There is an increasing need to be able to transmit high definition (HD) video over long cable runs, e.g., for security camera installations. In many cases the cable installation is pre-existing and uses low cost RG-59 cable but this cable is limited to low frequency use (<200MHz) for most applications; (at 1GHz its attenuation is 30dB/100m).
Existing methods to transmit HD video include separate analogue RGB/YPbPr, which requires three coaxial cables to transmit, or HD-SDI (and its derivative HD-CCTV), a serial digital transmission method which runs at a bit rate of 1.485MHz and can only achieve small distances with such cable.
Whilst analogue cable equalizers are available from some semiconductor manufacturers (e.g., Analog Devices and Intersil), they still require the transmission of three components (e.g., YPbPr) which involves the re-installation of additional cables, which can add considerably to the total system cost of a security installation.
Other methods, such as compression of the video prior to transmission, (the IP camera), add considerable cost and power budget to each camera as well as limiting options in the post-processing of the image.
In this article we will discuss a method of transmitting HD video across this medium using a modified form of the well-known NTSC analogue composite video standard to create the signal for transmission: we have christened the method HD-CVI, for High Definition Composite Video Interface.
Distances of greater than 300m are achievable and in excess of 500m at 720p/60Hz with some small signal degradation. As with most analogue transmission methods, the signal degradation is 'graceful' with no sudden cut-off of the signal that is encountered with digital methods. HD-CVI is compatible with most HD video standards and also non-standard video, (e.g., from HD-CCD cameras).
HD-CVI is also backward compatible to standard definition NTSC/PAL transmissions whilst also offering a considerable transmission distance increase.
HD-CVI is also designed to support a back channel allowing control information to be embedded in the vertical blanking interval for information transfer between the receiver/transmitter; for example to control camera functions, and between the transmitter/receiver for embedding information about the transmitter signal source, standard or the camera environment. The back channel also permits automatic cable equalization.
RG-59 Cable Specification
Exact specifications vary across cable providers but an average of these will show an attenuation of approximately 2.4dB/100ft at 50MHz and 3.5dB/100ft at 100MHz, the frequency range of interest to us; (7.8db/100m and 11.5dB/100m respectively).
These figures do not take into account cable installation issues or other issues such as hum pickup. The HD-CVI interface has some degree of hum rejection through its pseudo-differential receiver input stage.
For the target distance specification of 300m and at the highest transmitted frequency (for 720p/60Hz = 73MHz) we are therefore required to accommodate approximately 3dB/100ft, (9.8dB/100m), or 29.5dB of loss.
The loss at 100kHz for a 300m cable run is 0.9dB.
A typical cable specification is shown in Figure 1.
Figure 1: Typical RG-59 Cable Specification from Hosiwell Technology Co. Ltd. At 1GHz the attenuation of 30dB/100m makes it totally unsuitable for transmitting HD-SDI at 1.485GHz.