Difference between revisions of "VIN0 to HDMI latency measurement (Naon)"
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Several video applications need to acquire a video stream and to display it on an LCD panel or similar device. In some cases it is required that the latency measured between source stream and stream feeding LCD panel is within a specified time (often expressed in terms of fields). | Several video applications need to acquire a video stream and to display it on an LCD panel or similar device. In some cases it is required that the latency measured between source stream and stream feeding LCD panel is within a specified time (often expressed in terms of fields). | ||
| − | This article describes how | + | This article describes how the measurement of such latency has been performed on Naon platform. Texas Instruments provides documentation and software to perform similar measurements: |
http://processors.wiki.ti.com/index.php/Latency_Measurement_on_Capture_Encode_Decode_Display_Demo. Method suggested by TI is completely software-based. Since Linux is not an RTOS, the intrinsic error of such method might not be tolerable where the application has hard real-time latency requirements. Thus a hardware-based approach has been implemented to build ad hoc testbed as described in the following section. | http://processors.wiki.ti.com/index.php/Latency_Measurement_on_Capture_Encode_Decode_Display_Demo. Method suggested by TI is completely software-based. Since Linux is not an RTOS, the intrinsic error of such method might not be tolerable where the application has hard real-time latency requirements. Thus a hardware-based approach has been implemented to build ad hoc testbed as described in the following section. | ||
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Introduction[edit | edit source]
Several video applications need to acquire a video stream and to display it on an LCD panel or similar device. In some cases it is required that the latency measured between source stream and stream feeding LCD panel is within a specified time (often expressed in terms of fields).
This article describes how the measurement of such latency has been performed on Naon platform. Texas Instruments provides documentation and software to perform similar measurements: http://processors.wiki.ti.com/index.php/Latency_Measurement_on_Capture_Encode_Decode_Display_Demo. Method suggested by TI is completely software-based. Since Linux is not an RTOS, the intrinsic error of such method might not be tolerable where the application has hard real-time latency requirements. Thus a hardware-based approach has been implemented to build ad hoc testbed as described in the following section.
Testbed[edit | edit source]
The following picture shows a simplified block diagram of the testbed.
As source a NaonEVB-Lite/Naon system has been used playing an ad hoc video stream. The stream is output via HDMI port that, in turn, is connected to a HDMI splitter. One of the output of the splitter feeds a LCD display that is used by the operator to monitor the whole process. The other one is connected to an ADD-ON_DVI-RX board that is plugged onto J19/J24 connectors of a NaonEVB-Mid carrier board. This chain allows to convert video stream from HDMI to 24-bit RGB format in order to acquire it through VIN0 of Naon module populating NaonEVB-Mid carrier board.
The stream is then output in HDMI format again through HDMI port of NaonEVB-Mid board feeding a second ADD-ON_DVI-RX circuit.
The use of the two ADD-ON_DVI-RX boards allows to create two physical test points where the video stream under investigation in available in a easy-to-handle format, that is color components and syncs are available separately. At these points, it is possible to connect a standard oscilloscope through single-ended passive probes. Video stream contents are generated in order to embed a pattern that is easly detectable. Thus oscilloscope trigger can be configured in order to detect this condition and to measure precisely the latency between the two points of the chain where the probes are connecte as shown in the picture. This measure includes the delay due to the second DVI-to-RGB conversion that is neglectable with respect to the overall latency.
