Using the Video Quality Analysis Option DVQM-B4, the assessment of picture quality according to subjective criteria becomes an objective realtime measurement method. Picture quality is assessed from artefacts produced by digital compression. The method is based on the analysis of video data and can thus also be used where no reference video material is available. For complete display and analysis of all coding data as wel l as conveni ent remote control of the DVQM and display of the recorded quality data, the Elementary Stream Analyser PC Software DVQ-B1 can be used in addition.
The increasing use of digital, data-compressed TV signals calls for monitoring and assessment of the picture quality. Picture quality assessment is very strongly influenced by the subjective perception of the human eye.
The DVQM is a tool that ideally satisfies both requirements. It optionally determines the picture quality in relation to digital compression and evaluates the results according to the subjective criteria of visual perception.
The DVQM is the multichannel version of the successful Digital Video Quality Analyser DVQ from Rohde&Schwarz. The DVQM can combine the performance of up to twelve DVQs. The resulting large variety of configurations allows the DVQM to be optimally adapted to different requirement profiles.
For configuration of the individual analyser boards and for readout of the measurement results, the DVQM comes with the DTV NetView PC software running under MS Windows. This software enables remote communication with all instruments via the Ethernet interface where the instruments are not used at the same place or there are major distances between the measuring instruments and the PC.
The software can be individually adapted to different instrument configurations and provides a fast overview of the analysis results of all the instruments.
Video quality analysis is optionally available (DVQM-B4) for the individual analyser boards of the DVQM.
Characteristics
The basic unit of the DVQM comes with two analyser boards. The instrument can accommodate another 10 boards. These may be analyser boards for simultaneous monitoring of 12 channels, or descrambling boards for decoding of pay TV programs. Up to six scrambled programs can be monitored simultaneously (6 descrambling boards and 6 analyser boards).
Analyser board characteristics
Test parameters
Each analyser board can be used to monitor all the relevant parameters of the video and audio elementary streams of the selected program. Moreover, they determine whether a valid transport stream is present or whether there are failures. The hysteresis for the detection of transport streams or failures can be set by the user.
The video stream is checked for picture freeze and picture loss. The check is made using the threshold values for spatial and temporal activities as well as the period for which the threshold values have not been adhered to. All threshold values for the determination of picture freeze and picture loss can be set by the user. The audio stream, if available (audio sync), is checked for its volume separately for the right and left channels. If the volume is below a certain threshold for a defined period, this indicates a sound loss. Minimum volume and maximum period can be set by the user. AC-3-coded audio streams are also monitored in this way. For this purpose, the audio signal is downconverted to a stereo signal according to a method especially specified by Dolby so that this signal can be monitored in the described way. The DVQM-B4 option allows additional continuous monitoring of the picture quality.
Decoder
In addition to the analysis unit, each analyser board also has a built-in MPEG-2 decoder for audio and video data in the format Main Profile @ MainLevel and 4:2:2 Profile @ MainLevel. The program being analysed is decoded and can simultaneously be viewed on a connected video monitor (CCVS or ITU-R 601 or SMPTE259M formats). The audio signals are available at the connectors in both analog and digital form (AES/EBU).
On-screen display
For the analysis of an MPEG-2 transport stream, an on-screen display (OSD) can be switched on for each channel. The OSD is output together with the decoded picture via the CCVS or SDI output. Different views with a variety of information are available. The OSD is not available for measurements on SDI signals.
Alarm outputs
Altogether 12 relay contacts, which can be allocated to one or several (ORed) events, are fitted as standard. The switching mode (active when open or closed) can be set for the electrically isolated relays.
Scan mode for several programs
An MPEG-2 transport stream usually contains several programs made up of video and audio data streams. The advantage of the DVQM is its multichannel capability allowing several programs of a single transport stream or of various transport streams to be simultaneously monitored in realtime. Where continuous monitoring is not required, the scan mode allows all or selected programs of a transport stream to be successively analysed for picture quality and interference over a selectable period of time. The threshold values for the detection of picture freeze, picture loss and sound loss as well as the minimum value for picture quality can be set separately for each program in the scan mode. In addition, the user can select - for each of these tests and separately for each program - after how many scans with consecutive errors a given error is to be recorded and signalled. Thanks to these two setting facilities, monitoring can be optimally adapted to the specific characteristics of each program transmitted. The scan mode can be individually selected for each analyser board, and it can be configured so that upon detection of an error, monitoring of the program is continued until the error is remedied.
Clearly visible blocking effects on digitally coded TV picture and - by comparison - picture without blocking
Reference measurement
For comparative quality measurements, the quality analysis can simultaneously be carried out on two different signals.
Quality analysis is carried out completely independently for each signal and the final result is formed from the differences found. There is no pixel comparison of two video data sources in this mode, but the difference is formed on the basis of reduced video data according to the weighting algorithm. The reference signal is applied either as an uncompressed SDI video stream (to ITU-R 601/656 or SMPTE259M) or as a transport stream (ASI or SPI) to the DVQM input that is not occupied by the signal to be analysed. The DVQM automatically detects and compensates for any delay of up to ±5 s between the reference and the test signal.