7700 MultiFrame Manual
7736CEM Composite Encoder with Monitoring
Revision 1.5
Page - 11
*Grey background indicates un-implemented or un-tested features
5.
AUDIO LEVELS, HEADROOM, CLIPPING AND THE BAR GRAPHS
This section contains notes to understand how the 7736CEM-A4 relates analog audio levels, digital audio
levels, and the displayed bar graph levels.
Before you can calibrate the audio digital to analog converter, you must know a couple of system issues
specific to your application. What is your analog reference level and how much headroom is there in the
digital audio signal? By adding these two values together, you will get the analog output 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 0dB FS (FS stands for "full scale"). For instance, if your analog
program reference level is 4dBu and you have 20dB of headroom in the "digital world", then 0dB FS will
correspond to an analog level of 24dBu. Once the audio output level is calibrated, when you apply a digital
level of –20dB FS, the analog output signal will be 4dBu .
The SDI embedded audio, the AES input audio and the bargraphs are all based on the same digital
quantized signal level. The AES/EBU bargraph ballistics mode is scaled to 0dB FS while the other modes
have a user selectable headroom. For this reason, use the AES/EBU mode for calibrating audio levels.
When a full-scale digital signal is input, you will get a peak analog output signal level that is set by the
analog level
controls. The
Headroom
control sets the 100% program reference level (0 dB on the scale)
with respect to the Peak Output Level set by
the
analog level
controls. For example, if the Peak Output
Level is set to 24 dBu and the
Headroom
is set to 20 dB, then the 0 dB reference on the bar graphs will
correspond to an output level of 4 dBu. This level will be achieved when a digital input signal of –20 dB FS
is applied at the input. Therefore, the following relationship is always maintained:
PGM reference level + headroom = peak output level
NOTE: The VU and PPM standards were set before the digital revolution. These
standards deal with headroom levels that are quite a bit lower what should be used
in the digital world. Also, most of the bar graphs only display a few dB above the
program reference level. In a digital world, one needs to leave around 20 dB of
headroom to keep most material away from clipping.
For best results, follow these steps when setting up the audio bar graphs and output levels:
1. Set the
Headroom
control to the desired value. Remember that this is the value in dB between the
100% reference level (expressed in dB FS) and saturation level of the digital input word (0 dB FS).
It will also correspond to the value in dB between the analog output level when a reference level is
input and the maximum output level set by the
analog level
controls.
2. Pick the bar graph type and mode. This selection will configure the bar graphs with the defaults
dictated by the standards. It will also set the Peak Output Level and the 0dB reference level to
adhere to the standard and the desired headroom.
3. Adjust any of the bar graph configuration parameters to customize the display of the graphs.
4. Save the card configuration into a preset so that you can recall it if any card parameter is
tampered with.
