Bonjour à tous.
Voici enfin la copie de la partie qui nous intéresse dans le manuel d'encoding DD de dolby (ma version date de décembre 97).
J'ai un peu débordé de la section "sonomètre" car il y a des trucs intéressants. Au passage hrobien avait raison sur le 45°
.
@+
Emmanuel
2.4 ROOM LAYOUT
===============
The various possible room layouts used when Dolby Digital encoding all conform to
a basic standard. They all require Left, Center and Right channel speakers and at
least two Surround channel speakers. See Figure 2-7. Individual room
requirements will determine if additional speakers are required for the Surround
channels. Although the room layouts in Figure 2-7 show a mixing console, the
general room layout applies to other equipment complements as well. Both
listening positions, engineer and client, are compromised in larger rooms where two
Surround speakers are shared. When only a single pair of Surround speakers is
available, rooms should always be optimized for the engineer's listening position.
The preferred solution is for both the engineer and clients to each have their own
pair of Surround speakers.
2.4.1 Center Speaker Placement
Whenever possible, the Center speaker should be positioned so that its "acoustic
center" is in the same horizontal plane as that of the Left and Right speakers. In
near-field applications, this is usually a simple task. It is very important to keep the
three front speakers equidistant to the engineer's position.
With soffit-mounted speakers this task may be more difficult when a video
monitor(s) occupies the ideal location for Center speaker placement. If it is not
practical to position all three front speakers in the same horizontal plane, the Center
speaker should be located either above or below the monitor screen.
When it is not possible to align the "acoustic centers" of the three front speakers,
attempt to align the tweeters in as close to a straight horizontal line as possible.
This may require either inverting or positioning the Center speaker laterally, as well
as rotating the tweeter to maintain the proper dispersion characteristic.
Using soffit-mounted Left and Right speakers with a near-field Center speaker is not
recommended.
2.4.2 Surround Speaker Placement
For installations using one set of speakers, typically the speakers will be located
approximately two feet behind the engineerÆs position, at or above ear level and
angled so as to focus either directly overhead or on axis to the engineer. If four or
more speakers are used, the same guidelines apply for each set of speakers with
respect to the desired coverage. Speaker location and orientation may vary
depending on room layout, speaker type and personal preference.
2.5 CALIBRATION
===============
Calibrating your playback system to establish a balance between all five main
channels and to ensure that all speakers play back at the same level is important to
for accurate encoding/decoding.
For proper calibration, it is necessary to use a Sound Pressure Level (SPL) meter
and/or Real-Time Analyzer (RTA) when measuring (in decibels) relative and specific
playback levels of all six channels.
2.5.1 Playback Levels
Use an SPL meter and the Pink Noise Generator in your decoder to set your
system's playback level to a particular reference level.
There are three options to adjust monitor system playback levels:
À Amplifier gain trim controls
À Mixing console group outputs (one for each of the L, C, R, LS, RS and LFE
channels)
À Decoder output level trim controls (only if using the analog outputs)
The best option is to use your amplifier gain controls to set proper playback levels.
This option allows you to maintain optimum signal-to-noise performance from the
decoder and mixing console. If you decide to use either the console's group
outputs or the decoder's output level trim controls, both may sacrifice signal-to-noise
ratio, depending upon how you use these options.
In general terms, the industry standard for film work uses pink noise at 0 dB
reference level (i.e., -20 dB from digital full scale) producing a sound pressure level
(SPL) of 85 dBc (slow). When performing such measurements, it is important to
account for meter characteristics as well as the extent and accuracy of low
frequency response. In projects for theatrical release, the level should be set and
left there. For music mixing, speaker levels should be set for the same SPL in each
channel. (Some engineers prefer to mix louder than others do, but if the levels
between channels are correct, the overall level isn't as important.)
For very small mixing rooms, the Surround channel is generally set 2 dB lower than
the front channels (for example, 85 dBc in front and 83 dBc in back). This takes into
account the closeness to the Surround speakers and has proven to make the sound
in the home environment very close to what was heard by the mixer. For home
video releases, the overall level is usually set to 79 dBc. The lower level is used to
ensure that dialog will not be lost in a typical home environment.
Keep two important factors in mind when setting your playback levels:
1. Adjust your monitor system so that all five channels play back through their
respective speakers at the same level.
2. Set your monitor system's overall playback level so that the prominent material,
dialog or other, will be heard at the appropriate reference level. The reference
level will be either 85 dBc or 79 dBc, as described earlier.
2.5.2 Bass Redirection
Before you calibrate your monitoring system, you should decide whether bass
redirection of low frequencies is required in your system. Many decoders are
capable of redirecting low frequency information below a selected frequency (80,
100 or 120 Hz) to the channels capable of reproducing them. This is similar to the
normal/wide mode for the Center channel in a consumer decoder with Dolby
Surround Pro Logic.
For example, if you have five satellite speakers and a subwoofer, you could redirect
the low frequencies from the five main channels to the subwoofer outputs. If you
are using small Center and Surround speakers, you can take the low frequencies
from those channels and direct them to the Left, Right, or subwoofer outputs. If no
subwoofer is available, the low frequency effects signal (LFE) can be redirected to
the Left and Right channel outputs of the decoder.
There are generally two types of test noise found in decoders: Pink and filtered. If
no bass redirection is chosen, calibration of the monitoring system should be made
using pink test noise. If bass redirection is required, calibration of your monitoring
system should be made using filtered test noise (if available). This will compensate
for the absence of low frequencies in your main monitoring speakers.
2.5.3 Sound Pressure Level Meter
To properly calibrate speaker levels, an SPL meter is necessary. A suitable and
relatively inexpensive meter is readily available from Radio Shack stores (Tandy
Electronics outside of North America). Since in general, the relative level between
channels is more important than absolute level, the accuracy of this meter is
sufficient for channel balancing. For greater accuracy, more expensive meters may
be used. It is recommended however, that an inexpensive meter be left in the
control room for quick level checks.
2.5.4 Taking a First Measurement
1. Before you turn on the Pink Noise Generator in your decoder, be sure that your
playback system has been set to a moderate listening level. Adjust your
amplifier(s), self-powered speakers, or mixer only. Beware that if the playback
level is very high, you may run the risk of damaging your speakers or possibly
your hearing.
2. Pink noise should be cycled between the Left, Center, Right, Left Surround, and
Right Surround channels -- remaining approximately two seconds at each output
before moving onto the next. If you do not hear pink noise in any of the five
main channels as the noise has cycled through that channel, check your
system's connections and settings.
3. Ensure that you are sitting in your normal, proper reference listening or mixing
position. Pick up your SPL meter, turn it on, and set it for the proper SPL scale,
C-weighting and slow response.
4. As you face your front speakers, hold the SPL meter at chest level, with the
microphone facing up at an angle of approximately 45 degrees at the Center
speaker. Keep the meter at armÆs length to prevent any audio that may reflect
from your body. You should be able to take SPL readings, in decibels, as you
look down at the meter.
5. Keep the SPL meter at this same position. Make sure that the meter is aimed at
the Center speaker as you take readings for the Left and Right speakers.
6. When taking the SPL readings for the Left Surround or Right Surround
speakers, keep the meter at the same angle and position as you did for the front
speakers, but turn your body 90 degrees from the Center speaker towards the
wall closest to the Surround speaker you are measuring. This will minimize
"shadowing" or obscuring the meter with your body.
2.5.5 Individual Channel Calibration
The following steps describe the procedures for systems that have individual
amplifier gain controls, self-powered speaker gain controls, or less preferably,
adjustable console group outputs. If using a console, it is advisable to set the
master monitor volume control to a known marked position before proceeding with
calibration. This will allow you to easily return to the calibrated listening level at any
time.
1. Set the playback level for your Left channel. Generate test noise (pink or
filtered) so that it emanates from only the Left speaker.
2. Check your SPL meter and take note of the level. Adjust that channel's amplifier
gain control (or associated mixer group output or self-powered speaker gain
control) up or down as necessary. Unless you have an assistant, you may need
to leave your sitting position to do this. If you do, be sure to return to the same
position, with the SPL meter in the same position.
3. After you have set the channel to the desired reference level (85 dBc or 79 dBc),
you have properly set the level for the Left channel. Now return to Step 1, and
repeat the same procedure for the remaining Center, Right, Left Surround, and
Right Surround channels.
4. As a final check, cycle pink noise between all selected channels. Using your
SPL meter--and your ears--confirm that each speaker sounds equally loud from
your reference sitting position and that each speaker plays back at the exact
reference level (85 dBc or 79 dBc). If there are any discrepancies, repeat these
steps until everything is properly adjusted.
2.5.6 Subwoofer Calibration
Ideally, the test noise used for subwoofer calibration should use band-limited pink
noise, low-pass filtered at 120 Hz. To properly calibrate the subwoofer an RTA is
required. If an RTA is not available, you can approximate the settings with an SPL
meter.
When using an RTA, proper alignment of the subwoofer requires setting the
subwoofer within its bandwidth, typically 25-120 Hz, 10 dB higher than the same
bandwidth measured for the Center channel. For example, when measuring the
Center channel, if the average value of each third-octave band (between 25 and
120 Hz) is at the 0 dB datum on the RTA display, adjust the subwoofer so that the
average value of each third-octave band is at the +10 dB datum. See Figure 2-8
If an RTA is not available, setting the subwoofer channel 4 to 6 dB higher (e.g. 89
to 91 dBc for the subwoofer channel when the Center channel measures 85 dBc),
can give an approximate level on an SPL meter. This level varies with the quality of
your meter.
For future reference, if you have set up the level with an RTA, measure the level
with your SPL meter and take note of where your meter reads for the proper
calibration. You can then use this measured value for quick checks of the system
calibration in the future.
2.6 CENTER/SURROUND DELAYS
In addition to setting the proper monitoring levels, you must be sure that the sound
from each speaker arrives to your listening position at the correct time.
To achieve this, the delay times of the Center and Surround channels need to be
adjusted. Channel delay is determined by calculating the distances from each
speaker to the listener, as seen in Figure 2-9.
In the case of Dolby Surround Pro Logic, you want a sound common to both the
front channel and the Surround channel to arrive at the front channels a fraction of a
second before the Surround channels. This takes advantage of the Haas effect
(also known as the precedence effect), that states that if two similar sounds arrive to
our ears at slightly different times, our brain tends to focus on the sound arriving
first and ignore the second. Dolby Surround takes advantage of this effect to
reduce the perceived crosstalk between the front and rear channels. A time arrival
difference of 10-20 milliseconds is adequate to make the Haas effect work.
Since, Dolby Digital is a discrete system, there is no crosstalk between channels.
Although related, the delay times required for Dolby Digital are different from Dolby
Pro Logic.
In decoders, Surround delays may be set up separately for Dolby Pro Logic and
Dolby Digital, with the set up for the Center delay being the same for both modes.
2.6.1 Surround Delay for Dolby Pro Logic
To calculate the required delay in Dolby Pro Logic mode, you need to measure the
distance from your referenced listening position to each of the five speakers:
À Left speaker (L) or Right speaker (R)
À Center speaker (C)
À Nearest Surround speaker (LS or RS)
All of these measurements need to be made in feet. If you are measuring in meters,
multiply the metric measurements by three to get the approximate equivalence in
feet.
Once you have these three measurements, you can calculate the required delay
settings for the room. Using the formula below, apply the appropriate numbers and
calculate the delays.
Distance from C - Distance from S + 15 = Surround delay for Pro Logic
For example, if the listener's position is 15 feet from the Center speaker and 10 feet
from the Surround speakers, the delay setting for the rear channels will be 20
milliseconds for Dolby Surround Pro Logic. See the example below.
15 (distance from C) - 10 (distance from S) + 15 = 20 (Surround delay for Pro Logic)
If the answer is less than the minimum value available for the selected delay, use
the minimum value available. This value is the correct delay (in milliseconds) that
you would program into your decoder.
2.6.2 Surround Delay for Dolby Digital
Determining channel delays for Dolby Digital is similar in concept to Dolby Pro
Logic, but important considerations arise since every channel is discrete. Since
Dolby Digital delivers discrete signals for each channel, there is no leakage or
crosstalk between channels. Therefore, there is no need to delay the Surround
channels to take advantage of the Haas effect (precedence effect). The Surround
and front channel signals should arrive "coincidently," at the same exact time.
Consequently, the Dolby Digital mode uses approximately 15 milliseconds less
delay than the Dolby Pro Logic mode for the same speaker/seating arrangement.
To calculate the Surround delay for Dolby Digital, you need to measure the distance
from your listening position to each of the three speakers:
À Left speaker (L) or Right speaker (R)
À Center speaker (C)
À Nearest Surround speaker (LS or RS)
All of these measurements need to be made in feet. If you are measuring in meters,
multiply the metric measurements by three to get the approximate equivalence in
feet.
Once you have these three measurements, you can calculate the required delay
settings for the room. Using the formula below, apply the appropriate numbers and
calculate the delays.
The S represents the distance from the Left Surround or Right Surround speaker.
Distance from C - Distance from S = Surround delay for Dolby Digital
For example, if the listenerÆs position is 15 feet from the Center speaker and 10 feet
from the Surround speakers, the delay setting for the Surround channels would be
five milliseconds for Dolby Digital. This is the correct delay in milliseconds that you
would program into your decoder. See example below:
15(Distance from C) - 10 (Distance from S) = 5 (Surround delay for Dolby Digital)
2.6.3 Time Delay for Center Channel
The concept of "coincident arrival" can also be applied to the adjustment of the
Center channel in Dolby Digital. If the Center speaker can be placed at the same
distance from the listener as the Left and Right speakers (in an arc) to achieve
equal path lengths, i.e., distance C = distance L (distance from the listening position
to speaker), no delay is required. If the Center speaker is placed closer to the
listening position than the Left and Right speakers or further back, time delay can
be added to the Center channel signal to bring it into acoustic alignment with the
Left and Right speakers. This will electronically move the speaker to its proper
position in the room. For example, if distance C is two feet less than distance L,
Center delay is set to two milliseconds. If distance C is two feet more than distance
L, Center delay is set to -2 milliseconds.
To make the C-time delay negative, the decoder is actually setting C-delay to zero
and adding delay to the Left, Right, Left Surround, and Right Surround outputs of up
to three milliseconds. This keeps the Surround and front arrival times aligned.
» 17 Jan 2002 11:03 
