wm (11/06/82)
Aren't people being a little fast and lose with the meaning of dB? I thought dB's were only for comparing the levels of two different sounds. How can people talk about 90dB of dynamic range and a sound level of 90dB in the same sentence? Another thing that is bothering me. I thought the whole controversy about digital recording is that it introduces some insideous new form of distortion. I have seen recording engineers claim that they get headaches listening to digitally recorded sound. The person who collected these negative attitudes about digital recording claimed it was because of quantitization problems, which makes a certain amount of sense since it is a characteristic of digital recording and would not show up in traditional measurements of distortion. In a response to this, the point was made that some early digital recording equipment were bad in this area, but it had been corrected. But what do I know? I haven't ever heard something played back on a digital system. Hmmmm. I don't suppose there are any people on the net who use digital audio equipment on a regular basis? But I do welcome the digital revolution in audio. Not because of 90dB of dyanmic range or wow or anything like that. There are analog systems around today that totally blow me away as it is. I welcome it because digital systems will be CHEAP!!! Digital records cost about 10 cents in materials. Digital IC's are much cheaper than critical linearized analog circuitry. Switching power supplies and switching amplifiers are much cheaper, and lighter, and smaller. And all of this cheapness will not affect that 90dB of dynamic range or 0% wow. Everyone will be able to have one hell of a good stereo. You don't believe me? Anyone remember what calculators and computers used to cost? Maybe some day we will look back on current audio technology with the same amusement we reserve for vacuum tube computers. Wm Leler - UNC Chapel Hill
ark (11/06/82)
It is true that strictly speaking one can only use dB to refer to a ratio. However, one can indeed talk about the absolute loudness of a sound by mentioning dB SPL, where SPL stands for "sound pressure level." While I do not know the exact reference points, 0 dB SPL has been established as (approximately) the quietest sound that can be perceived by someone with perfect hearing. On that scale, I believe the threshold of pain is about 130 dB SPL. In other words, talking about the loudness of a sound in dB is not really incorrect, just a bit sloppy. On the subject of new distortions from digital recording: I am no expert in this, but I believe the most severe form of distortion comes when you overload the thing. After all, you are recording the signal level as a binary integer of some size or other (I think 14 bits has become more or less standard for "home" equipment) and you have no way of representing a value that's too big. Given that you do not overload, the remaining distortions in a well-designed digital system are small indeed. Note that I said "well-designed." I am fairly certain that the biggest pitfall is something called "aliasing" which comes when you try to reproduce signals with a higher frequency than half the sampling rate. In this case, it is no longer possible to tell what frequency you started with, and terrible things happen. In order to prevent aliasing, I think it is necessary for digital reproduction systems to have a rather sharp low-pass filter in the chain. Typical sampling rates are between 40 and 50 kHz, so your garden-variety digital system has a bandwidth of 20 kHz and NO MORE. In trade for this you get tremendous dynamic range, distortion figures that look more like an amplifier than a tape recorder, and unmeasurably small wow and flutter. Is the trade-off worth it? Time will tell. I would love to see some of the people who claim digital recordings have strange distortions in the following test: let them listen to two recordings. One is a conventional analog master tape of something, say an orchestra. Two microphones, quarter-inch half-track, 30 IPS, Dolby A, best equipment available. The other is a digital copy of the anaog tape. Match levels accurately, and then let neither the experimenter nor the subject know in advance which is which. (psychologists call this a double-blind experiment) Now do it again, but instead of a digital copy of the analog tape use a digital master tape. Now the subject will know which is the digital tape -- it will be quieter -- so said subject will be tipped off. This is why I like the digital copy of the analog tape as comparison -- if there are "subtle distortions" this is the way to bring them out.
tony (11/07/82)
#R:unc:-422700:pur-ee:12000001:000:487 pur-ee!tony Nov 6 14:36:00 1982 The reference used in dB measurements is 2x10^-4 dyne/cm^2. There`s nothing at all sloppy about this. You just need to know what the reference is. Quantization can be viewed as simply another source of noise. If 14 bits are used, and one makes some reasonable assumptions about the probability density function of the sound, (i.e. Laplacian) then the SNR due to quantization works out to be around 77dB. If you use all 16 bits, the SNR jumps to 89dB. Not too shabby. pur-ee!tony
marr (11/11/82)
Does anyone have any idea whether or not the Canadian record companies have also refused to pay the royalty to Philips for digital disk recordings? I suppose some of our friends north the border might know the answer to this one. --Leon Marr