williams@kirk.DEC (John Williams 223-3402) (08/14/85)
To answer some of your questions: Companding DAC's are neither linear nor logrithmic. This makes computation a major pain in the neck. Companding DAC's have an exponent and a mantissa. A true logrithmic converter would be the ultimate choice for modulation techniques, whereas a linear DAC would be best if you wanted to add alot of waveforms together. The advantages of a companding DAC would not be significant inside a closed box, the major advantage being noise immunity, and hopefully you could control this fairly well inside the box. The best realistic S/N ratio you should ever want is 95 dB. This is the limit for the human ear. This works out to 18 bits. 12 bits will yield about 70 dB, which is probably good enough for alot of applications. You can use a notch filter to improve this by setting it at the sampling frequency. There is very little you can do about the lower harmonics that are generated by the quantization error. I would seriously look into MIDI. Any piece of equipment that is worth anything will have this interface, because: 1) It allows digital recording 2) It allows synchronized playback 3) It allows modularity of different components. 4) It's *REAL* cheap to impliment. Some Suggestions: 1) Set the sampling rate at something that fits nicely with the temperment. Most modern keyboards are set at equal temperament, which is a compromise of harmony for transposability. 2) Do some research on the MIDI standard and include it. 3) Use lookup tables for all your functions except the really simple ones. This will probably mean a rather large ROM. Reconstruction just doesn't cut it at those speeds unless you have a CRAY 1. 4) Play test it. Get some professional musicians to try it. Good Luck, John Williams
jeq@laidbak.UUCP (Jonathan E. Quist) (08/16/85)
In article <3613@decwrl.UUCP> williams@kirk.DEC (John Williams 223-3402) writes: > The best realistic S/N ratio you should ever want is 95 dB. This >is the limit for the human ear. This works out to 18 bits. 12 bits will >yield about 70 dB, which is probably good enough for alot of applications. Wrong. The range of the human ear (from just audible to threshold of pain) is 120dB. 18 bits = (6dB/bit) * 18 = 124 dB. 12 bits is 72dB. In practice, once you add analog noise on top of everything else, 12 bits is pushing the low side. Many folks insist that the 16 bit standard is still too small a word size. One large box (the Lucasfilm ASP) uses (correct me if I'm wrong, anyone) 20 bit converters (both D/A and A/D) and a 24 bit data path. >2) Do some research on the MIDI standard and include it. >4) Play test it. Get some professional musicians to try it. Excellent advice. Some of the best hardware has been packaged under some of the worst software/control layouts... (MIDI products are not exempt from this!) Even giving it to the kid next door for a week or two will tell you lots about what to improve; a musician who has torn his/her hair out with a frustrating product may be able to point out weak spots more quickly.... Jonathan E. Quist Lachman Associates, Inc. ihnp4!laidbak!jeq ``I deny this is a disclaimer.''