HD-CVI Technical Overview
HD-CVI Technical Overview
The following is a brief overview of the HD-CVI interface for the transmission of 720p/60Hz HD video. The parameters may be altered to adapt to other resolution requirements.
The basic concept of the HD-CVI interface is to build on the proven and reliable transport method of NTSC; (the advantages of PAL – v.v. multi-path reception – is not relevant to a cable system so the simpler NTSC standard is used as the model). NTSC transmissions are capable of being transmitted over more than 1km across RG-59 cable but the bandwidth is limited to 5MHz. NTSC also has chroma/luma crosstalk issues because of the interleaved nature of the signal that are difficult to resolve at the receiver end.
Because the cable system is a closed system it is only necessary for the transmitter and receiver to 'understand' each other and we can modify the basic NTSC method to suit HD transmissions.
The first thing to overcome is the bandwidth restrictions of the cable. HD 720p/60Hz transmission requires a luma bandwidth of 30MHz. Because we have only a single coaxial cable for the transport we have chosen to transmit luma and colour difference signals, (as opposed to component red, green blue), as the colour difference signals, because of the visual perception of the eye being less acute to colour, can be sent at half the bandwidth: i.e., 15MHz each. This is in accordance to the SMPTE-296M specification.
To further reduce the bandwidth of the transmission the colour difference signals are modulated onto a carrier in quadrature so they effectively use the same bandwidth. However, to avoid the signal recovery problems of NTSC, (and as we have no backward compatibility issues), the carrier is set above the luma bandwidth so there is no interference between the two signals.
For 720p/60Hz transmission the carrier is at 58.559489MHz which ensures there is no beating with the horizontal or vertical scan frequencies; (the line frequency/carrier phase relationship of NTSC does not have to be adhered to because the chroma and luma are not interleaved). The carrier frequency is suitably far above the luma maximum frequency to allow inexpensive (in terms of silicon area) filters to be used in the transmitter and receiver.
The effective bandwidth of the complete signal is therefore approximately 15MHz + 58.6MHz or about 73MHz. This also sets the minimum sampling frequency at 2x 73MHz or 146MHz. For convenience we choose 148.5MHz as this is related to the 720p/60 SMPTE standard.
For 300m of RG-59 cable we can expect a 30dB loss at this frequency. However the synchronizing signals are at a much lower frequency where the loss is only about 1-2dB so reliable rastering of the received signal should always be assured.
To improve the signal to noise ratio (SNR) of the transmission pre-emphasis is used. The degree of pre-emphasis is programmable to allow for different cable lengths. The maximum pre-emphasis is set at 30dB for our evaluation and the frequency response is set to approximate the RG-59 cable loss.
A further improvement in the SNR is achieved through transmitting a peak to peak video level of 2V which maintains compatibility with any legacy SD equipment on the network and also allows common low-power 5V drivers to be used.
Beyond 300m the bandwidth will start to further fall off. The chroma signal will be the first affected by this being the highest-frequency component. However automatic colour control in the receiver can maintain the colour saturation over a further 20dB signal attenuation (to approximately 500m).
Fall off in the luma bandwidth can be compensated by a small boost in the receiver circuit. At extreme distances the signal will revert to monochrome as the chroma signal falls below the receiver's range of compensation but synchronizing signals should be able to be received at greater than 1km distance. The luma bandwidth will be 'gracefully' reduced as the distance is increased.
Because of the similarity in the transmission method to NTSC both the transmitter and receiver can easily be made to accommodate conventional NTSC/PAL transmissions.