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7700 MultiFrame Manual
7736CD2 Dual Composite Decoder
7.
AUDIO LEVELS, HEADROOM, AND CLIPPING (-A4 and –A8)
This section contains notes to understand how both the 7736CD2-A4 and 7736CD2-A8 relates analog
audio levels, and digital audio levels.
Before you can calibrate the audio analog to digital converter, you must know a couple of system issues
specific to your application. What is your analog reference level and how much headroom do you want to
have in the digital audio signal? By adding these two values together, you will get the analog input level
that will just begin to saturate the digital word (This is the highest level that can be represented without
distortion with the digital numbers). This level is called 0dBFS (FS stands for "full scale"). For instance, if
your analog program reference level is 4dBu and you want 20dB of headroom in the "digital world", then
0dB FS will correspond to an analog level of 24dBu. Once the audio input level is calibrated, when you
apply a 4dBu analog signal, the digital level will be –20dBFS.
The SDI embedded audio, and the AES output audio are based on the digital quantized signal. Use either
digital audio output signal to calibrate the analog input levels.
7.1.
INPUT AUDIO LEVEL CALIBRATION
The analog audio input circuitry has two gain control stages before the audio is digitized; a coarse range
(settable using jumpers in –A4 or miniature DIP switches in –A8), and a software (menu system) control.
One of the two coarse ranges is optimized for when peak audio levels are above 18dBu and one for levels
below. The jumpers, when not installed, allow input levels of up to 24dBu. When the jumpers are installed,
best performance is achieved when input peak levels are below 18dBu. The OSD menu system has
independent +/- 10dB audio level control of all four channels (see section 8.5.5 for OSD control details).
The following is an example calibration procedure:
1. Take your analog program reference level and add the amount of desired headroom in the digital
signal (SMPTE standard is 20dB). If this number is greater than 18dBu, then remove the 8 jumpers
near the rear of the 7736CD2-A4 module (on 7736CD2-A8 you would slide coarse range switches
as pointed by +24dBu arrow on the module). If the number is less than 18dBu, then install the 8
jumpers (on 7736CD2-A8, slide coarse range switches as pointed by +18dBu arrow).
2. Apply an analog audio signal of the level calculated above (maximum peak level).
3. Apply composite analog video to the "ANALOG IN" BNC.
4. Monitor either the embedded audio or the output AES audio with appropriate level measuring gear.
5. Adjust the level adjust controls in the audio menu so that the digital level just starts to clip.
6. Verify by applying program reference level. The output level should read at the desired reference
level.
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Revision 1.0.4
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