TIMING IS EVERYTHING
Over the years ‘Black Burst’ has been the primary video synchronization signal. The simple analog black signal is a real workhorse. It provides vertical timing that allows switching between two video signals. Without this, whenever a signal was switched, the picture would roll until the new signal locked up. I hate it when that happens.
In addition this signal locks the color phase between the switched signals. Using the sine wave in the Burst portion of the waveform, the phase relationship of the two signals can be locked. Without this, whenever a signal was switched, the colors would rainbow until it relocked.
Black Burst was so good that when digital video came along it was still working hard. The primary use in digital video signals is to give a reference for horizontal alignment. Since digital video is a component format, the burst phase was no longer important. As long as the frame rate (29.97 or 25) and line number (525 or 625) was the same for black burst and SDI, everything was fine.
Unfortunately for black burst, high definition video was riding into town. Just one of the standards (SMPTE 274) added almost a dozen new frame rates and active line combinations.
Black burst still gave it a good try. In cases where the frame rate coincided with the HD frame rate, it could still be used for vertical timing. PAL black burst at 25 fps, still lined up quite well with 1125/50/I, 25/PsF, 25P, 750/50/P, 625/50/P and 625/50/P.
NTSC at 29.97 fps worked well, with a minor adjustment, in 1125/59.94, 29.97/PsF, 29.97/P, 750/59.94/P, 525/59.94/P and 525/59.94/I. The minor adjustment is that the horizontal reference points of line 1 of 1125-line, line 1 of 750-line aligns with line 4 of 525-line black burst signals. Additionally in mixed format studios, the line count disparity can cause problems in the line phase relationship. Some adjustments may be required. This is dependent on the studio system architecture.
Further details of switch point location are available in SMPTE RP-168 ‘Definition of Vertical Interval Switching Point for Synchronous Video Switching’. This Recommended Practice “defines the line number and line timing for the switching point of serial digital and analog interfaces carrying television and data payloads to minimize any disturbance in the active payload area”. Table 2 shows black burst compatibility with some line and field/frame rates.
You may also notice that in the vertical interval where these counts are being calculated, black burst moves between two voltage levels. This is why black burst, when used to time digital signals, is sometimes referred to as bi-level sync. Tri-level sync uses three voltage levels.
EAV - SAV
In the digital video data stream itself there is no analog reference to either black burst or tri-level. The timing information is included in the ancillary data as four consecutive code words. These code words are called either EAV (End Active Video) or SAV (Start Active Video).
The first word is all ‘ones’, the second is all ‘zeros’ and the third is again all ‘zeros’. The fourth word bits include:
F-bit (field/frame) – Always 0 in progressive systems, 0 = field one and 1 for field two in interlaced systems.
V-bit – Shows a 1 during vertical blanking and 0 during active video lines.
H-bit (horizontal) – Shows a 1 for EAV sequence and 0 for SAV sequence.
Parity bits – Error correction data
The frame rate and number of lines determine the exact placement of the EAV first word in relation to the analog 0H.
A line of digital video extends from the first word of EAV through the last word of video data. 0H is the analog horizontal timing reference point and in the analog domain is regarded as the start of the line.
Tri-level sync is becoming a required part of HD system timing.
One reason is that tri-level sync can be created to exactly match any of the standard formats. Black burst only comes in two flavors 29.97 fps (525 line) and 25 fps (625 line).
Another reason is that black burst (gen-lock) is measured at the halfway point of the leading (falling) edge of the pulse. Tri-Level sync uses the halfway point of the trailing (rising) edge of the pulse. These points are used to time the digital video. They are determined by means of a sync separator and voltage comparator.
Usually determining the 50% point of the falling edge entails measuring the total height and divide by two. Unfortunately, the trigger point has past by that time. Another method is to infer the total height from previous sync pulses. This involves some averaging process. In addition, the amplitude of the pulse can vary due to attenuation in the cables. These effects cause some uncertainty in the final positioning. This uncertainty leads to jitter in the output of the sync separator / comparator.
Tri-level sync was created to avoid this uncertainty. The target 50% point is on the rising edge of the pulse. This point corresponds to the original blanking level. This means that the 50% voltage level is a known voltage. There is no integration or averaging involved. This leads to lower jitter from the sync separator / comparator.
Tri-level sync has advantages and should be used whenever possible in all digital facilities. But that workhorse ‘Black Burst’ will be around as long as standard definition analog equipment is in use and requires timing.
Jim Alfonse, owner of Tri-Sys Designs, is a Systems Integrator with twenty-five years experience in the Broadcast Industry. He's designed, built and commissioned installations from Satellite News Vehicles to Production Suites to OB vans. Jim has been involved with several equipment manufacturers performing video standards compliance and signal integrity testing.
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