SingMai Electronics
SM06 User Manual Revision 0.4
Page 10 of 26
4. Automatic Cable length compensation.
aCVi automatically compensates for the cable length. It does this using a special test signal that is
inserted into the aCVi video source signal.
The aCVi transmitter inserts a video test signal into line 9 of the vertical blanking interval (VBI).
This test signal comprises a reference white and black bar and a high frequency sinewave burst
signal. The amplitude difference between the white and black references and the peak to peak
frequency burst are measured by the aCVi receiver (SM08) – the difference indicates the high
frequency loss of the cable.
To compensate for the high frequency loss the aCVi transmitter uses a pre-emphasis filter which
attenuates the lower frequencies with a frequency response characteristic similar to that of the
cable. The aCVi receiver transmits data to the transmitter (on line 8 of VBI) which the transmitter
decodes and uses to control the amount of pre-emphasis. The pre-emphasis is increased until the
high frequency amplitude and the DC amplitude are equal. The transmitter also echoes back the
pre-emphasis value to the receiver to check the transmission is working correctly (on line 7 of the
VBI).
Figure 6 shows lines 7, 8 and 9 of the VBI.
Figure 6 aCVi cable length compensation (lines 7,8 and 9 of the VBI).
Adjusting the pre-emphasis reduces the amplitude of the lower frequencies, which includes the
synchronizing pulses. This is compensated for by an analogue automatic gain control (AGC) at the
receiver. Up to 18dB of analogue gain can be applied on the SM08, allowing full compensation
beyond 300m of RG-59 type coaxial cable.
Beyond this distance, or with cable that attenuates high frequencies more, the SM08 can apply
automatic chroma gain to maintain saturation of the colours. At these extreme distances the luma
frequency response will be reduced below the specification and the transmission may switch to
monochrome.
Figure 7 shows a screen capture of a camera test pattern image (HD-SDI) and Figure 8 shows that
same image after aCVi transmission through 300m of RG-59 coaxial cable.
