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re:cursive
Joined: 04 Jan 2006
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 Posted: Sun Aug 27, 2006 12:35 am Post subject: |
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He has got somewhat of a point. There is a tendency in modern mastering to have near constant hot levels across the frequency spectrum (squashed signals/compression etc.). It's got to sound loud and jump out of the speakers more than the last record the radio/dj/whatever played. The dynamic range and musicality can suffer from this process.
I'm curious as to whether mixing/mastering styles will change to suit mp3's like they seemingly have to suit radio play and CD's. |
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re:cursive
Joined: 04 Jan 2006
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| Posted: Sun Aug 27, 2006 12:57 am Post subject: |
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Some sonograms....The top white line is 20kHz and the bottom white line is 30Hz.
Led Zeppelin "Stairway to Heaven":
Red Hot Chilli Peppers "All Around The World"
Notice the difference? |
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JongnoGuru
Joined: 25 May 2004
Location: peeing on your doorstep
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| Posted: Sun Aug 27, 2006 2:14 am Post subject: |
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Of course many interpretations are possible (so fuggin' Dylanesque), but I got the sense that all he was really saying was:
"Get outta my yard, damn punk kids!  "
As to the pro-/anti-MP3 debate, it's adequate for background noise during long boring commutes and on airplanes provided they still allow them. And for parties where everyone's talking & walking around. But if we're talking about pure music appreciation, at home, seated in an optimally positioned chair (rather than giving the dog a bath), over a decent though not necessarily expensive hi-fi system, then MP3 really has no place and nobody's even going to consider it. I'm not anti-MP3 at all, and it works for most people who want music on the go. For people seriously into "experiencing" their music in all its rich detail -- i.e., seated in a quiet room -- I personally believe that correct speaker placement (height, distances & angles controlled down to the milimetre) will result in a much more dramatic and clearly discernible improvement in sound quality -- on anyone's system -- than using this or that compression format. |
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Demophobe
Joined: 17 May 2004
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| Posted: Sun Aug 27, 2006 3:15 am Post subject: |
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| wrote: |
The debate of CD vs. vinyl is a popular religious issue amongst music fans, audiophiles, DJs and techies. The arguments are usually peppered with scientific factoids ranging from the pedantically correct to the hilariously mistaken. Here I intend to explore the technical issues in a balanced manner. Reasoned scientific feedback, debate and corrections are most welcome.
I shall try to avoid the quagmire of subjective aesthetic judgement; having never owned any vinyl records I couldn't say which format sounds "better".
The Frequency Domain
Frequency is pitch - the difference between a low, deep sound and a high piercing sound. As we hear sounds as a sum of frequencies, most of the important information is in this domain, and accurate reproduction is correspondingly important.
The sample rate of a CD is 44,100 samples per second. According to the Nyquist theorem this is sufficient to capture all the information in a signal that contains no frequencies above 22,050 Hz.1 This bandwidth is more than enough. The human ear is most sensitive at around 3,000 - 4,000 Hz. (The fundamental tones of a piano range roughly 25 Hz to 4,200 Hz) The upper frequency limit for human hearing is roughly 20,000 Hz, while for a middle aged male this figure may be nearer 15,000 Hz. At both extremes of the audible frequency range the perceived loudness relative to the signal power is less than that for moderate frequencies.
The reason for this extra sampling overhead is actually to make the CD player's job a little easier. When recreating an analogue signal from digital data it is necessary to filter artefacts generated above the Nyquist frequency (in this case 22,050 Hz). Analogue filters with sharp cut-off slopes are expensive, so a 2,050 Hz gap to ramp down is quite helpful. Still, a poorly designed filter might unduly attenuate the audible high end or have a "ringing" frequency response across the full range.
Sound behaves linearly, which means that frequencies above 20,000 Hz cannot somehow affect, alter or "shape" the lower, audible sounds. Nevertheless, audiophiles often believe that these high frequencies actually do make a difference. The best scientific support I have seen for this view is a paper2 which suggests that brain activity may be somehow affected by the presence of hypersonic sounds.
Frequency domain issues with vinyl have an entirely different nature. First off, vinyl records are not recorded with a constant frequency response. To properly reproduce low frequencies would require larger grooves, so records are pressed with the low frequencies reduced. In addition, as a strategy to drown out noise, higher frequencies are boosted on the recording. A RIAA equalisation amplifier mostly corrects the frequency response on playback. Problems introduced by cheap turntables include "wow and flutter", which refers to frequency shifts caused by variation of playback speed and "rumble", low frequency noise from the motor.
Dynamic Range
The dynamic range is the resolution of the recording - the ratio of the loudest possible sound to the softest, expressed in the logarithmic decibel (dB) scale.
For CDs, each sample has a 16-bit range (65536 values). This might not seem to be a great deal but it actually translates to roughly 96 dB dynamic range. I would argue that this should be sufficient for any music, when we consider the range of human hearing.
By definition, decibel levels are always comparative - the difference between the loudness of two sounds. When you see an absolute figure quoted, ("A lawn mower measures 90 dB") this is in relation to a defined zero point - 0 dB is defined as the softest sound audible to a person with perfect hearing. This means that if you set the volume of your CD player such that you can hear all the sound it contains, the loudest point on the track will measure at least 96 dB on the absolute scale. Listening at a higher volume than this for too long will reduce the need for a large dynamic range.3
The biggest general misconception about analogue technologies is that they have infinite resolution, which is never true. (In this example, a PVC molecule is small, but not infinitesimal). In practice, the dynamic range of vinyl records is affected by the age and quality of the recording, and the density of the tracks. The best-case seems to be around 60 dB. The dynamic range will be adversely affected at low frequencies because of the equalisation issue detailed above.
Errors and Noise
CDs are recorded with interleaved redundant information built in. This allows the decoding machinery to correct or interpolate bad data caused by damage to the CD surface. However, should the damage be too great, the CD will simply skip.
Any wear to the surface of a vinyl record will cause a gradual deterioration in the quality of the recorded signal. This manifests as hiss and reduces the effective dynamic range. Scratches and other damage can result in pops and clicks that momentarily drown out the recorded sound.
Conclusions
CDs technology is mostly an improvement on vinyl technology. Stop worrying and just enjoy the music. |
| wrote: |
The subjects of sampling frequency and dynamic range are important for any sampled time domain signal. In the CD audio format, these are set in stone and cannot be changed. I think these issues give some credence to the claims that vinyl is better than CDs.
According to the Nyquist Theorem, to capture a time domain signal in discrete form and reconstruct it from these samples without loss, the rate of sampling must be 2 times the highest frequency present in the time domain signal. CD audio is sampled at 44100 Hz. Therefore, a audio signal could have frequencies up to to 22050 Hz and be captured perfectly in sampled form. This signal can then be reconstructed mathematically using a series of weighted sinc functions. I won't go into the details, but I really mean perfectly (you audiophiles can argue this point all day long but you'll be wrong). The Nyquist Theorem is discussed in excessive detail in most signal processing textbooks.
There are two problems with a fixed sampling rate though, higher frequencies present in the audio signal and the electronic system used to recreate the signal from its samples. Sometimes there are frequencies higher than 22050 Hz in audio; the lucky among us may be able to hear them. When a signal like this is sampled, these frequencies which are higher than the Nyquist Frequency are aliased and wrap around in the frequency domain, smearing together with other, lower frequencies. This causes distortion. Secondly, even thought you can mathematically reproduce the signal with sinc functions and so on, the actual electronics (your CD player, using sample and holds, filters, etc) don't do it perfectly. Add in more distortion.
Dynamic range is another issue with CD Audio. It's 16 bit, so the samples themselves can only occupy a discrete value between 0 and 65535. The entire set of waveforms in an audio signal (parts in an orchestra, for example) has to be sampled and fit to a value in that range. Measurable sampling noise is a result of this operation. This is yet another source of nastiness in the reproduced audio. And think about it, is 2^16 values really enough to capture all the ranges in full bodied sound?
In my opinion, CD audio is very good but is probably limited for some types of music. When an audiophile says they can tell the difference between vinyl and a CD, they probably can. The super-CD format (22 bits I believe) is far better in terms of dynamic range and sampling. This may be able to fool even the best ear. |
These guys present a fairly convincing argument. |
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eamo
Joined: 08 Mar 2003
Location: Shepherd's Bush, 1964.
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| Posted: Sun Aug 27, 2006 4:06 am Post subject: |
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There's another argument in the hi-fi world.
Which is more important? The signal source (i.e. CD player, turntable, tape, MP3 etc).....or the reproduction (i.e. Amp and speakers/headphones).
I've had some very average CD players and turntables in my time. I was able to make them listenable by using good amps and speakers. Then again, I've had poor amp and speakers at other times. Even a Rockport Master Reference.....
.......(list price 75,000 dollars) would sound awful.
Don't know what point I'm trying to prove here actually. Just that audio equipment has a lot of variables. |
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IlIlNine
Joined: 15 Jun 2005
Location: Gunpo, Gyonggi, SoKo
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Demophobe
Joined: 17 May 2004
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| Posted: Sun Aug 27, 2006 2:36 pm Post subject: |
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Good post, and finally right on-topic (my posts included). So, the culprit is who, not what. |
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