Conference hips::uk_audioo

208.0. "Full Digital Hi-Fi?" by HAMPS::LINCOLN_J (Where sheep dare) Fri Sep 21 1990 13:51

	Now that digital sources are becoming commonplace why not, I wonder
	,extend digital control to the entire hi-fi system?.

	Audio reproduction was an entirely analogue, open loop system,
	and this is where the errors creep in. With an open loop system
	the individual components need to be of precise quality in order
	that each stage of the process introduces as little error, ie
	distortion, as possible. 

	Microphone produces analogue signel
	Signal recorded on tape
	Tape transcribed to disc
	Disc applies movement to stylus
	Stylus motivates cartridge
	Cartridge generates analogue signal
	Amplifier amplifies signal
	Speaker converts signal to air movements.

	Plenty of scope for degradation as comparisons between the best
	and the worst will show.

	Nowadays the microphone may produce digital codes and these are
	committed to CD discs without degradation but that's where the
	story stops. The digits are converted to analogue and the rest
	of the process remains unchanged.

	However it seems to me that it would now be possible to close
	the loop by use of computer technology.

	Monitor the position of the speaker diaphragm with a digital 
	encoding transducer, probably optical for zero mechanical contact,
	and then it's possible to replace all of the intermediate stages
	between disc and diaphragm movement with a positioning servo
	system. The D-A convertors which appear to be the main weakness
	of current CD players would go and the amplifier no longer needs
	to be of great precision since within a closed loop system such
	things aren't critical. With the current sampling rate for CDs
	of 44.1 kHz you'd need a microprocessor capable of at least 2
	mips in order to execute the servo code in the time available.
	Such processors are commonplace today.

	Given that even top class sytems produce some 10% distortion
	(mostly from the speakers) there would also appear to be an
	enormous improvement to be gained here. If one wanted to be 
	truly pedantic you could place a microphone in the room at
	the listening position and compare it's output with the source
	data. This might also reduce speaker characteristics.
	
	Far fetched?, I don't think so. I once worked on some software
	not so very far removed from what's required here and it would
	be simple enough, though one imagines that some programs might
	perform better than others!. 

	I wonder whether this is being worked on now. I notice that
	many CD players are available with digital outputs nowadays.
	It's said they're so you can do A-D within the amp and avoid
	the intermediate analogue stage but does it have to stop there.

	-John

I seem to remember that some people are working on Digital speakers, with
16 different 'binary' speakers.

You've got to remember though, that digital audio equipment is incapable of
fully rendering the continuous nature of music.

You can compare the original digital input, and the resulting digital output,
but this, of course, may bear no resemblance to the analogue input and the
analogue output.

Cheers.

Mark.

    Re 1 & 2.
    Keep your eye (the good one that is!) on a UK firm called MERIDIAN
    (they already have an outboard D-C converter) who are currently working
    towards your idea of room compensation and digital signaling to the
    last stage (the amp/speaker). Interesting stuff......which is why I am
    holding back on preamp purchase to see what the future turns up....it
    just stupid to convert to analogue so early in the chain if digits are
    going to become the future storage medium......my experience to date is
    that digit players cost a lot more than the old analogue information
    retrieval units (turntable!!) of similar 'picture'/event reproduction
    quality...forgetting the CD versus LP wear/sound arguments as well as
    the consumer costs for CD (3x in my case) versus LP. I have both but
    digit hardware is so young and underdeveloped as of yet!
    
    cheers fjeff who's-keeping-his-LPs-anyway-!