Datasheet
THS8135
SLAS343B –MAY 2001–REVISED APRIL 2013
www.ti.com
Table 7. Full-Scale or ITU.BT601 Reduced-Scale Mode Selection and Impact on Blanking Level
Positioning
AVAILABLE CHANNEL OUTPUT LEVEL DURING
IN VIDEO BLANK ACTIVE CORRESPONDING
DAC (V) AND TO DAC INPUT CODE:
M1_INT M2_INT BLNK_INT OPERATION MODE
GENERIC
DAC (G)
AGY ABPb ARPr
MODES?
L L L V, G GBR 3x10b 4:4:4, full scale range 0 0 0
GBR 3x10b 4:4:4, ITU-R.BT601-
L L H V 64 64 64
compliant range
L H L V, G YCbCr 3x10b 4:4:4, full scale range 0 512 512
YCbCr 3x10b 4:4:4, ITU-R.BT601-
L H H V 64 512 512
compliant range
YCbCr 2x10b 4:2:2, ITU-R.BT601-
H L L V 64 512 512
compliant range
H L H V, G YCbCr 2x10b 4:2:2, full scale range 0 512 512
YCbCr 1x10b 4:2:2, ITU-R.BT601-
H H L V 64 512 512
compliant range
H H H V, G YCbCr 1x10b 4:2:2, full scale range 0 512 512
In full-scale range, the DAC is driven with input codes 0-1023 to the desired video level, set by the resistor
connected to the FSADJ terminal (for example, a full-scale video amplitude of 700 mV when terminated into 37.5
Ω and when using the nominal R
FS
value).
In reduced-scale ITU-R.BT601 range, it is the intention that full-scale video amplitude is reached when the device
is driven with digital inputs within the input code range shown in Table 8. Note that the code range is unequal
between RGBY on one hand and CbCr on the other hand. Figure 1 through Figure 4 illustrates the difference
between ITU-R.BT601 reduced-scale and full-scale code range operation. In reduced-range configuration, the
B/Cb and R/Cr components are digitally amplitude scaled internally. Note that there is no scaling on the G/Y
component. Therefore, to accommodate the 700-mV video compliance on all components, the DAC full-scale
output current needs to be increased between full-scale and reduced scale modes by a factor of 1023/(940-64)
by decreasing R
FS
in that proportion.
This implementation has the advantage of avoiding amplitude scaling on the most critical G/Y component, while
still providing the possibility for instantaneous overshoot/undershoot on the analog component video output when
illegal signals according to ITU-R.BT601, such as super-black or super-white, are applied to the device.
When using reduced-scale range, the output sync:video amplitude ratio is still 7:3, but now takes into account the
reduced code range, not the full-scale range, to determine this ratio. Therefore, proper sync amplitudes are
preserved in either mode, when the full-scale current is modified as explained higher. When changing DAC full-
scale current using R
FS
, the sync amplitude level always scales proportionally with the video output compliance.
Note that even when using reduced-scale range, the midscale blanking level on ABPb and ARPr channels still
corresponds to code 512 = [64+(960-64)/2] when using YCbCr color space configuration. Table 8 shows the valid
reduced input code ranges for RGB and YCbCr operation on each of the input data buses. While the THS8135
allows reduced-scale code range with RGB data, video systems normally use it only with YCbCr type data.
Table 8. Input Code Ranges for ITU.BT601 Modes
OPERATING MODE GY[ ] RPr[ ] BPb[ ]
RGB 64-940 64-940 64-940
YCbCr 64-940 64-960 64-960
DAC Operation
The analog output drivers generate a current of which the drive level can be user-modified by choosing an
appropriate resistor value R
FS
, connected to the FSADJ pin.
All current source amplitudes (video, blanking, and sync) are derived from an internal voltage reference such that
the relative amplitudes of sync, blank, and video are always equal to their nominal relationships.
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