Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!cmcl2!lanl!lambda!crs
From: crs@lambda.UUCP (Charlie Sorsby)
Newsgroups: sci.electronics
Subject: Re: Digital/Analog converter help needed
Summary: What 100w DACs?
Message-ID: <14454@lambda.UUCP>
Date: 1 Aug 90 21:52:06 GMT
References: <3550.26a1d2d7@ccvax.ucd.ie> <1839@island.uu.net> <1990Jul24.215643.22718@amd.com>
Lines: 61

In article <1990Jul24.215643.22718@amd.com>, ncpjmw@brahms.amd.com (Mike Wincn) writes:
> In article <1839@island.uu.net> rich@island.uu.net (Rich Fanning) writes:
>>In article <3550.26a1d2d7@ccvax.ucd.ie> b_haughey@ccvax.ucd.ie (Brian J Haughey) writes:
>>>What I am looking for is info on D-A converters that can directly drive
>>>speakers. My query is : would the scaling factors of such a D-A have
> [...]
>>There are at least two ways to do this: convert D/A at a one-volt, high
>>impedance level, and amplify in analog. 

> Well, yeah, maybe... though I haven't heard of any 100w DACs anywhere yet.

Whoa!

What 100w DACs?  "one volt, high impedance" doesn't imply 100w at
all.  My reading of the above suggests that the power comes from
the analog amplification.  Granted that doesn't address the
original question of driving speakers directly from DACs, but....

That isn't to say that DACs *couldn't* be built to drive
speakers...

To drive speakers directly, you would need DACs able to provide
from a few hundreds of milliwatts to a few hundreds of watts, in
which case, the criticism may be valid when speaking of commercial,
existing, DACs.

>>Or amplify the digital signal pulses,
>>send through a filter to round off all those nasty sharp edges, and out
>>to the speaker.

Filtering won't do it.  The output of a DAC *isn't* pulses; it's a
level corresponding to the *value* of the digital data.

> THIS idea hasn't a snow-balls chance in hell of working!  Think! ...how in
> hell do propose to sum up all those digital pulses to reconstruct the 
> analog program material before sending it to the load?

In principle, the same way that one does so at micro-milli-watt
levels in any other DAC application.

>>Either way, it would probably be desirable to multiply the 16-bit audio
>>coming in by a scale factor which represents the "volume".
>>Given fast enough hardware, it would probably be reasonable to do a
>>"brute force" approach:  represent the volume by a 16-bit value, and multiply
>>the 16-bit audio signal to get a 32 bit result.  Take the top 16 bits,
>>and there's your scaled digital signal.

Use a "multiplying DAC" where the analog voltage input represents a
funcition of the volume and the digital input represents the signal
and the analog output represents the scaled analog.

The problem remains that (to my knowledge) no commercial DAC is
available that will drive other than *very* small speakers, if any,
directly.

NOTE: that my disagreement is not that 100 w DACs are not
commercially available but that the point where the criticism is
applied is non sequitur in that, at that point in the existing
suggestions, a 100 w DAC is not needed.

Charlie