@sun.acs.udel.edu:hamilton@sun.acs.udel.edu (hamilton) (02/26/90)
I read in the current issue of CD Review that it is possible to adjust the trim pots on the D-A convertors of a CD player to improve linearity. They adjusted the MSB on the converters of the Vector Research VCD-410R so that the linearity error was "essentially 0 error to well below -100dB." This happens to be the CD player I own, and I opened up my player, and sure enough, next to each D-A chip was a small trim pot. Obviously there is great potential here to improve sound quality for _FREE_. I imagine that it would work with any CD player with trim pots. Linearity below -100dB is something found only in the most costly CD players, and maybe in the new 1-bit machines. Does anyone know what is needed to perform the adjustment? Can it be done by the average joe? J. P. Grenert hamilton@sun.acs.udel.edu
mark@decwrl.dec.com (Mark G. Johnson) (02/27/90)
In article <2589@uwm.edu> @sun.acs.udel.edu:hamilton@sun.acs.udel.edu (hamilton) writes: > >I read in the current issue of CD Review that it is possible to adjust the >trim pots on the D-A convertors of a CD player to improve linearity. > ... Linearity below -100dB is >something found only in the most costly CD players, and maybe in the new >1-bit machines. Does anyone know what is needed to perform the adjustment? >Can it be done by the average joe? Isn't the human ear an acceptable test instrument? It would appear that either (i) the ear can't hear these effects, in which case adjustment is unnecessary; (ii) the ear can hear them, so adjustment by ear is praciticable. You could even try it a week at a time at each of ten equally-spaced positions on the dial, thereby getting the advantage of long-term exposure and relaxed listening on a wide variety of musical signals -- not just test waveforms. -- -- Mark Johnson MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086 (408) 991-0208 mark@mips.com {or ...!decwrl!mips!mark}
tjr@cbnewsc.ATT.COM (thomas.j.roberts) (03/01/90)
> > In article <2589@uwm.edu> @sun.acs.udel.edu:hamilton@sun.acs.udel.edu (hamilton) writes: >> >>I read in the current issue of CD Review that it is possible to adjust the >>trim pots on the D-A convertors of a CD player to improve linearity. >> ... Linearity below -100dB is >>something found only in the most costly CD players, and maybe in the new >>1-bit machines. Does anyone know what is needed to perform the adjustment? >>Can it be done by the average joe? This is a difficult adjustment to make, and one that only "purists" would bother with (by "purist" I mean someone who is more concerned with talking about a system than listening to it ... :-). There is no hope of making this adjustment on normal music. You will need a CD with special test tones (namely a very quiet sinewave, -60 to -80 dB below full scale). In virtually all cases, leaving the trimmer alone (or at its center position) will suffice. I have a 16-bit homebrew music synthesizer, and a 16-bit ADC, both installed within a PC. The DAC has an MSB trimmer. I cannot hear the effect of the trimmer on normal-volume music or sinewaves. The ADC (used as an oscilloscope) cannot see it on normal-volume sinewaves. When I run the DAC on a -72dB sinewave (wrt full-scale), and crank up the gain in the speaker amp, I can barely hear the MSB adjustment when it is at its extreme ends (pulling the MSB out of true by about 4 LSB counts) [note this "sinewave" has only 10 levels]. The ADC oscilloscope (single sweep) has considerable noise at this level (1 LSB = 0.08 millivolts), but I can infer the steps of the "sinewave"; I have not bothered to write the multiple-sweep averaging software required to reduce the noise to the level needed to see the effect. This is with the ADC Sample/Hold connected directly to the DAC output, and differing sampling rates of the DAC (50 kHz) and ADC (33 kHz) complicate the observation; looking after the DAC output filter washes out the steps, and makes the trimmer effect invisible. The DAC is specified to have THD <0.1% WITHOUT TRIMMING; I believe it (I installed the trimmer when I was having linearity problems - they turned out to be completely unrelated to the DAC or its trimming). I simply leave the trimmer in the middle of its range, and am happy. Tom Roberts att!ihlpl!tjrob