mat@hou5d.UUCP (M Terribile) (02/18/84)
I have a Phillips (Magnavox) CD player -- the cheapest I could find at $499 -- and I have noticed no unusual qualities in the sound. On the other hand, I should point out that on very quiet passages, the mechanical noise of the player (about the volume of a cat breathing) is noticeable, so the more interesting sonic effects may be getting overwhelmed. The motor is quietest at the beginning and end of a disk, and noisiest about 3/5 the way through a 40-minute recording. The noise on the signal with nothing playing, or in a silent passage is below the noise floor of my pre-amp. The pre-amp is spec'd at 86 dB below a reference input of (I think) 1 mV, so things are pretty quiet. My machine is the Magnavox top-loader, and I am not sure if it has the filtering scheme that has gotten so much discussion here. I have heard at audiophile salesman at Tower Records (check this place out, by the way) say that ``Phillips botched the digital filters''. It IS very easy to use. Incidentally, I have disks by Telarc, CBS, Denon, and DG and the Telarc disks seem consistantly to be the best. One Denon disk came in a German-language case, but with a Japanese brochure! I can eventually make sense of some of the German, but I am have no hope on the Japanese. All of them have good disks, but only Telarc seems to avoid really botching the recording from time to time. Mark Terribile hou5d!mat
gregr@tekig1.UUCP (02/20/84)
Yes Shaun, you are correct the Sony doesn't use Bessel filters. I will severely discipline my fingers for typing this error for which my brain now steadfastly refuses to acknowledge responsibility on its part. Geez, how do these things happen? Anyway, as I later pointed out the phase response of the Phillips approach is indeed closer to ideal than the Sony approach. (Although not necessarily audibly so.) Also I now see Sony advertising claiming an 11th order filter rather than 9th in their units. Does anyone have a reference to a technical article by Sony filling in the details of their filtering system? Since you mentioned a desired out of band rejection of -70 dB in your comments it started me thinking about the rejection capabilities of the two systems. According to Phillips the out of band rejection for its digital filter is about -50 dB at cutoff with rejection decreasing fairly linearly to only -30 dB at 88.2 kHz. The D/A converter holds the sampled values at its output between samples thereby introducing another filter response of the shape (sin x)/x with its first zero at 176.4 kHz. This contributes only an additional -3 dB of rejection at 88.2 kHz. Since the 3rd order Bessel filter (yes Bessel) used by Phillips following the digital filter has a -3 dB frequency of 30 kHz the combined rejection of these three filter sources must be fairly constant between 30 kHz and 88 kHz at approximately -53 dB. This nearly constant rejection occurs because the shape of the 3rd order Bessel filter increasing rejection with frequency is nearly inverse of the shape of the digital filters decreasing rejection with frequency (up to 88.4 kHz). The minor rejection of the (sin x)/x response up to 88 kHz simply fills minor differences between the other two. Between 88.4 kHz and 154.3 kHz things are much better as both the digital filter and the Bessel filter provide increasing rejection and the (sin x)/x response is around -17 dB at the upper end. Above 154.3 kHz however the original audio signal reappears as sidebands around 176.4 kHz without any attenuation due to the digital filter. At this point and beyond the Bessel filter and the (sin x)/x response will continue to provide at least 50 dB of attenuation. So whats the point of all this? Simply this, unlike the Sony approach where the analog filters have a constantly increasing rejection with frequency (up to some practical limits of the components of course), the Phillips approach provides a fairly constant rejection of only -53 dB or so over the complete bandwidth of the audio aliases. Since the aliases are not harmonically related to the original the presence of these signals must certainly be more objectional than harmonic components at an equivalent amplitude. The aliases themselves are too high a frequency to hear but they can create intermodulation products that could be audible if they have sufficient amplitude. So at what level does this become audible? Could it be heard if the aliases were only attenuated by 50 dB or so? When we perform standard THD tests (which really measure all undesirable components, not just those harmonically related) we must also be measuring these components. Is it not possible to assume then that we could find the same measured THD value more objectionable when associated with a digital player of the Phillips type, than with a player of the Sony type which would be more likely to have its THD comprised of the conventional harmonically related components as with analog equipment? I really don't know the answer to these questions, YET! I intend to try and find out. I don't know if this explains why some people think the Phillips approach sounds inferior to the Sony approach or not. I, like Shaun, have heard these comments but I have no idea if they are factual or not. I have not yet even heard a Phillips type player. For all I know the Phillips and Sony players may be sound audibly identical and all this is simply an engineering curiosity. My reference for information about the Phillips system is the Phillips Technical Review, Vol 40, No 6. Has anyone else with more of a digital signal processing background thought about these issues? What say the experts on the two systems? I invite technical corrections, and technical comments. I'm bored with the rantings of the subjectivists. Greg Rogers