ODP: DSP Stuff

[Roman Sowa]

Hi Harvey and all
see my comments below:

> -----Oryginalna wiadomość-----
> Od:	Harvey Devoe Thornburg [SMTP:harv23 at leland.Stanford.EDU]
> Wysłano:	7 stycznia 1999 07:31
> Do:	mwmjr at ix.netcom.com
> DW:	synth-diy at mailhost.bpa.nl
> Temat:	Re: DSP Stuff
> 
> > 
> > >Beware, there are two EZ-Kit's: one with the ADSP2181, which is about
> $100.  
> > >This is a fixed point DSP. Then there's another one with the floating
> point  
> > >Sharc DSP, but it is more expensive. The floating point capabilities
> should  
> > >make it easier to program, but I have no experience with that.
> > >
	[Roman Sowa]  
	I have seen that one (SHARC that is) in action,
	and I almost bought it then. I costs about 300$.
	As for 2181 ezkit - well, it has software tools included and
manuals,
	but programs I develop on another platform

> Floating point is really the most natural for audio, because our 
> perception of frequencies and amplitudes is logarithmic, and also 
> because the most efficient attempts to simulate floating point
> with long ints in fixed point instructions almost always generates 
> unreadable code (reliance on shifts etc.) for ex. to divide by 255 
> (common in graphics and image processing) on a fixed point 
> machine it is easier use the 
> relation 1/255 = 1/256 + 1/256^2 + 1/256^3 + ... and use shifts and
> accumulates.  I have seen, but thankfully never had to write, this kind
> of stuff in audio applications.  Do you really want to be thinking 
> about this stuff (hint: the compiler never helps you) or thinking 
> about the algorithm?
	[Roman Sowa]  
	floating point is helpful when for example
	filter coefficients are more than 1 (and they like to be very much).
	In fix-pt you have to scale the coefs and signal, process it and
	rescale again. Besides SHARC's architecture is *much* easier
	to write especially in assembler. There are multi-purpose registers
	instead of ALU or MAC oriented. And there are more of them.
	more instructions and better cyclical addressing.
	I could go like this for hours...
	btw 2181 can divide. It takes 16 cycles, but can be done with
	2 instructions
>  
> One thing might be disturbing with floating point is the generation 
> of underflow errors. let's say you're using a one-pole filter for an 
> envelope generator (like for a 303 model) and there's a long time 
> between gates, then sooner or later you will underflow. 
> some processors (Pentium) react very poorly to this, causing much 
> slowdown (or maybe this is due to Microsoft DirectSound standard?) 
	[Roman Sowa]  
	That's why I cancelled subscribtion to music-dsp list. Everybody
	were Pentium-oriented, while most of the stuff made with real
	DSPs is simpler.
	In no way underflow will affect float-DSPs speed or funcionality.

> Traditionally the audio industry has used Motorola 56K's but many new
> companies are transitioning to ADSP21061 or 21160 (600 mflops) sharc.
> The main issue seems to be floating point, the availablity of
> C development environments, and parallelization (hardware support
> for multiprocessing stuff like semaphores). I think Digidesign still 
> uses 56K (which isn't much consolation for the $$$$$ for protools
> systems)!
> There is some effort underway to port Csound to the sharc but I am
> not up on this.
	[Roman Sowa]  
	SHARC is gaining because of $$$ droppings. When I first heard about
it
	it cost about 300-500$. Now I can buy it for 50$ in single
quantities.
	(10$ for OEMs)
	I no nothing about paralel processing, so this quad SHARC chip was
	for me only a cool looking device.

> Something which interests me is the TI stuff like TMS320C60 floating
> point series.  Performance is supposed to be in excess of 1000 mflops.
> But the evaluation kit costs $1500 not to mention 3rd party development
> tools.  I've been considering it (just to be different and to work
> with a more powerful machine) but I don't know many in industry who
> are adopting TI.
	[Roman Sowa]  
	I'd be interesting to see someone DIY with TMS320C6xxx.
	It's BGA package. Not to assemble with known to me tools IMHO.
	I was also considering buying this development system (fixed point)
	(I had an discount offer for just 1k$), but I'd rather spend it
	on Yamaha AN1x and tweek it ;-)
	TMS320C6xxx floating point so new that it's too hot for me.

	Roman
>