DVL-Digest 1373 - Postings: Index AUDIO FUNDAMENTALS (was: Speakers for use with FCP) What DV Cams Are You Using? AUDIO FUNDAMENTALS (was: Speakers for use with FCP) - "Don Mitchell" From: bounce-dv-l-5963@dvcentral.org [mailto:bounce-dv-l-5963@dvcentral.org]On Behalf Of Danny Grizzle Sent: Friday, December 20, 2002 2:03 PM To: DV-L Subject: [dv-l] AUDIO FUNDAMENTALS (was: Speakers for use with FCP) On 12/19/02 12:04 PM, "Don Mitchell" [runtime@wzrd.com> wrote: Can you run through the difference between > monitoring speakers such as the ones you mention > and, say, good quality stereo speakers? The idea behind good audio monitoring is loudspeakers and listening conditions which reveal the recorded signal in a precise way. You are looking for reproduction accuracy so you can "see" what you've got. This is no different than avoiding an inferior display when evaluating video... nobody here would consider using a handheld Casio LCD TV to evaluate 3-chip DV footage, would they? Good quality stereo speakers might be useful. 20 years ago, there was more clarity in audio, consumer vs. HiFi. Today, thanks to CDs, consumer audio is vastly improved, while HiFi has all but disappeared. I would characterize mass-market audio as more or less uniformly mediocre. The public's rush to digital CD audio was about convenience, not quality. I'm not familiar with the speaker you named, and don't want to get into a debate about the merits of various real loudspeakers, which is entirely subjective. My only intention is to badmouth low fidelity computer speakers, completely inappropriate for critical decision making. A quality loudspeaker's job is to reproduce sound accurately and with no coloration. The measurement of success is frequency response. (Human hearing is from 20 cycles to 20,000 cycles per second.) The difference between good speakers and bad speakers is how "flat" the response curve is -- less variation is good, because it shows the speaker is not adding to or taking away from the sound signal. HUMAN HEARING There are several important things to know about how your ears work. All things being equal, when comparing two speaker systems, most people will believe that louder of the two sounds better. In fact, the louder speaker may deliver far less performance. Bottom line: when evaluating speakers, it is important to compare at the same volume level, even if this requires separate amplifiers and settings. All speakers should be evaluated "flat", with no tone controls and no loudness setting engaged. In recent years, electronics designers have been researching and exploiting the "psychoacoustics" of human hearing. This goes beyond straight performance measurements and gets into perception. This research is closely related to video compression, both involving issues of how to eliminate data without detection. In the case of audio, the technology of psychoacoustics is used to either add or subtract. For instance, MP3 files - compressed audio for the Internet - are based on lossy data compression, making files smaller with little perceived loss in quality. Principles of psychoacoustics can also be used to add sound perception, such as a video producer who wants to mix audio so that a cheap 2-inch TV speaker is perceived to make low bass notes that the laws of physics will not allow such a device to make, in reality. Thus, psychoacoutics are useful going & coming for people who know what they are doing, but they have no place in a critical listening environment. Avoid trick consumer-type stereos with built-in tricks. Tone controls. Most people never exposed to HiFi audio are surprised to learn that, at the high end, audiophile equipment provides less controls, not more. The theory goes like this: anybody committed to good audio will own superb speakers that do not color the sound. Tone controls -- bass and treble -- are there only to compensate for inadequate performance of an inferior loudspeaker or a poor listening room. Good sound systems do not have tone controls or equalizers because these indicate a deficiency in another part of the system that should be corrected. Masking Effect. Means this: when more than one sound is present, the human ear will tend to hear the loudest. You can hear something whispered into your ear normally, but not standing next to a jet engine. SOUND PRODUCTION VS. REPRODUCTION In the sound world, production & reproduction are two completely different animals. For an audiophile, the objective is to realize the sound created by an artist, reproduced in the listening room as if the artist were present. Thus, no tone controls, etc., because use of these would be to introduce changes in the sound, thus not respect the artist's wishes for how the performance was to be rendered. Sound production, on the other hand, has no rules. It is not important if a guitar amplifier loudspeaker is flat or not, so long as it produces a sound that pleases the artist. Extreme example: Jimi Hendrix deliberately used amplifier feedback and distortion, overdriving electronics, etc. Cool -- these became part of the instrument he was playing, and thus the job of the audiophile's megadollar sound system is to reproduce Hendrix's distortion and feedback perfectly, exactly how he played it. Fidelity is not a primary consideration for sound reinforcement, and public address (PA) type equipment should never be used for critical listening. In fact, speaker designs for sound reinforcement frequently "creatively degrade" fidelity for specific purposes. For instance, intelligibility of spoken words is not enhanced by full fidelity. That's why both telephones & AM radio have a particular sound quality, to enhance intelligible speech, and to conserve bandwidth (telephones only). Similar design optimizations are made in churches and large halls. This may be hard to discern since there are so many ignorant amateur implementations, but top professional installations are tuned, and not necessarily for perfect accuracy, but towards a specific set of objectives in a particular venue. BASIC LOUDSPEAKER DESIGN There are different types of speaker design. Two primary approaches are acoustic suspension and bass reflex. With acoustic suspension, speaker drivers are enclosed by a sealed box. Bass Reflex has a ported box, with an opening allowing air to move to & from the inside of the speaker enclosure. Acoustic suspension speakers are easier to design for accuracy, but require a lot of amplifier power. Bass reflex are louder with smaller amplifiers, but much more difficult to design for flat frequency response. There are many other loudspeaker designs, some more exotic than others. None of which you are too likely to see. Horns are sometimes used for tweeter elements, but become too big at low frequencies. Interestingly, Bose Acoustic Wave, if you ask me, holds a lot in common with Klipsch folded horn designs. True horn designs are excellent, but not commercially feasible to package and ship since they require a speaker something like 20 or 30 feet long. There are a lot of exotic speaker technologies on the market, too much to get into here. One design variation that you *are* likely to see anymore is various cross-over networks and subwoofers. JBL was the first to popularize this design in a pricy HiFi speaker called the L212 (if I remember correctly). Basically, an electronic crossover network strips low frequencies from the signal sent to stereo speakers (or a surround system), and routes low frequencies to a dedicated sub-woofer. Bass sounds are not directional, so 1 speaker is plenty, no need for stereo imaging. Mackie is using sub-woofers with their studio monitors. I like (but can't afford) Bob Carver's Sunfire cubes, a really innovative design. PHASE CANCELLATION Sound is a vibration in the air, molecules being jostled about, to and fro, push and pull. The "push" portion of the vibration is a miniscule amount of compression, like a microscopic high pressure front. The "pull" portion of the vibration is called rarefaction, like a microscopic low pressure front. If push and pull were to occur at the same point in time, they would net out to zero, no different than summing +1 and -1. The major problems with speaker installation -- listening room and speaker position -- is due to the fact that reflecting surfaces such as walls make sound waves bounce and rebound. Once sound is arriving at any given spot via two different routes of different lengths, it is inevitable that phase cancellation begins to occur. Factor into this that various sound frequencies have various wavelengths, and you will see that phase cancellation is not uniform, with dead silence at particular listening positions, but rather frequency dependent. Nearfield monitoring (below) addresses this problem in a direct manner based on the human ear's masking effect. Problem is, nearfield monitoring is highly tuned for a specific listening sweetspot, a single human head. Anyone else, see acoustic treatments, below. NEARFIELD MONITORING The terms "active" and "nearfield" monitors are often used in the same breath, but they are not related. Active loudspeakers contain built-in power amplifiers, and have become very popular in recent years. They are not superior to non-amplified loudspeakers in any way, other than perhaps the convenience factor of requiring less wiring and fewer devices to clutter your desktop or equipment rack. Nearfield is a design strategy in how you position loudspeakers in your listening room. It relies on a basic fact of physics (hope I get it right): at twice the distance from a sound source, volume will be 1/4th as loud. By locating monitors very close to your listening position, away from walls & hard surfaces, sound enters directly into your ears. Reflected sound, the reverberation present in all rooms, is vastly diminished as a factor in coloration of what you hear, because distance from reflecting surfaces is a much greater distance, resulting in much more volume fall-off, with the consequence that your ear's masking effect no longer hears the lower volume reverberant sound. In essence, nearfield monitoring splits the difference between headphones feeding sound direct into your ear, and conventional loudspeaker placement where the reverberant qualities of the room combined with specific speaker position introduce excessive coloration (frequency dependant phase cancellation) into the audio. The thinking behind most active monitors such as the Mackie HR824 and Behringer Truth is that they will be used in a nearfield application. Absolute volume levels are not critical, but location in relation to your listening position is. ACOUSTIC TREATMENTS We've all seen photos of acoustic foam and other sound treatments in recording studios and listening rooms. How do these work? See various Auralex websites for details: Typical problem for video producers: nearfield works great for critical listening, but not as well for playback to groups of people -- like clients, for instance. Either that, or the video producer simply does not have the space to position speakers away from walls appropriate for nearfield monitoring. Situations like these are where acoustic treatments can benefit a studio. Here's a short, simple, & practical way to determine basic acoustic treatment of a given room. It will be perfectly clear to anyone who has ever banked a shoot on a pool table. With your loudspeakers in final position and also your listening positions defined, take a mirror and hold it flat to the wall. Anyplace on the wall that you can see the speakers in the mirror from a listening position needs to have acoustic treatment applied to that portion of the wall. This deadens direct reflective sound from reaching your ears along two paths, one direct one reflected, producing inevitable phase cancellation. In addition to wall treatments, you will probably also want to treat corners (vertical -- wall-to-wall -- & horizontal wall-to-ceiling). This traps bass frequencies. Remember, bass is non-directional, so line of sight is not a factor at low frequencies. ===========================Read Jay Rose book for details and insight on audio, not just studio, but production issues, too. BTW - Jay Rose had a recent column thoroughly trashing the idea of using computer monitors such as the clear plastic stick speakers for video production, although he admired them for industrial design and general utility. Some people expouse the theory that you should do everything on a "typical end user TV". Besides the fact that there is no such animal, I think this is generally a sloppy compromise that tends to make a fool out of the producer. The Clint Eastwood line, "A man has got to know his limitations" is more important for producers than it is for the general populations. It is inexcusable for anyone claming to do professional-level video work to be clueless about audio, yet we see this constantly. Sorry so long, and errors. I'm out of time, so no editing or fact checking. Danny Grizzle --- You are currently subscribed to dv-l as: runtime@wzrd.com To unsubscribe send a blank email to %%email.unsub%% -- (cut off when replying) ----- Subscribe, unsubscribe, etc.: http://dv-l.org Sponsors: http://dvcentral.org/Sponsors.htm Critical problems: mailto:postmaster@dvcentral.org -------------------- What DV Cams Are You Using? - "Perry Mitchell" From: Bill P Here we go again. Have you actually tried a 250 in the electronic 16:9 mode, or are you just repeating what you have read or seen from compressed still pics? I find a very small dropoff in quality, with the footage still very acceptable. It's better in 4:3, but the 16:9 degradation is so small that most people probably wouldn't notice it. --- Enrico Quaglia > > Remember the DSR250 doesn't support true widescreen > like the DSR500. What > you're getting is cropped 4:3 with an unacceptable > compromise in terms of > vertical resolution loss. Don't use it unless you > really need to.... (diese posts stammen von der DV-L Mailingliste - THX to Adam Wilt and Perry Mitchell :-) [up] |
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