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Instruction Manual: evolution 5000 E57xx DSNG and DENG Voyager Encoder Page I-1
ST.TM.E10076.3
Annex I
I. Accuracy of Frequency Sources
This equipment is based around ISO/IEC 13818 specifications (commonly
known as MPEG-2) and within these specifications all timing is derived
from a 27 MHz system clock. The system clock is required to have an
accuracy of better than ±30 ppm.
An oven-controlled crystal oscillator (OCXO) within this equipment
achieves the ±30 ppm accuracy within five minutes of applying power. This
accuracy is maintained over the specified operating temperature range for
the life of the product without further adjustment.
Composite television systems such as PAL and NTSC have traditionally
used high precision oscillators for colour sub-carrier. Many different
specifications are in common use and a required accuracy in the range
±0.2 ppm to ±2 ppm is common. Typically an entire TV studio runs from a
central frequency standard, with all equipment being fed with a Black and
Burst reference signal.
Generally, individual items of equipment are not capable of the required
accuracy in the absence of this reference. Where a suitable reference is
not available (e.g. outside broadcast or intercontinental programme
exchange) the specifications allow a relaxed accuracy.
When this equipment is used to source a timing reference which is used to
generate a composite video output (for instance the PAL or NTSC output of
a TANDBERG Television Receiver/Decoder) the accuracy of the resultant
sub-carrier is directly traceable to the 27 MHz system clock in this
equipment.
To ensure continuing accuracy, the system clock in this equipment can be
locked to an appropriate frequency reference by feeding a Black and Burst
signal to the H SYNC input. Alternatively, the system clock can be locked
to the video input. In either case, the system clock is frequency-locked to
the source sync pulses, and hence the composite video sub-carrier is as
accurate as the frequency reference.
Where an accurate reference signal is not available, the OCXO in this
equipment must be used. The OCXO is adjusted to better than ±0.2 ppm
during manufacture, but due to natural ageing of the OCXO, regular
calibration is required to keep the OXCO within ±0.2 ppm if composite
video accuracy is to be maintained. Calibration intervals depend on the
requirements of the particular composite video specification in force.
Please contact TANDBERG Television Customer Services for advice.