Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.ARPA Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!spp From: spp@ucbvax.ARPA (Stephen P Pope) Newsgroups: net.audio Subject: Re: David Mohler is completely correct Message-ID: <10068@ucbvax.ARPA> Date: Wed, 21-Aug-85 19:48:24 EDT Article-I.D.: ucbvax.10068 Posted: Wed Aug 21 19:48:24 1985 Date-Received: Sat, 24-Aug-85 14:21:05 EDT References: <19@drune.UUCP> <4162@alice.UUCP> Organization: University of California at Berkeley Lines: 41 Summary: David Mohler is not completely correct Sorry, Dave, since you clearly know quite a bit about signal processing, but there are some misstatements in your recent posting. First off, a 14-bit quantizer, oversampled at 4x, will give you the resolution of a 15 bit quantizer, not a 16 bit quantizer. Each time you double the sample rate, half as much quantization noise energy lies in-band as previously did, so you get a 3dB S/N improvement. A 4x oversampling gives you a 6dB improvement, which is the same improvement you would get from adding ONE bit to your quantizer. A 16-bit quantizer has 12dB better S/N than a 14 bit quantizer. Also, you are commingling two partially related issues -- D/A implementation, and reconstruction filter design. All consumer CD players process operate at a 44 kHz sample rate (for obvious reasons) but some interpolate up to a higher sample rate, filter digitally, D/A convert at this higher sample rate, and then filter the analog signal. The advantage of doing this is that the sharp cut-off you need at about 20 KHz is implemented by an EXACTLY LINEAR PHASE digital filter, while the analog filter does not have a lot of phase shift in the audio band since its rolloff occurs much higher. The implication (I'm not sure you stated it explicity) that a CD player that implements its reconstruction filter in this way has a higher S/N than one that has an all-analog reconstruction filter is completely false. The statement that the D/A implementation might be more linear is true, however, the D/A conversion could be performed after interpolation to a higher sample rate even with an all-analog reconstruction filter. To summarize: You can eliminate most of the phase shift in a CD player's reconstruction filter by using a sharp-cut-off, linear-phase digital filter followed by a gradual-cut-off analog filter. As a side benefit, this possibly allows a more linear and/or lower cost D/A implementation; but the same D/A implementation could be used without the digital filter. steve pope (ucbvax!spp)