DVB-C Solutions
Product Note
HP 8594Q QAM Analyzer
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Introduction
All cable TV network operators planning the introduction of DVB-C
(Digital Video Broadcast via Cable) services have increasing competitive pressure to install these new services - quickly. With the added
complication of migrating to a new technology, measuring new modulation types and learning how digital signals are affected through
the system, it is important to be able to quickly measure system
performance. The HP 8594Q QAM analyzer is a comprehensive
and powerful test solution for installation and maintenance test
on DVB-C systems. This product note describes the capabilities of
the HP 8594Q QAM analyzer, where it can be used and why the
measurements it performs are important.
Before describing the measurements in detail it is important to
understand the flow of data through the system from transmitter
to receiver.
The layered structure for the flow of information
Transmitter (Headend)
Video Image
Digital Compression
FEC Coding
Bit Stream Format
Pre-emphasis
Digital Modulation
RF Up-conversion
Cable Network
RF Down-conversion
Receiver (Set top)
FEC Decoding
Digital Demodulation
Adaptive Equalization
Bit Stream Deformat
Digital Decompression
Video Image
Figure 1: The layered structure for
the flow of information
Power and
Spectrum
Tests
Modulation
Quality
Tests
Data
Quality
Tests
Picture
Quality
Tests
Measurement Layers
The video image is digitized, converting the analog signal to data
bits. The data bit stream is compressed, using MPEG-2, into packet
data. These packets are then organized into the transmission format
called an MPEG-2 transport stream (TS). Forward error correction
(FEC) coding is added to protect the MPEG-2 transport stream as
it passes through the system. The digital video signal may be preemphasized to compensate for known problems in the transmission
path. Finally, the data stream is digitally modulated and upconverted to the carrier frequency.
The digital video receiver, or set top box, reverses the signal processing layers performed in the transmitter. The key advantages of a
digital receiver are the signal distortion compensation provided
in the adaptive equalizer, and the bit error correction provided
in the FEC decoder. These two signal processing blocks remove
impairments from the received signal.
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What measurements
are important?
Digital video signals differ from traditional analog signals in that
forward error correction and coding are designed to conceal transmission errors and linear transmission medium distortions. Therefore, different measurements are appropriate for different stages
in the transmission process.
Power and spectrum tests are applied to the RF digital video signal.
Modulation quality is assessed after digital demodulation, around
the adaptive equalizer. Data quality tests examine the integrity of
the bits recovered from the digital modulation, including the bit
correction effect of the FEC.
• Power measurements are key to adjusting levels and minimizing
inter-channel distortions throughout the cable distribution system.
• Spectrum measurements give a clear view of the RF channel
quality.
• Direct measurements on the digital modulation are useful tools
for troubleshooting the source of signal impairments.
• Data quality is the key product delivered to the subscriber.
Data quality tests are overall, end-to-end checks on the integrity
of the digital cable system.
Figure 2: A simplified view of a
cable TV system
Test from the headend through to the subscriber drop
Transport
Coax Feeder
Coax Drop
System
Hub
Headend
Amp
Amp
The HP 8594Q QAM analyzer addresses the power, modulation and
data quality measurement layers from the headend through to the
subscriber drop. Whether measuring high-level signals at the output of a modulator or measuring low-level signals in a crowded
spectrum at the subscriber drop, the HP 8594Q performs the key
RF and modulation measurements quickly and accurately.
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