T-VIPS CP560, User Manual

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Estimated Transmission Time 187
ID: um_t2gateway CP560 DVB-T2 Gateway User’s Manual Rev. 2.2 (3686)
Appendix F Estimated Transmission Time
The value input in the Estimated Transmission Time (ETT) field in the T2-Gateway defines an
upper limit for the transmission delay in the whole network. The transmission delay is defined
from the CP560 output until the signal is emitted on air. The ETT therefore needs to be large
enough to account for the network delay between the CP560 and the T2-modulator and the
processing delay required in the modulator. For an SFN with several modulators the ETT needs
to be set according to the largest network delay. Keep in mind that IP networks will jitter, and
worse case jitter also needs to be accounted for in the Estimated Transmission Time.
The CP560 generates a T2-MI stream which carries a DVB-T2 timestamp. The DVB-T2 timestamp
defines the transmission time for when the first bit of the T2 Super Frame of the pertaining T2-
signal is to be emitted. The modulator then needs to compare the DVB-T2 timestamp with its
external reference to calculate the remaining time until the signal is to be emitted on air. For
more information on the DVB-T2 timestamp, have a look at chapter 5.2.7 of
[9]: ETSI EN 102
773 .
F.1 How the CP560 uses the configured ETT value
When the CP560 starts building a T2 Super Frame, it samples and stores the phase of the current
external reference (1PPS). The ETT is then added to this value, and then entered into the DVB-T2
timestamp. The ETT has no other implications in the CP560 besides being added to the current
time.
F.1.1 Example with 2 modulators
1. CP560 starts building a T2 Super Frame. External 1PPS is sampled at T2 Super Frame
start to 200 ms.
2. ETT is defined by the user to 450 ms.
3. DVB-T2 timestamp set to 200+450 = 650 ms for the T2-Super Frame
4. T2-MI signal transmitted to Modulator1 over a link with a total delay of 350 ms. Modu-
lator1 compares the DVB-T2 timestamp (650 ms) with the external reference (currently
200+350 ms) and finds that the given signal should be emitted in 100 ms.
5. T2-MI signal transmitted to Modulator2 over a link with a total delay of 300 ms. Modu-
lator2 compares the DVB-T2 timestamp (650 ms) with the external reference (currently
200+300 ms) and finds that the given signal should be emitted in 150 ms.
F.2 Implications of adjusting ETT
If the ETT is set larger than the actual network delay the modulators needs to buffer the T2-MI
stream until the transmission time. The signal is delayed more than necessary.
If the ETT is set less than the actual network delay there is an error, and the outcomes are as
follows: