Le Multi-canal - Stephen`s Audio Site

Transcription

Le Multi-canal
Contents :
L’historique du multicanal:
comment s’est développé le Multi-canal
Exemples de diffusion
Medias et la chaine de production
Les Formats de production et diffusion:
evolution
LCR / LCRS / LCR Ls Rs LFE
Le notion de matricage
Methodes d’encodages et compression
Bande passantes a l’enregistrement
Bande passantes a la diffusion
Bass Management
Downmixing
Environement d’ecoute
Emplacement des monitors
Considerations accoustiques
Considerations de productions
Compression et Codecs
L’Authoring
Fantasia Le Parrain du multicanal
En 1938 Walt Disney a produit le film Fantasia utilisant un system de diffusion a 8
pistes
Le film a tourne aux US avec son propre equipe technique
Fin premature des projections quand USA entre dans le 2eme guerre mondiale
Le procedure ne sera plus jamais utilise.
Son Cinema
1938 Fantasia
1950s
Mag stripe 35 / 70 mm
4/6 canaux
1976
Dolby Stereo Optical
1978
Stereo Surround
70mm magnetic
1987 Dolby SR Optical
Wider Dynamic range
4 Ch
1992 Dolby Digital Optical
5.1 chs
1999 Dolby Digital surround EX Optical
6.1 Ch
L’audio Multi-canal - notes - Stephen Taylor
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Formats Publiques
1958 Stereo LP 2ch
1961 Stereo FM 2ch
1970 Dolby B Cassette 2Ch
1972 Video Cassette (Mono)
1978 Stereo Video Cassette
1980 Laser CD
1982 Dolby Surround
3 Ch L R S
1986 Stereo TV
1987 Dolby Surround Pro Logic
4 Ch L C R S
1993 1st Dolby Digital IC
Dolby chosen for ATSC Digital TV (5.1ch)
1995 Dolby sur disques laser
Dolby Digital choisi pour DVD
5.1 Ch
1996 FCC (US) Adoptent ATSC DTV avec Dolby Digital (5.1)
1997 DVD-Video, DVD ROM with Dolby Digital reaches market
5.1 Ch
1998 Premier diffusion DTV en US
5.1 Ch
…En termes du multicanal:
Dolby
Dolby
Dolby
Dolby
Stereo 4.0 Ch (L / C / R / S) depuis Lt Rt
Surround au cinema - (L / C / R S) depuisLt Rt
Pro Logic (L / C / R / S) depuis Lt Rt
Pro Logic II (L / C / R / Ls / Rs LFE) depuis Lt Rt
Dolby Digital (L / C / R / Ls Rs / LFE
Exemples de diffusions
Cinema: Dolby 70mm magnetique
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Pistes numeriques et analogique sur pellicule 35mm
Dolby Stereo Optical
Dolby Digital au Cinema
•Source arrive en “bitstream” numerique en AC 3
•Numerique = canaux “discret” – pas de diaphonie.
• De-compression a travers un “Cinema Processor”
•Vers les 6 canaux du 5.1
Dolby Surround a la maison
•System recoit divers medias
encodes vers une code PCM Rt
Lt
•Decodage a l’intereur du
“Home cinema”
•Emulation du system Dolby
Digital ou DTS
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Les facteurs figurant dans le media multicanal
Surround processing
Processing en temps reel
A/D, D/A Compression
Processing numerique “Codecs” etc
Record Specification
Specifications d’enregistrements – Mise
en place des enceints dans le studio
Playback Spec
Specifications de lecture: Disposition des
enceintes en cinema
Downmixing
Retro compatibilite
La chaine de production multicanal
Chaine de production
Enregistrement
Haut resolution
5.1
Mixage
Haut resolution
5.1
Encodage
Compression
PCM or optical
Decode
reconstruction
5.1
Diffusion
Film / Cinema
DVD / Home theatre
DVD ROM
Formats Multicanal
L’evolution des formats:
2CH – 4CH – 5.1 CH – 6.1CH
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Presentatioon des divers formats dans la production multicanal
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Videocassettes en 4.0
Films en « Stereo »
Format 5.1 numerique
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Formats « Etendus »
Format DVD Video 3-1
Format DVD Video 5.1
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Format DVD Video 6.1
Formats DVD Audio et SACD
Format de diffusion numerique
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Formats Jeux Videos
Le processing du surround
Il y a 2 types de processing multicanal – Le matricage et le “discrete”
1
Le matricage
•Utilise la technologie de synthese a base de phase pour enregistrer plusieurs canaux
sur un support d’enregistrement ou l’espace est limite (film / VHS etc)
••Il y a reduction de bande-passante sure quelques canaux
••Il peut y avoir des restrictions au niveau de la separation (diaphonie)
••Surtout utilise dans le domaine analogique
La piste enregistre en
matrice s’appel LtRt (Left
total /Right total)
Quand vous mixez en
utilisant ce procedure, il
est important d’ecouter la
sortie
de l’encodure pour eviter
le “Magic surround” et les
stereos phantoms
instables
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Dolby Prologic encode
Equation de dematricage:
Lt=Rt (signaux en phase) Signal au centre
Lt – Rt (singaux en opposition de phase) si le resultant passe le seuil de declenchement,le
signal passe en surround
Lt et Rt differents: Lt passe a gauche, Rt passe a droite
Decodage Pro-Logic
Decodage Pro-Logic II
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5.1 Discrete
6.1 Etendu
6.1 Discrete
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Methodes d’encodages
Il y a 2 grands methodes d’encodages
PCM - Pulse Code Modulation Methode de numeriser ou A/D et D/A sont determine
par Fs (frequence d’echantilliage) et Qb (quantization bit depth – profondeur de bits)
Utilise dans beaucoups de media
Non compresse, on l’appel LPCM (linear PCM)
DSD - Direct Stream Digital L’utilisation de 1 bit avec echantillonage a haut vitesse
pour enregistrer numeriquement une forme similaire a l’onde de compression trouve
dans le monde phtsique
Utilise en SACD
Reponse en frequence: DC 100KHz
Dynamique a 120dB
Methodes de compression
Il y a 2 grands methodes de compression
Lossy compression
Le signale originale ne peut pas etre reconstruite completement
Compression est irreversible
Utilise souvent des phenomens psychoacoustiques pour reduire “les redondances”
dans le signal originale
Examples de codecs: Dolby AC-3, DTS Coherent acoustic, MPEG (-AAC), etc.
Media: Film, DVD-Video, digital broadcast, games, etc.
Lossless compression
Permet au signale originale d’etre reconstruite completement
Compression reversible
Utilise souvent pour compresser des fichiers d’ordinateurs (.ZIP .RAR styffit, etc…)
Utilise des moyens mathematiques pour reduire les redondances
Est moins economique que le lossy compression
Examples of lossless Method MLP (PPCM: Packed PCM), DST (Direct Stream
Transfer)
compression Media: DVD-Audio, SACD
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Bande passantes a l’enregistrement
Recording response
By “recording response” we mean the response when the master tape produced by
the studio is recorded
onto the production target media.
The response of each channel recorded on the media will depend on the encoding
method and compression method as described above.
In the case of analog recording, the response will depend on the specifications of the
recording media. In other words for most types of media, all channels are recorded
with full-range recording. However in the case of LFE and surround channels, there
will be differences depending on the media.
Surround channels (S, LS, RS, BS)
For 3-1 matrix, the recording bandwidth of the S channel is restricted to 100 Hz–7
kHz.
For DTS in film (5.1, 6.1), the recording bandwidth of the surround channels (LS, RS,
BS) is specified as 80 Hz and above, but since sound that lies below this point on the
master tape is collectively recorded on the LFE channel, the resulting playback is fullrange. This is what is known as “bass management”
LFE channel
For media that is recorded in Dolby DIGITAL, such as film and DVD-Video, the
bandwidth is restricted
to 120 Hz at the time of recording.
This also applies to DTS. (However in film, the range to 80 Hz is the DTS recording
range.)
For other media (DVD-Audio, SACD, MPEG2, MPEG2-AAC), full-range recording is
allowed for the LFE channel in the same way as for the main channels.
3-3. SUB
When placing the sub-woofer, we must take the acoustics of the room into account.
For example if we place the sub-woofer in the corner of the room, we will get plenty
of power, but the frequency response will tend to be disrupted by standing waves.
When placing the sub-woofer, we must consider both the playback power and the
frequency response.
2-3-2. Surround channels (S, LS, RS, BS)
For 3-1 matrix, the recording bandwidth of the S channel is restricted to 100 Hz–7
kHz. For DTS in film (5.1, 6.1), the recording bandwidth of the surround channels
(LS, RS, BS) is specified as 80 Hz and above, but since sound that lies below this
point on the master tape is collectively recorded on the LFE channel, the resulting
playback is full-range. This is what is known as “bass management”
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Bande Passante a la diffusion
2-4. Playback response
By “playback response” we mean the desired (recommended) response of the
playback system that plays back the media. For example, this corresponds to the
frequency response of each speaker and the level
balance. It is important to be aware that depending on the media and the channel
format, playback response may not be the same as the recording response.
The following pages describe playback response for typical media.
DVD Video Dolby DTS
Front channels
Level
L = C = R (= 85 dBC)
Match the playback level of all channels.Playback bandwidth
Full-rangeSurround channels
Level
3-1: S (LS+RS) = L/C/R
Set the LS and RS playback levels lower than for 5.1 (LS = RS =: 82 dBC)
5.1: LS = RS = L/C/R (= 85 dBC)
6.1: LS = RS = BS = L/C/R (= 85 dBC) Playback bandwidth
3-1: In the case of matrix, 100–7 kHz (it is best to use full-range speakers)
In the case of discrete, full-range
5.1: Full-range
6.1: Full-rangeLFE channel
Level
“Band level” is +10 dB compared to the main channel.Playback bandwidth
(20 Hz) – 120 Hz
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Playback response for film
Differences with DVD-Video are as follows.
[Level balance of the surround channels]
For film productions, set the playback level of the surround channels at –3 dB relative to the
front
channels.
In the case of L = C = R = 85 dBC,
3-1: LS = RS = 82 dB; in other words, S (LS+RS) = 85 dBC
5.1: LS = RS = 82 dBC
6.1: LS = RS = BS = 82 dB
Music DVD A et SACD
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L = C = R Full-range
Ls=Rs=L/C/RLFE +/- 0dB Full range
PBR pour Broadcast et jeux
Broadcast: Dolby DIGITAL, MPEG2, MPEG2-AAC
nDolby Digital = a DVD-Video
nMpeg 2 et Mpeg2-AAC: specifications sont determine par la societe de diffusion,
particulierement dans la facon de traiter le LFE
Jeux Videos
Selon la mode de processing utilise par le fabriquant
Dolby DIGITAL etDTS sont les plus utilises
Dans ces cas on utilisera la norme DVD VideoBass Management
Note: Verifiez toujours vos mixes avec un system de Bass Management
Parce que un grand nombre de systems “grand publique” envoient les frequences
graves de leurs enceintes vers le SUB, il est important de reproduire cette phenomen
en studio pour verifier si’ily a des problemes.
Verification que les graves sont en phase.
Plus la frequence “crossover” est bas (80Hz), plus il y aura flexibilite dans le
placement du SUB, et plus la localisation restera invisible.
•Bass management permet
l’utilisateur de rediriger les
frequences graves vers le sub.
•Important car les chaines type
satallite / sub ne peuvent reproduire
les frequences < a 80Hz dans les
enceintes.
• On verifie l’interaction des
frequences graves avec la piste LFE
•Emulation des systemes Home
Theatre
•Bien que LFE peut contenir jusqu’a
120Hz, Bien des systemes ont de la
bass management a frequences fixes
(le plus souvent a 80Hz) en utilisant
un filtre apres la sommation de l’LFE
et des canaux principaux. Ceci cause
la perte des frequences entre 80 et
120 Hz dans le LFE.
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Pour cette raison il est
recommende de filtrer a
80Hz pour mixes musique
Systems peuvent avoir
d’autres fonctions:
Quand un SUB n’est pas
connecte, la piste LFE est
redirige vers L et R.
Bass management est une
fonction de MONITORING.
Les avantages de l’utilisation d’un system de Bass Management sont:
La reponse des basses en L/C/R/Ls/Rs peuvent etre rendu “consistantes”
Tous les cannaux peuvent emettre plein spectre – particulierement pourles
studios travaillant le mixage de films
Filtres :
Low Pass
nSlope: -24dB / octave
nCutoff freq: 80Hz
Hi Pass
Cutoff: Doit etre la meme frequence que LPF. (80Hz)
Type de filtre doit etre pareille aussi (butterworth, Linkitz, etc…)
Slope: meme dB/oct que LPF
Si vous utilisez l’LFE a hauteur de 120 Hz, il faut utiliser 2 filtres LPF. Regardez le
schema de Bass Management suivant
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Bass Management « Overlap »
Attention!! Il peut y avoir une desequilibre dans les frequences entre 80 et 120Hz
(Redirige dans les canaux L et R @ +10dB!
Bass Management et l’acoustique
Dans les studios de petits ou moyens tailles, les ondes stationaires souvent
deviennent un problem, et il est facile que des inegalites se developpent dans les
basses frequences des chaque enceinte.
Ceci peut fausser les besoins fondamentaux de l’enregistrement multicanal:
1Tous canaux ont une reponse consistante de 20 a 20KHz
2Que le LFE maintiens +10dB de gainde 20 a 120 Hz
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Downmixing
C’est la gestion de la retro compatibilite
“cross platform compliance” Travaillez le mixage de facon que le resultat peut jouer
dans les differents formats:
5.1 – 3.1 – 4.0 – 2.0
On peut soit:
Creer des mixes separes (surtout pour le stereo)
Reassigner electroniquement les pistes vers le format cible
Most multi-channel media requires two-channel playback. There are two possible
ways in which a multi-channel production can be mixed to two channels. One way is
to generate a separate two-channel mix using the individual musical materials
(stems) that were used for multi-channel mixing. The other way is to use electrical
circuitry to forcibly create the two-channel program (fold down). The fold-down
algorithm is defined for each type of media, and the production side must store
attenuator values etc. on the media as meta-data.Typical examples of fold-down are
shown below.
Downmix MPEG2
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Downmix pour DVD Audio et SACD
5.1 vers 3-1
5.1 vers stereo “custom”
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System de Monitoring 5.1
Voici un exemple
d’un systeme de
monitoring. (Les
envois vers
l’enregistreur
sont situes avant
ces etapes
Il y a 3 sections
distincts
•Matrice de
Monitor
•Bass
management
•Alignement de
monitors
Environment d’ecoute studio
Placement des enceints
Le placement ideale des enceintes depends de la taille de la piece, la distance entre
les enceintes et le point d’ecoute et la traitement acoustique de la piece.
Le placement est determine largement par 2 facteurs:
L’angle L R
Le placement des enceintes surrounds
Rec ITU-R BS. 775-1
Rec=recommendation, donc pas standard.
ITU= International Telecomunication Union
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Enceints avants
Les 3 enceintes doivent etre equidistante du point de reference
En se referant a “C”, l’angle des enceintes C- L et C- R doivent etre +/-30 degrees
Les 3 enceintes doivent etre places a la meme hauteur (1.2 m)
Reference verticale = 0 degrees
Si un monitor TV vous empeche d’aligner de facon identique LCR, aligner les
tweeters
Enceintes surround Ls + Rs
Il y a 2 types d’enceintes en surround
Son diffuse – enceintes dipole
Son direct – un pair d’enceintes est dirige directement vers l’auditeur.
Note: Les 5 enceintes (L, R, C, LS, RS) devrait etre identiques.
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Le son direct et le son diffuse en Ls Rs
Diffused surround
The most common method of creating diffused surround is to use several surround
speakers (side + rear).
With such a configuration, it is possible to create a monitoring environment that
allows both a sense of stereo and 360° surround panning.
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Direct surround
In the case of direct surround, the placement of the surround speakers involves a
trade-off between “surround panning” and “sense of rear stereo.”ITU-R: 100°–120°
In the ITU-R placement, which locates the surround speakers at the “side” rather than
at the “rear,” there is good left/right separation between the surround speakers, and it
is easy to produce a phantom image. However, surround panning is typically limited
to expressions such as blurring the image behind the listener’s head, and it is not
easy to produce surround panning expressions with depth. (In other words,
sound-source movement via surround panning does not describe a circle.)
135°
In order for a sound source to be perceived as being “behind” rather than “beside”
the listener, it is said
that the surround speakers need to be placed at least 135° to the rear.
In most households, it is common for the speakers to be placed not at the “side” as in
ITU-R, but rather “behind” at approximately 135°. If the surround speakers are to be
placed “behind,” or if end-user compatibility is to be emphasized, it is best to place
the speakers at the 135° position.
150°
If you require that the surround L and R have the same acoustical conditions as the
main channel L and R, placing the surround speakers at 150° will produce a
placement that is completely equivalent between front and rear. With such a
placement, L/R and LS/RS will have the same angle of separation, and it will be easy
to move the sound in a 360° path by surround-panning. However, as the surround
speakers are placed farther to the rear, the surround sound field will tend toward
monaural, and there will be a more distinct separation between the front and rear
sound fields.
Son direct
Facilite de creer des images phantoms precise a travers 360 degrees
Souvent utilise en DVD-A SACD
Compatiilite complete est necessaire pour la restitution de ces images
Son diffuse
Ambiente, Vague / flou
Permet paning a travars 360 degrees
Environment de production video
“General Purpose”
Le Sub
Il faut prendre l’acoustique de la piece en compte
Dans le coin, il va avoir de la puissance mais va etre interrompu par des ondes
statioaires
Consiterez a la fois puissance et aussi bande passante
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Alignement des monitors
Filtre en peinge
Si le meme son est emise par 2 enceintes don’t la distance differe d’environ 8mm ou
plus, de creux vont apparaitre dans le spectre. En se basant sur la vitesse du son,
une distance de 8mm ou plus= 0.025msec.
Haas
Si Ls et Rs sont place a plus de 30cm plus pres du distance LCR le son va etre “tire”
vers l’arriere lors de panning en surround (joystick). L’avant ne va pas etre entendu
de facon naturel.
Avec un environment Ls Rs diffuse, un desequilibre de distance va favoriser un
enceint au dessus de l’autre. La perception va etre localise autour d’un enceinte.
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Crossover avec sub
S’il y a plus d’1 metre d’ecart de distance entre le sub et les autres enceintes, il y
aura des creux qui vont apparaitre dans le spectre “combine”, (souvant autours de la
frequence de cutoff)
En cas d’utilisation de Bass Management ces problemes sont plus importants
Lors du design du system d’ecoute il est important d’inclure la possibilite de placer
GEQ ou PEQ dans la chaine de monitor de CHAQUE enceinte.
Considerations Acoustiques
Il y a des differences significants entre un studio “stereo” et un disposition
multicanal.
Enceints multiples diffusant dans differents directions affectent des facteurs de
bases tel la taille optimum et geometrie, les besoins en equipement, les methodes de
construction, cablage, HVAC (la clim’) lumiere, et ergonomiques.
L’addition et le placement d’equipement nessesaire pour la diffusion multicanal
souvent affect l’accoustique de la piece aussi.
Le but primaire de mettre en place une piece d’ecoute de reference est de faciliter
ds jugements interchangable entre differents studios.
Les references qui suivent sont bases autour du studio de petit a moyen taille.
Dimensions…
Bien des formes de studio marchent, le studio ideale est symetrique sur la l’axe zero
degree.
Hauteur minimum de 3 metres est desirable
En stereo, les enceintes sont habituellement installe pres de mur “court” et diffuse
dans le sens “long” de la piece
Beaucoup de studios 5.1 maintenant placent les enceintes LCR pres du mur long et
diffusent dans le sens de la largeur de la piece. Ceci fournit une utilisation efficace de
symetrique de l’espace d’ecoute.
Les studios multi usages – ou qui ont deja une utilisation stereo n’ont pas besoin
d’etre re-oriente. Seulement si l’opportunite se presente…
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Acoustiques
Reflections primaires
(Les premiers 15ms) devront etre au moins 10dB en dessous du niveau du son direct
pour les frequences entre 1KHz et 8KHz
Champs reverbere
Temps de reverbe est dependante des frequences. Le valeur nominale, Tm, est la
moyenne des temps de reverbe mesure en 1/3 octaves entre 200Hz et 4kHz et
devrait etre contenu entre 0.2 et 0.4 s
0.2 <Tm<0.4
Tm devrait augmenter avec la taille de la piece
Surfaces reflechissantes et surfaces absorbants
Grands surfaces reflechissantes devraient etre evitez dans l’environement de
mixage
Le placement des portes, fenetres devraient etre prise en consideration
Une combine de reflecteurs et d’absorbeursdevraient etre utilise pour atteindre un
Rt equilibre a travers le spectre
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Bruit de fond
La position d’ecoute devrait atteindre un coeffinient de NC 10 ou moins avec
tous equipement eteinte
Power ON devrait rendre un coefficient de NC 15
NC = Noise curve
THX
THX Pm3 a ete annonce en 1999. C’est une norme pour aider le design et
construction de studiosde petite ou moyenne taille.
En date c’est le seul vrai norme existant specifiquement pour le multicanal
Calage du systeme
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Speaker Calibration Procedure
Along with physical placement, correct speaker calibration is the single most
important factor in ensuring an accurate monitoring environment. The tools
required for calibrating a surround sound speaker system are much the same as
those required for a stereo system: a source for pink noise and/or tones, and a
Real Time Analyzer (RTA) or Sound Pressure Level (SPL) meter, with a
reasonable quality omnidirectional condenser microphone. RTA metering is the
preferred method for calibrating a surround sound speaker system because an
SPL meter will yield a less accurate result since only the peak of one band is
measured. If an SPL meter is used, set it to C-weighting on the slow scale.
It is also helpful to have a number of well-recorded commercial surround sound
releases on hand to play back in order to do final “tweaking” by ear.
In addition to console oscillators and onboard pink noise generators, there are a
number of surround sound calibration tapes and CDs available which can be very
helpful in calibrating speakers. These range from professional releases (such as
those available from MLSSA, Tomlinson Holman, and Dolby) to numerous
consumer DVDs and CDs. The key is to have access to both full bandwidth and
band limited pink noise (low-pass filtered at 80 - 120Hz) and a sine wave at the
subwoofer crossover frequency (80Hz in most instances).
The need for precise speaker calibration should be obvious: Only if the system
plays back accurately can it be used to produce surround sound mixes which will
translate well in other listening environments. The converse is also true: If the
speaker system used for mixing is not correctly calibrated, there is a greater
likelihood that the resulting surround mixes will only sound good in the studio in
which they were mixed, and nowhere else.
After making sure that the signal path from the console is in proper electrical
phase to all speakers (a near-certainty if using powered monitors and correct
balanced wiring), the first, and perhaps most important calibration is the acoustic
phase alignment of the subwoofer(s) at its crossover point (again, 80Hz in most
instances); incorrect alignment will cause a drop in the frequency response of the
entire system at the crossover point. Some subwoofers have built-in phase
matching controls which allow adjustment in precise 90° steps.
Following is the recommended procedure for subwoofer phase alignment: (Note
that this procedure assumes that all main speakers in the system are full range
and not satellite.)
1. Route a sine wave at the crossover frequency (generally 80Hz) at a
moderate listening level to the left front and right front speakers and to the
subwoofer.
2. Using an RTA or SPL meter, note the signal level at the mix position.
3. If the subwoofer provides phase controls, toggle the switches, noting
the signal level at the mix position each time. Leave the switch at the
position which yields the maximum signal level.
4. If the subwoofer does not provide phase controls, simply rotate it by
hand at 90° increments until the signal level at the mix position is highest.
The next step is to set the reference level for the main speakers. We
recommend a nominal reference level of 79 - 85dB SPL (see section 3.5). The
important thing here is not so much the actual SPL chosen, but that all five main
speakers are set to that same chosen level. Following is the recommended
procedure for reference level adjustment for the main speakers: (for purposes of
illustration, a reference level of 85dB is assumed)
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1. Turn off all speakers and subs except the front left speaker.
2. Place the calibration microphone at the center of the mix circle, at ear
height, facing directly towards the center speaker. If an RTA is being used
for measurement, set it to read 85dB. If an SPL meter is being used for
measurement, set it to C-weighting on the slow scale, and then set it to
read 85dB.
3. Route pink noise at 0 vu to the front left speaker and raise the
speaker’s amplifier level until all bands of the RTA (or the SPL meter) read
85dB.
4. Continue by routing pink noise to each remaining speaker in turn (front
right, then center, then rear left, then rear right), adjusting their amplifier
levels so that the RTA or SPL meter reads 85dB for each. In each
instance, leave the calibration microphone at the same fixed position as in
step #2 above.
Some professionals instead suggest pointing the calibration microphone towards
the phantom center when measuring the rear speakers. Others recommend
pointing it at 90° left and 270° right when calibrating the rear speakers. Though
these techniques will yield slightly different results, the fundamentals remain the
same.
The final step is to set the level of each subwoofer relative to that of the main
speakers. Common practice is to calibrate the subwoofer approximately 4dB
above the reference level of the main speakers. This procedure differs
somewhat depending upon whether the subwoofer is receiving the LFE channel
only or whether bass management is being utilized to route signal to it from some
or all of the main channels.
The recommended subwoofer calibration procedure when no bass management
is being used is as follows:
1. Turn off all five main speakers.
2. Route band-limited pink noise (low-pass filtered at 80 - 120Hz) at 0 vu
via the LFE channel bus to the subwoofer and raise its amplifier level until
the RTA or SPL meter reads +4 dB over the selected reference level (i.e.
89dB if the selected reference level for the main speakers is 85dB).
3. Turn on the front left and right speakers.
4. Route full frequency pink noise at 0 vu to the front left and right
speakers as well as to the sub. Adjust the subwoofer amplifier so that the
gain boost when adding the subs to the mix does not exceed 4 - 6db, as
measured by the RTA or SPL meter.
The recommended subwoofer calibration procedure when bass management is
being used is as follows:
1. Route band-limited pink noise (low-pass filtered at 80 – 120Hz) at
-10 vu via the LFE channel bus to the subwoofer. (The 10dB of
attenuation compensates for approximately 10dB of “in-band gain” in the
LFE channel as compared with a main channel.)
Note: “In-band gain” refers to the fact that the level in each 1/3-octave
band within the frequency range of the subwoofer is 10dB above the level
in each 1/3 octave band in each of the main channels, averaged across
the main frequency range. This does not mean that the LFE channel is
10dB higher in SPL than the main channels, however, due to the broader
bandwidth (and correspondingly greater energy) in the main channels.
2. At the same time, route band-limited pink noise (low-pass filtered at
31
80 - 120Hz) at 0 vu via a single bus that is sending signal to the subwoofer
via the bass management circuitry.
3. Raise the amplifier level of the subwoofer until all bands of the RTA (or
the SPL meter) read at the selected reference level.
Note that, following this calibration procedure, the subwoofer level may need to
be adjusted up or down by a dB or two to compensate for heavy bass trapping
(or lack thereof) in the room. The best way to do this is to listen critically to some
commercially recorded surround sound music while seated at the mix position.
Some subwoofers have DIP switches that allow their level to be adjusted in fine
increments of plus or minus 1 or 2dB.
Also note that, following speaker calibration, the overall gain of the system is
mathematically up to 12db louder, excluding the sub (i.e., a reference level of
85db for each individual speaker actually sums to 97db at the mix position with
all five main speakers driven).
Use of Delay
As noted in section 3.3.1, delay may be used to compensate for non-coincident
(differing) arrival times if it is not possible to place all five main speakers
equidistant from the mix position, or if it is not possible to angle them correctly
(for instance, if there is a "flat" front wall, with the L, C, and R speakers in a
straight line).
The following formula should be applied: For each foot of distance disparity, add
.88 milliseconds of delay. (For each meter of distance disparity, add 2.94
milliseconds of delay). For example, if the rear speakers are two feet further
away from the mix position than the front speakers, the signal going to the front
speakers should be delayed by 1.76 milliseconds. In the case of a "flat" front
wall, delay should be added to the center speaker.
It is important to note that the use of delay is not recommended unless absolutely
necessary. It provides a far less satisfactory solution than actually positioning
and angling speakers correctly!
32

Le Multi-canal - Stephen`s Audio Site

Transcription

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