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REPORT: Input lag Part 4

This singularity of the V-Sync signal, which always announces the start of a new frame, makes it extremely important for the measurements carried out later. It servers as a temporal reference, the so-called trigger point, for most of the measurements of the digital signal.

All other 560 combinations which can be formed from 10 bits are reserved and their use is forbidden.

The detailed algorithm for the discovery of the bit patterns can be referred to in the official specifications for the DDWG on Page 29. See: DVI 1.0 Specifications.

Data rate and electrical refinements

8b/10b coded serial bit streams, which have an extremely high data rate at up to 1,65 GB/s, are transmitted on the each of the three "channels" via a "single-link" DVI cable. This bit rate is sufficient for resolutions of 1.600 x 1.200 at 60 Hz and/or 1.920 x 1.200 at 60 Hz with reduced blanking. In accordance with DVI specifications, these signals are TDMS coded, but sometimes to not confirm to the normal TDMS signals and therefore could not be decoded automatically.

As well as studying the specifications available to the public, some creative advice was also necessary, therefore, in order to make out clear bit patterns in the transfer, since the largest graphics card manufacturers remained silent when we asked specific questions about the clear bit patterns or declared simply and honestly that this information is not made available.

The transmission of the signals described above takes place with a voltage of 3.3 V differentially via twisted pairs of wires. The differential transmission type causes many induced interference signals to arise in opposition since the interference voltage in both connections is at least close to identical and thus the difference between the individual cables remains constant.

Figure 4: Interference induced from the outside on a differential signal.

For the display and examination of signals with a data rate of 1,65 GHz, high-end oscilloscopes with bandwidths of at least 4 GHz are needed, which are ideally also capable of recognising serial bit patterns and triggering on freely provided bit patterns. Through the friendly support of Tektronix, an outstanding DSA71254 oscilloscope was provided for the test, which far exceeded our requirements with a lofty bandwidth of 12,5 GHz and which will be presented in more detail later in the report.

The analogue signal

In direct comparison with the digital signals, the analogue signals are easy to record, with their regular bandwidths of less than 200 MHz at an amplitude of 0.7 V peak-to-peak at 75 Ohm, and do not present a challenge for such an oscilloscope. Furthermore, the examination of the V-Sync signals is made easier by the fact that they are transmitted on a separate connection and therefore to not need to be separated from the other image information first.

Interim conclusion

With this basic knowledge, the V-Sync signals in the image format, for example, can be used to examine the relative positions of the allegedly parallel image outputs to each other or also to examine the actual refresh rates more closely. Further details on how to carry these actions out precisely can be found in the sections for the respective measurements.

Furthermore, in the course of the test, we will naturally check the difference in time between image content played back with he screen output, both on the CRT and on a range of LCDs at various resolutions in order to determine the true input lag times as precisely as possible. To this end, the respective input signal is used as a time reference with the oscilloscope and is compared with the signal of a precise and very fast responding photo receiver, the FEMTO SI-200-OE, which was provided by the manufacturer.

Measurement setup and devices

The oscilloscope

Tektronix DSA71254:

Tektronix provided a type DSA71254 oscilloscope for the entire duration of the test which has four channels, a bandwidth of 12,5 GHz and a sampling rate of 50 GS/s. This high-end oscilloscope far exceeds all requirements which a test of this kind could have. In addition, as a DSA model, it has special software extensions which allow not only for the triggering of serial bit patterns but also for their analysis.

The Tektronix DSA71254 offers a 12,5 GHz bandwidth.

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