Chapter 7 VBI
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237
Ellipse 3000 Release 3.0, Rev. B
VBI Bit-Rate Calculation
7.4 VBI
Bit-Rate
Calculation
The VBI elementary-stream’s bit-rate consumption is dynamic; it changes according to the
number of lines and fields in use. The bit-rate consumption is also affected by the Video Format
mode.
The VBI bit-rate calculation is currently most important now, as the encoder cannot add
elementary-streams if the total bit-rate of the ESs will exceed the transport-stream’s output rate.
If it is required to add another line to the VBI, it must be known how to calculate the VBI’s current
bit-rate and future bit-rate (after adding the new line).
NOTE:
Each VBI odd or even line is considered as a single line. This means that in order to enable a line’s odd
and even rows calculation need to be done for two lines.
Each VBI line’s bit-rate consumption is calculated according to the following formula:
Each row takes 46 bytes from the packet, each byte is made of 8 bits; The number of total bits is
timed by the number of frames per second. The number of frames per second is different in the
two video formats, for PAL it is 25 frames, and for NTSC it is 30 frames.
Raw Data lines (lines that are enabled as Raw Data) are calculated as 18 single lines. In other
words, a Raw Data line’s bit-rate is calculated as a regular line’s bit-rate then timed by 18.
To calculate the total bit-rate consumption of the VBI elementary-stream:
1. Sum all active non-Raw-Data lines (NRD Lines) in the VBI.
2. Sum all Raw-Data lines (RD Lines) in the VBI.
3. Sum the total Lines using the following formula: (NRD Lines)+18*(RD Line)
4. To the sum received in step 3, add 5 more virtual rows. These virtual rows are used for stream
operation and technical requirements.
5. Round down the received number (calculated in Level 4) to the closest number that can be
evenly divided by 4. For example, if the value calculated in level 4 is 15, round it to 12.
6. Time the number received in step 5 with the calculation formula (Table 7–1). For example, if
using PAL, calculate 12x46x8x25.
NOTE:
Due to rounding down the total row number (see Step 4), the user must recalculate the bit-rate
consumption every time when trying to add a new row, as it may change the final numbers. The user cannot
just add (+1) to the final number (timed by the formula).
Table 7–1: VBI Bit-Rate Calculation Formula
46 bytes per row
X
8 bits per byte
X
video format’s
frames per second
46
X
8
X
25
46
X
8
X
30
PAL
NTSC
