, and people will say its resource hungry. It would then 
> be very hard
> to dispell that impression.

I'm not sure you understood my comment.  I said it would be faster to port
Vorbis to floating point hardware, but fixed point would be better for
portables.  The best option depends upon the goal: time-to-market and
maintainability vs. power and cost efficiency.

I don't presume to know the market well enough to think there is only one
correct answer.

> It can be a Royal PITA programming fixed point DSPs (I've used many),


> but they are they only kind of DSP acceptable for consumer 
> products, and

Depends on the product.  Example: in-dash car audio player.

> the only ones low enough in power consumption for a portable 
> player.

Somewhat agreed.  In products that already have high power needs for
amplifiers, moving parts, etc; the power requirement of a floating point DSP
might be negligible.

>  The MP3 people already had mature implementations on energy 
> efficient fixed
> point DSP architectures when the first MP3 players appeared - they had
> been working on them for ages for the MPEG video player field. That is
> what Vorbis needs to catch up on now. Monty said many MP3 players are
> currently using StringARM. That would be the easiest route to port
> Vorbis to a portable platform, as you can get a workable 
> solution with a
> few tweaks and a recompile. Since it avoids the need for a separate
> management/control processor, StrongARM may be the most cost effective
> platform overall.

Either I underestimate the amount of floating point math that is done, or
you underestimate the problems with dynamic range that will be encountered.
I think it will take more than "a few tweaks and a recompile".

I admit I am rather ignorant of the ARM. But I do know that it is more of a
GPP than a DSP.  
Why is it popular for MP3 decoding?  Is it mainly a case of overall
popularity of the chip and availability of tools?

Gregory Maxwell said, "There is a specific product based on ARM which vorbis
has been ported to. "
What is this product?  Is the source code available?  Is the Piccolo
coprocessor required?

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