ADV7401/03/ ADV7188/84 ADV7180
Free-Run Blue Screen
Introduction
Evaluation Setup
Customer used and ADV7403-ADV7321 with a Golledge 28.63636Mhz Crystal.
Scenario 1:When using a 28Mhz Xtal there is a slight offset on the LLC Free-Run output
Clock for PAL
Decoder has previously been locked to an Input signal but has now lost-lock. The decoder should
enter free-run and produce a default Blue screen. However when the standard is PAL the setup
produces a stable grey screen. This problem was not observed in NTSC. This loss of colour
affected certain monitors.
Figure 1. PAL Blue Screen has lost color.
Scenario 2:When working with 28Mhz Xtal the Part will not Free-Run Correctly unless a
Source has been detected.
Decoder has been programmed and no input has been applied. The decoder is Out-of-Lock in
this situation and therefore should enter free-run and produce a default Blue-screen. However
this may produce an unlocked picture with scrolling or tearing lines across the screen. This
occurs regardless of the standard. Below are the results for a JVC BM-H1400PNKA (Figure 2.)
and Sony KX-14CP1 (Figure 3)
Figure 2. JVC Free-run output without ever been In
Lock
Free-run Blue Screen Issue
nnicheallaigh
Analog Devices B.V.
Figure 3. Sony Free-run output without ever been
In Lock
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Scenario 1
The issue in Scenario 1 relates to an offset in the 27Mhz output Line Locked Clock, LLC,
provided by the decoder when the part enters Free-Run Mode. As the LLC output is used to
clock the encoder any offset in it will also result in a corresponding offset on the encoder color
subcarrier output. Due to the offset the colour-subcarrier may not be accurate enough for a
backend device to decode the colour correctly resulting in Loss of colour and flicker.
The frequency of the LLC was measured once the decoder had lost lock and the results are
tabulated in Table 1.It is clear from these results that when the standard is PAL and the decoder
looses lock there is a significant offset, ~9KHz, in the LLC. It is apparent from these results that
the offset is much lower when the standard is NTSC. This explains why blue screen is achievable
in NTSC. Please note that this offset may vary with XTAL cct and Crystal Attributes.
Standard
CVBS PAL
CVBS NTSC
XTAl Cct. Load Caps
47pF*
47pF*
LLC MHz
27.00883
26.997845
Table 1. LLC Frequency Values for various Crystal Circuits for Scenario 1.
During testing we noticed that some monitors maybe more robust to this offset than others. The
Sony KX-14CP1 was not able to achieve blue screen with this offset.
To achieve blue screen, one must compensate for the offset. This is done by disabling the
SFL/RTCO function and programming the Subcarrier Frequency registers in the Encoder with the
expected offset.
Scenario 2: Freerun Bug.
For the ADV7403/01 & ADV7188/84 decoders the pixel and sync outputs are invalid if the
decoder has not previously been locked to an input.
We conclude that it is not possible to achieve a blue screen free run with this configuration. The
alternative is to use the Test Pattern function in the encoder to produce a good locked image in
the event of no input being applied.
In addition to this the LLC output will have and offset as shown in table 2 below.
Standard
CVBS PAL
CVBS NTSC
XTAl Cct. Load Caps
47pF*
47pF*
LLC MHz
26.910735
26.910735
Table 2 LLC Frequency Values for various Crystal Circuits for Scenario 2.
*Note: To achieve accurate oscillator performance it is important that the proper load cap values are used ont eh
Xtal Support circuitry. Please see applications note note_on_xtal_rev0.pdf
Free-run Blue Screen Issue
nnicheallaigh
Analog Devices B.V.
Video Group Limerick
Page 2 of 3
Current Proposed Solution
This solution will provide a Colour Bars Test pattern every time the system loses its input signal.
We propose that the system monitors the IN_LOCK status bit ( Reg. 0x10 bit 0) provided by the
decoder. When the system is achieves lock , IN_LOCK has a value of 1b, the systems control
registers this fact and stores the standard provided by the decoders AD_RESULT[3:0] (Reg 0x10
bits 4,5,6).
When IN_LOCK has a value of 0b, the system has lost lock to an input signal the systems control
disables RTCO (Reg 0x44 bits1&2), manually programs the color subcarrier registers ( Reg 0x4C
to 0x4F) within the encoder to compensate for the offset in the LLC and finally enabling colour
bars on the encoder.
Figure 4 Flowchart to produce Encoder Test Pattern when Input is not Connected.
Free-run Blue Screen Issue
nnicheallaigh
Analog Devices B.V.
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