le multi-canal - freestephen.taylor1.free.fr/assets/surround/le multicanal - notes.pdf · 1958...

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 1

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 Stereo 4.0 Ch (L / C / R / S) depuis Lt Rt Dolby Surround au cinema - (L / C / R S) depuisLt Rt Dolby Pro Logic (L / C / R / S) depuis Lt Rt Dolby 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


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

L’audio Multi-canal - notes - Stephen Taylor

•System recoit divers medias encodes vers une code PCM RtLt •Decodage a l’intereur du “Home cinema” •Emulation du system Dolby Digital ou DTS

3

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

EnregistrementHaut resolution

5.1

MixageHaut resolution

5.1

EncodageCompression

PCM or optical

Decodereconstruction

5.1

DiffusionFilm / Cinema

DVD / Home theatreDVD ROM

Formats Multicanal

L’evolution des formats: 2CH – 4CH – 5.1 CH – 6.1CH


Presentatioon des divers formats dans la production multicanal


Videocassettes en 4.0

Films en « Stereo »

Format 5.1 numerique


Formats « Etendus »

Format DVD Video 3-1

Format DVD Video 5.1


Format DVD Video 6.1

Formats DVD Audio et SACD

Format de diffusion numerique


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


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


5.1 Discrete

6.1 Etendu

6.1 Discrete


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


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 full-range. 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”


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


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


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.


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


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


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


Downmix pour DVD Audio et SACD

5.1 vers 3-1

5.1 vers stereo “custom”


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


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.


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.


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


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.


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…


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


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


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)


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


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!


le multi-canal - freestephen.taylor1.free.fr/assets/surround/le multicanal - notes.pdf · 1958...

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