b_haughey@ccvax.ucd.ie (Brian J Haughey) (07/16/90)
[Repost of a query that died somewhere...] Hi there. I'm planning on doing some work with D-A converters, and I need some info before I start. 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 to change as the volume went from very low to very high ? Any advice, tips, or references welcome. Thanks a lot. -- bjh B_Haughey@ccvax.ucd.ie || "There's no future in time travel" b_haughey@ccvax.bitnet
rich@island.uu.net (Rich Fanning) (07/19/90)
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 >to change as the volume went from very low to very high ? I'm not exactly sure what you want here. You say "D-A converters that can directly drive speakers". By this do you mean a "digital amplifier"? There are at least two ways to do this: convert D/A at a one-volt, high impedance level, and amplify in analog. Or amplify the digital signal pulses, send through a filter to round off all those nasty sharp edges, and out to the speaker. 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. No doubt, there are more efficient ways to do it. And a simple multiply does not take care of rounding problems. How do professional digital mixers do it? -- Rich Fanning {sun,ucbcad,uunet}island!rich
damos@otc.otca.oz (Daniel Amos) (07/20/90)
in article <3550.26a1d2d7@ccvax.ucd.ie>, b_haughey@ccvax.ucd.ie (Brian J Haughey) says: > Xref: otc sci.electronics:3089 rec.audio:747 > > [Repost of a query that died somewhere...] > > Hi there. > > I'm planning on doing some work with D-A converters, and I need some > info before I start. > > 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 > to change as the volume went from very low to very high ? > In all my experiences with DACs, I haven't seen one that could drive speakers directly. Generally, DACs are designed to produce a reasonably accurate analog representation of a digital number - they don't include high power amplifiers to drive low impedance loads. A possible reason for this is that amplifiers usually dissipate a fair amount of heat, which is not really good for resistor chains inside DACs - you would prefer temperature stability. In practice, (ie if heat problems could be solved), a high resolution DAC (eg 22 bits) could be used, and a digital multiplier would multiply the bit stream from the digital source by a constant (the volume setting), and the DAC would output the result. Naturally, the DAC would require a peak-to-peak output voltage of at least 50V (to provide a reasonable output power). Dan Amos ||||OTC|| Services R&D
bill@vrdxhq.verdix.com (William Spencer) (07/20/90)
in article <3550.26a1d2d7@ccvax.ucd.ie>, b_haughey@ccvax.ucd.ie (Brian J Haughey) says: > 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 > to change as the volume went from very low to very high ? AND most important: > Any advice, tips, or references welcome. Thanks a lot. What are you trying to accomplish? Low cost? Amplifier chips start about 50 cents. This does bring up a good subject though... first, no, it's hard enough to do 16 bit D/A, much less put power handling into the same chip. But a higher output D/A would be nice. Best would be a high voltage output. This output would be essentially passive: a resistor ladder connected to the output by (FET) switches. Digital volume contol could be used but controlling volume in analog would be better. Passive filters could be used. If the voltage was high enough no voltage amplification would be required. Current amplification would be needed in ther form of unity gain buffers. Merely increasing the voltage ratings of a D/A chip is very possible. If digital could provide a means of eliminating almost all analog circuitry this would answer many audiophile concerns. Digital is complicated enough on its own without following it with complex analog. A "digital speaker" has been built (Meridian). All it really eliminates is analog cabling. This is not enough to justify it, especially if there's any chance of using analog sources. ANOTHER POSSIBILITY: "One bit" conversion at power. This is simialar to a "switching" or "class D" amplifier but there's also big differences. Despite the high effieciency and power output possible this type of amplifier is not popular and for good reason. Analog type switching amplifiers rely on loop feedback. "One bit" D/A conversion if a forward only process. Don't know what this would do with a complex load though. "One bit" conversion could also be used for the high voltage D/A concept above. Hope someone can benefit from these ideas. I'm not finished mucking with conventional analog yet. bill S.