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10. Expanded 5.1
Calibration Guide
INSTRUCTIONS FOR ELECTROACOUSTICS CALIBRATION OF A 5.1 AUDIO
SYSTEM USING A SPL METER AND BLUE SKY'S TEST FILES
Before starting this procedure you will need to download BlueSkyTestFiles.zip
(18MB) by going to www.abluesky.com/calibration. To download the test file,
"Right Click" and select "Save Target As" and the file will begin downloading.
Once downloaded, either burn the test files to a CD or import them into your
DAW and follow the instructions below.
ADDITIONAL REQUIRED ITEMS
1.
5.1 Monitoring System
2.
SPL Meter - such as the SPL meter sold by RadioShack
in the U.S.
3.
Small screw driver for adjusting trim pots
BlueSkyTestFiles.Zip Includes 4 files:
•
1000Hz SINEWAVE -20dBFS.wav – a 1kHz file recorded at -
20dBFS for electrical calibration
•
40-80Hz PINK NOISE -20dBFS.wav – a 40Hz to 80Hz bandwidth
limited pink-noise file recorded at -20dBFS
•
500-2.5kHz PINK NOISE -20dBFS.wav – a 500Hz to 2.5Hz
bandwidth limited pink-noise file recorded at 20dBFS
•
Pink Noise full bw -20dBFS.wav – a full-bandwidth pink-noise file
recorded at - 20dBFS
These test files are all mono files. Please make sure you hard assign
them to the left and then the right, not both channels at the same time.
If you are using a CD player use only one channel of the CD player.
THEORY
The purpose of calibration is to adjust the overall electroacoustics system gain
so that 0dBVU of electrical signal level equals a certain acoustic level at the
listening position. Since most recording media is now digital, the reference
electrical signal level is usually –20dBFS with 20dB of headroom. The reference
SPL level however can vary based on the delivery media and speaker type.
Please note that the bandwidth limited signals that have been provided, limit
many of the room interaction affects often associated with measuring SPL and
broadband pink noise.
*Also, please note that the LFE channel gain in a 5.1 system varies
from 0dB to +10dB, depending on the encoding format that is being
used. Since the LFE channel is not calibrated as a separate entity, the
LFE gain will not affect system calibration. For more information about the
LFE channel refer to page 6 [System Signal Connection / A note about the LFE
Channel / LFE Input].
The common calibration levels
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All test signals are recorded at –20dBFS including the 1 kHz sine wave tone.
The sine wave tone is used to set the electrical output level throughout the
signal path, right up to the point you get to the speakers, while the various pink
noise signals are used for acoustic measurements and calibration.
The following procedure assumes you are calibrating the system to 85dBC SPL.
If you are calibrating to TV, etc. substitute the appropriate level from the chart
titled "The common calibration levels" shown above.
Page 10
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Step 1
TURN OFF THE MONITORING SYSTEM (until step 4)
Step 2
Remove all eq and dynamics from the signal path and set all
controls to zero / unity gain. Play the 1kHz Sine Wave, hard assign
it to the left channel only, and adjust the output fader so that the
output meter reads -20dBFS. If you are using an analog console, set
the output level to 0 VU. Then hard pan the signal to the center
channel output and repeat for the center channel (repeat for the
each of the remaining channels). Once calibrated do not move
the output faders.
Step 3
Mute everything and make sure the 1kHz tone is OFF .
Step 4
Now that the system has been electrically calibrated
turn ON the MediaDesk 5.1 System.
Step 5
Assign the 500-2.5kHz pink noise signal to the center channel only
and set the remote master gain control to a relativly low level. Make
sure there is nothing coming from the left or right channels (or any
other channels). Because this signal is bandwidth limited, you
don't have to worry about turning the sub level down or off.
Note: SPL should be measured at the mix position, with the SPL
meter at arms length, the microphone at seated ear height, angled
at approximately 45 degrees, and pointed at the center point
between the left and right speakers .
Now, increase the level of the master gain control (on the remote)
until you measure 85dBc. Once you reach 85dBc make a note of
where the volume knob is relative the markings around the knob
- this is your "reference level" gain setting (do not move the knob).
If the level is relativly low and you are using the XLR inputs, you
may need to adjust the "input attenuator switch" on the back of the
MediaDesk amplifier so that you don't overdrive the inputs and so
you achieve the best signal to noise ratio.
Note: If the "reference" level is not perfectly near a marking on the
dial, you may choose to adjust the level up or down and use the
center channel gain trimpot to compensate.
Step 6
Once the center channel is set to 85dBc, assign the 500-2.5kHz
pink noise signal to the left channel and use the trim pots to make
the adjustments between the channels. (+/-6dB in 1dB increments).
Make sure that the master gain setting has not been changed from
the "reference level" setting. Repeat this step for the remaining
main channels - Right, Left Surround & Right Surround.
Step 7
Feed 40-80Hz pink noise signal to the center channel only. Adjust
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the subwoofer level control (on the back of the sub) and the
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SUB trim pot, until the subwoofer reads 85dBC (slow) at the mix
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position. The meter will bounce around a little, so you will need
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to do a mental average (I tend to filter out the peaks in my mind,
so I don't set the sub too hot). The other channels should measure
about the same and no additional adjustments need to be made.
Step 8
You can play the full-bandwidth pink noise, assigning it to any of
the main channels (not at the same time). You should measure
about 85dBc. It may be a little higher, because below 30Hz the
room may have a little extra gain. No adjustments should be
made with Full Bandwidth pink noise, unless you have an RTA
(real time analyzer).
Step 9
You are finished and the calibration process has been completed
– enjoy!
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